varaibles

Variables and Data Types A variable is a storage location in the memory of the computer. A variable is usually given a name to make it easy to refer to (meaningful variable names). Data can be stored in a variable at any time during the execution of a program.

When variables are created they are normally able to store one particular type of data. A variable cannot normally be used to store data of any type other than the type for which it was created.

Data Types To avoid silly mistakes like trying to multiply two letters together, languages tend to classify data into distinct types, and restrict the range of operations which can be performed on any given type.

Different data types are used for two reasons. Firstly, each data type is represented differently internally and the compiler or interpreter can tell immediately how much memory to set aside for each one. Secondly, using of data types allows the programmer to validate data and this aids the robustness of the software.

There are two main families of data types available to the programmer, simple (or scalar) data types and structured (or complex) data types.

Simple Data Types Integer An integer value may be any positive or negative whole number. Examples of integers are: 0, 17, -3, 876 Real The definition of a real value is not so clear cut as the integer. A real value can be any number. In most programming environments, a real value is taken to mean a number with a fractional part. Examples are: 2.0, 3.9, -789.5432 A real number is normally represented internally by the machine as a floating point binary number. Some environments actually name the type 'floating point' to avoid ambiguity. Some environments allow integers to be treated as a subset of real numbers.

For example, the number 4, stored as a real number rather than an integer, might be represented by 3.9999999 or 4.0000001 Character A character value is taken to mean any single character, usually from the ASCII character set. Characters are usually differentiated from number values by enclosing them in quotes. Examples of character values might be: a, $,, %, Z Boolean A boolean value is a two-state value. It can either be True or False. This data type is a useful tool for terminating loops and satisfying conditions within a program.

Most software development environments provide facilities for storing simple data types such as, integer, real, character and boolean.

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Infosheet 1.1

Variables and Data Types Structured Data Types Structured data types are features of high level programming languages. They are for storing items of data that are grouped or structured in a particular way so that you can manage all the data items as one.

Strings A string is a sequence of characters which can include letters, both upper and lower case, numbers, punctuation marks etc. The beginning and end of a string must be marked with quotation marks e.g. “aAb_123”. Different languages implement strings using single or double quotes.

Strings can be assigned to variables. Some environments include string as a simple type. For example the language BASIC, allows us to group individual characters together and treat them as a single item of data.

Arrays Often we use the computer to store and manipulate sets or lists of data. For example we might want to search through a list of names for one particular name or sort a group of numbers into an order. An array is a data structure which makes it easier to deal with a list of data.

Using only simple data types we could store a group of five names each in a different variable like this: LET name1$ := “John” LET name2$:= “Peter” LET name3$:= “Mary” LET name4$:= "Alex" LET name5$: = "Louise" In this case, the program is using 5 separate variables – name1$, name2$, name3$, name4$ and name5$.

The program treats 'name1$' as an isolated variable and it does not connect to any other variable even if it looks similar.

The program has no way of knowing that name1£ is the 1st item, name2$ is the 2nd item, name3$ is the 3rd and so on. However using an array structure the items can be seen as a list and every item on that list is identified in a particular order.

Arrays can be declared as any data type. In the example above the array would be declared as string. So the data could be declared as: Array data is usually stored internally in a contiguous set of storage locations.

An array is a set of data all sharing the same variable name which can be accessed by means of a subscript. This subscript identifies the position of the element within the array. Subscripted variables allow the programmer to loop through the variables in the array by changing the subscript. They cut down the number of variable names needed in a program. And hence make the program more efficient.

An array therefore is a list used to 'hold' data values. The data values could be real numbers, integers, characters, strings etc.

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Infosheet 1.2

Variables and Data Types Using the example above this diagram represents the array called name, with space for 5 data items or elements: Subscript

1

2

3

4

5

name Each element of the array can be referred to using the array name with the subscript in brackets after the name.

Example name$(1)= “John” This will assign the string “John” to array element at position 1 of the array called name i.e.

Subscript

1

2

name

John

3

4

5

The other elements in the array could also be assigned depending on the data values they had to store.

Using the example: name$(2) = “Peter” name$(3)= “Mary” name$(4) = “Alex” name$(5)= “Louise” This will assign the string “Peter” to variable number 2 of the array called name, “Mary” to variable number 3 of the array called name etc .

Subscript

1

2

3

4

5

name

John

Peter

Mary

Alex

Louise

Each variable in an array can be treated in exactly the same manner as a single variable. It can be assigned a value, the value can be altered, printed out, and used in a program statement.

This type of array is called a one dimensional array. Arrays with more than one subscript can be used.

For example a two dimensional array would have two subscripts etc. Using the example below: name$(1,1) = Susan, name$(1,2) = Peter, name$(1,3)= Margaret, name$(2,1)= Sarah, etc.

1

2

3

1

Susan

Peter

Margaret

2

Sarah

Careywyn

Calum

3

Tara

James Two dimensional array

Murdina

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Infosheet 1.3

Subprograms, Variables and Parameters Types Of Subprograms Subprograms are used by programmers to give modularity to programs. In other words the program is broken up into smaller parts using these subprograms. There are generally two kinds of subprograms, procedures (or subroutines) and user-defined functions. Procedures and user-defined functions are a series of instructions within the program that have been given a name and can be called at any time within the program code. The difference between a procedure and a user-defined function is that a procedure produces an effect, whereas a user-defined function produces a value. In practice, knowing which to use, usually comes down to a mixture of experience and personal preference.

Calling Subprograms Into Action The sequence in which subprograms are carried out is decided by the control structures used in the program itself. It is normal to have a top level algorithm which controls the execution of the subprograms.

This is sometimes called the control module or main program.

Using Variables Within A Program Some languages allow variables to be set up at any point in a program other languages need variables to be declared at the start of a program before they are used within the program. This allows the translator program to reserve suitable areas of memory to hold the data structures which will subsequently be used by the program. The declaration of variables also serves as a discipline to programmers because they have to create a list which details the name, type, and purpose of each variable used in their program. There are two different types of variables – one that is used throughout the whole program and one that is restricted to a subprogram.

Local Variables Local variables are variables which exist only within a subprogram. They are created when the subprogram is called or invoked, and are destroyed when the subprogram terminates. They cannot be accessed or assigned a value except within that subprogram.

Local variables are safer to use because they avoid potential conflict between a subprogram and the calling program.

Global Variables Global variables are variables which can be accessed and altered from any part of a program including subprograms. They should always be used with care, their values should not be changed or they should not be used within subprograms if at all possible.

So the use of global variables should be avoided where possible, as there is a danger that their values may be reassigned accidentally.

A variable can store many different types of data. When assigned, a variable cannot change its data type.

Data Flow Through Parameter Passing Data flow within a program is handled by parameters. A parameter is a variable or value that is passed into, or out from, a subprogram. If a subprogram is defined in such a way that it only uses local variables or values passed to it as parameters then this can be of benefit as:  it is unlikely to change accidentally the value of variables used elsewhere in the program  the subprogram can also be used elsewhere if it is completely self-contained  the use of parameters to pass information to and from a subroutine improves the generality of a subprogram and is likely help the reliability and robustness of the program as a whole.

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Infosheet 2.1

Subprograms, Variables and Parameters The parameters which are passed into a subprogram when it is called from another part of the program are called the actual parameters. The parameters which are used in the subprogram definition are called the formal parameters.

Passing Parameters By Value And By Reference There are two methods of passing formal parameters: by reference and by value. Some programming languages such as Pascal use different syntax for each of these.

Parameter passing by reference is used when the value being passed in can be updated and passed back out again. This means that the data that entered the subprogram could leave with a different value. The program uses the memory address of where the parameter is stored as its reference. So any updating of the value within the subprogram would ensure that the changes would be made to the parameter before it left the subprogram.

Parameter passing by value is used when a value is passed in to a subroutine but does not require to be passed out. This means that the data that entered the subprogram would leave with the same value that it entered with. In order to achieve this, the program makes a second copy of the parameter and uses that copy when the subprogram is executed. This ensures that the original is not changed. Passing by value is not used for large data structures such as arrays as that would be very inefficient i.e. unnecessary RAM could be taken up storing the second copy. So in this case, the need for efficiency is more important than the need for good design. So all arrays are passed by reference to subprograms.

Examples Using Pascal, the syntax is different for each: procedure calculate(var area :real)

the var indicates that the parameter area is passed by reference

procedure calculate(area :real)

no var command indicates that the parameter area is passed by value

Using TrueBasic, the syntax is also different for each: CALL calculate(length,breadth) the single brackets makes this call passing by reference CALL calculate((length),(breadth)) the double brackets makes this call passing by value Scope The scope of a variable is the parts of the program it can be used in. For example if a variable is declared within a subprogram then it can only be used with the code within that routine. This will be the variable’s scope. A variable cannot be used outwith its scope. This enables variables with the same name to be used in different parts of a program without causes problems with reassignment.

This concept is useful when large programs are being constructed with various programmers working on separate modules. It prevents conflict between variables happening when the complete system (all modules) is run together.

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Infosheet 2.2

Using Algorithms Most software development environments provide a fairly standard set of facilities. These facilities are for storing data, for exercising control over the execution of a program, and for passing data from one program or part of a program to another.

Before a program is can be written though, an algorithm must be determined. An algorithm is the detailed sequence of operations to be performed to solve the problem. Often there may be several alternative algorithms for a problem and the program designer must select the most efficient algorithm for the circumstances of the problem.

Pseudocode Pseudocode is a cross between a programming language and English. It is sufficiently like English to allow the programmer to concentrate initially on the program design without worrying about the complexities of the programming language. However pseudocode does contain enough programming language features to code the finished design quickly in the chosen program language. The lines in a pseudocode algorithm should in theory have a one-to-one mapping with the actual code. Although the description can be easily converted into high level language code, pseudocode is not simply a set of language statements.

A top-down design approach should be used when producing an algorithm. This involves splitting the overall problem into smaller tasks. This allows the problem to be understood and ensure elements are not missed out. Stepwise refinement involves splitting these smaller tasks into even smaller ones until they are more manageable and therefore easier to be understood and solved.

Example Typical pseudocode for a program has to be written to find the average of a set of data items.

Algorithm

Refine step 1 Refine step 2

Program Inputs: data Program Outputs: average 1 initialise variables (out: total, count) 2 work out average (in: total, count out: average) 3 print the average of the data set (in: average) 1.1 initialise the total and count of data items to zero 2.1 get the first data item 2.2 while there are more data items do 2.3 add data item to total 2.4 increment the count of data items 2.5 endwhile 2.6 divide the total by the count of data items

When writing pseudocode it is important to use indentation and keywords to represent the design structure. Every program should be broken down into modules and then each module should be described and again broken down fully. This shows the top-down design and stepwise refinement. The data flow should also be indicated to show what parameters are being passed in and out of each refinement.

The algorithm will be a one-to-one mapping with the main program or control module of the code. The refinements should indicate the number of subprograms that will be used, what parameters are being used and whether they are passed by reference or by value i.e. in would be value parameters and out or in/out would be reference parameters.

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Infosheet 3.1

Using Algorithms Flowcharts A flowchart is often used to illustrate an algorithm (set of instructions) to solve a problem. A flowchart is a diagram that uses different shapes and arrows to show the steps in a problem solution. Particular box shapes are used to represent different aspects of coding. A flow chart is an example of a graphical design notation.

Using the same example, but with flowcharts this time: Example A program has to be written to find the average of a set of data items.

A typical flowchart for this problem could be:

Start

Initialise variables

Get first value Add 1 to count

Any more values?

Yes Add value to total

No

Calculate average

Print out average

Stop

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Infosheet 3.2

Using Algorithms Structure Diagram A structure diagram shows the different sections of the program. It is a treelike diagram, representing component modules and their interrelationships. It also shows the hierarchy of each module. A structure diagram is another example of a graphical design notation.

Using the same example , but with structure diagrams this time: Example A program has to be written to find the average of a set of data items.

A typical structure diagram for this problem could be:

Finding the average of a set of data values

Get first data values

Repeat until no more data values

Add data item to total

Calculate average

Print average

Add 1 to count

Notes about structure diagrams 

The box at the top describes what the whole program does.



The boxes below are arranged in logical order from left to right.



Each box shows a different section of the program.



The box with a double line at each side shows a subroutine or procedure.



The flow of data may be indicated by an arrow labeled with the data name.

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Infosheet 3.3

Simple Selection A simple selection (or conditional statement) allows the programmer to let the program select which of two different sets of steps to do next. The general form of the IF….THEN….END IF is:

IF condition is true THEN the program performs sequence of instructions ELSE the condition is false the program follows another sequence of instructions END IF

Example 2.1 This example is part of an input validation routine - it is checking to see whether the data entered at the keyboard is valid. In this example it is validating an age by checking it is not a negative number.

Algorithm 1.

2.

3.

4.

get age of user if age less than 0 then display suitable error message end if

Notice that there is no 'ELSE' in this one - if a suitable age is entered, there is no need for any further action so the program simply continues with whatever comes next in the sequence. Note also that lower case is used for the keywords in the pseudocode.

Program PRINT "How old are you" INPUT age IF age<0 THEN PRINT "Error - please re-enter" END IF Example 2.2 This example validates an age by checking it is not a negative number or greater than 100. In this case we do use the 'ELSE'.

Algorithm 1.

2.

3.

4.

get age of user if age less than 0 OR age > 100 then display suitable error message End If

Program PRINT "How old are you" INPUT age IF age<0 THEN PRINT "Too young - please re-enter" END IF IF age>100 THEN PRINT "Too old - please re-enter" END IF

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Infosheet 4.1

Simple Selection Example 2.3:- This example is part of a payroll program for calculating wages. The employee earns £5.65 an hour. Any hours worked in excess of 40 are paid at the overtime rate of “time and a half”. Notice that there are two steps being executed between the IF and END IF. Algorithm

1.

2.

3.

4.

ask user for number of hours ask user for rate of pay calculate wages display wages

refine step 1

1.1 1.2

ask user for number of hours worked store response in hours

refine step 2

2.1 2.2

ask user for rate of pay store response in rate

refine step 3

3.1 3.2 3.3 3.4 3.5 3.6

if hours greater than 40 calculate overtime using (hours-40) calculate wages using (40 x rate)+(overtime x 1.5 x rate) else calculate wages using (hours x rate) end if

Program CALL get_hours_routine(hours) CALL get_rate_routine(rate) CALL calculatewages((hours),(rate),wages) CALL display_wages(wages) END SUB get_hours_routine(hours) PRINT”Please enter the hours worked this week-”; INPUT hours END SUB SUB get_rate_routine(rate) PRINT”Please enter the rate of pay-”; INPUT rate END SUB SUB calculatewages((hours(,(rate),wages) IF hours>40 THEN LET overtime=hours-40 LET wages=40* rate + 1.5*overtime* rate ELSE LET wages=hours*rate END IF END SUB SUB display_wages(wages) PRINT”Your wages are - “;wages END SUB

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Infosheet 4.2

Multiple Outcome Selection Often when producing a solution to a problem there are not just two alternatives as in the previous example but many. This is sometimes known as multiway selection and can normally be accomplished in one of two ways. One way uses the IF structure from the previous examples and the other uses a CASE structure which is specially designed to deal with multiple outcome selection.

Example 3.1 This example uses nested IFs to calculate the grade which corresponds to a student mark.

Algorithm 1. get mark from user 2. if mark from 70 to 100 3. set grade to A 4. elseif mark from 60 to 69 5. set grade to B 6. elseif mark from 50 to 59 7. set grade to C 8. elseif mark from 40 to 49 9. set grade to D 10. else 11. set grade to FAIL 12. end if 13. end if 14. end if 15. end if 16. endif

Program SUB Getgrade INPUT mark IF mark>69 THEN Pass$="A" ELSE IF mark>59 THEN Pass$="B" ELSE IF MARK>49 THEN PASS$="C" ELSE IF MARK>40 THEN PASS$="D" ELSE PASS$="FAIL" END IF END IF END IF END IF END SUB

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Infosheet 4.3

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Multiple Outcome Selection CASE Structure Another variation of the selection algorithm allows the programmer to select one action or a set of actions, when one particular condition is true. This is called a CASE structure. You will find that different languages implement this structure in significantly different ways.

Example 3.2 This example uses CASE to calculate the grade which corresponds to a student mark.

Algorithm 1.

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4.

5.

6.

7.

8.

Refine 1 1.1 1.2

get mark from user select case mark when from 70 to 100 when from 60 to 69 when from 50 to 59 when from 40 to 49 when from 0 to 39 end select

set grade to A set grade to B set grade to C set grade to D set grade to FAIL

ask user for mark store mark

Program

SUB Getgrade INPUT mark SELECT CASE mark CASE 70 TO 100 Pass$="A" CASE 60 TO 69 Pass$="B" CASE 50 TO 59 Pass$="C" CASE 0 TO 49 Pass$="Fail" END SELECT END SUB

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Infosheet 4.4

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Repetition It is often required that a part of a program be repeated several times. A section of program which is repeated is called a loop. The loop structure is a sequence of instructions which is repeated zero or more times. There are several loop routines for repeating a section of a program.

Fixed Loop (Unconditional Loop) A fixed loop is a structure in which the programmer sets the number of times a loop must be repeated.

The loop structure usually has a counter variable which is incremented each time a loop is executed. The general form is shown below: FOR counter = start value to finish value step stepsize execute commands NEXT counter Example 4.1:- This example shows the fixed loop at its simplest. The words “Hello World” are displayed 10 times.

Algorithm

1.

2.

3.

loop 10 times (use index) display the message end loop

Program

FOR nooftimes= 1 TO 10 STEP 1 PRINT "Hello World" NEXT nooftimes

Example 4.2:- One of the most useful aspects of a loop, is the ability to make use of the loop counter variable. In this example, the square of the numbers from 5 to 10 will be calculated and displayed.

Algorithm

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loop from 5 to 10 (use counter) calculate square using counter * counter display square end loop

Program

FOR numbers= 5 TO 10 STEP 1 LET square = numbers * numbers PRINT "The square of the number "; numbers; " is "; square NEXT numbers

Example 4.3:- In this example the odd numbers from 1 to 9 are printed.

Algorithm

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loop from 1 to 9 in steps of 2 display number end loop

Program

FOR oddnumbers= 1 TO 9 STEP 2 PRINT "The number is "; oddnumbers NEXT oddnumbers

Just as the conditional statement IF can be nested, so loops can be nested, i.e. loops can contain loops.

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Infosheet 5.1

Repetition Example 4.4 In this example we are going to compare 3 numbers inputted by the user with 5 numbers generated by the computer. This uses a nested loop to compare the user numbers against all the computer numbers.

Algorithm

1.

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4.

get user numbers get computer numbers Compare user numbers with computer numbers display number of winners

refine

3.1 3.2 3.3 3.4 3.5 3.6 3.7

loop 3 times loop 5 times if computeno = userno then increase winners by 1 end if end loop end loop

Program

DIM userno(3), computerno(5) CALL get_user_nos(userno()) CALL get_computer_nos(computerno()) CALL compare_numbers(userno(),computerno(),winners) CALL display_winning_numbers(winners) END SUB get_user_nos(userno()) FOR counter=1 TO 3 STEP 1 PRINT "Please type in number ";counter INPUT userno(counter) NEXT counter END SUB SUB get_computer_nos(computerno()) FOR counter = 1 TO 5 STEP 1 LET Compterno(counter) = INT(RND * 100) + 1 NEXT counter END SUB SUB compare_numbers(userno(),computerno(),winners) LET winners=0 FOR usercounter = 1 TO 3 STEP 1 FOR computer counter= 1 TO 5 STEP 1 IF userno(Usercounter)=computerno(computercounter) THEN LET winners=winners+1 END IF NEXT computercounter NEXT usercounter END SUB SUB display_winning_numbers(winners) PRINT"The number of winning numbers is ";winners END SUB

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Infosheet 5.2

Repetition Conditional Loops The fixed loop is limited in that the programmer has to know in advance how many times the program has to go round the loop. The conditional loop is a more flexible loop structure and is used when the number of times a section of program is repeated is not known beforehand, and depends on a certain condition being met. There are two kinds of conditional loop, one with the condition at the end of the loop, the other with the condition at the start of the loop.

DO .. LOOP WHILE (In some languages REPEAT .. UNTIL) The DO .. LOOP WHILE structure is a conditional loop in which the condition comes at the end of the loop. The general structure of a doâ&#x20AC;Ś..until loop is shown below.

DO execute commands LOOP WHILE some condition is true Notice that in this structure, the condition is at the end of the loop. This means that the steps inside the loop must be executed at least once.

Example 4.5:- This algorithm makes use of a sentinel value. A sentinel value is used to tell the loop when to stop. In this example the sentinel value is the string 'banana'. This program will keep asking for name until Banana is entered.

Algorithm 1.

2.

3.

Program

start loop get password until the password is Banana

DO PRINT"Please enter the correct password" INPUT password$ LOOP WHILE password$<>"Banana"

Example4.6 This example is the same as the one above except it mixes conditional and unconditional repetition by only allowing 3 attempts.

Algorithm

1.

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3.

4.

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6.

set counter to 0 start loop add 1 to counter get password until counter = 3 OR password is banana display total

Program

LET counter=0 DO LET counter=counter+1 PRINT"Please enter the correct password" INPUT password$ LOOP WHILE password$<>"Banana" AND counter<>3

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Infosheet 5.3

Repetition Example 4.7 This is an example of a standard routine which checks that a number entered is within a given range. This type of loop is called an input validation loop.

Algorithm

1 2 3 4 5 6 7

Program

DO

start loop display suitable message get number and store in get_number if number less than 1 or greater than 10 display error message end if until number is one from 1 to 10

PRINT "Please type in a number between 1 and 10" INPUT number IF number<0 OR number>10 THEN PRINT "This number is not acceptable" END IF LOOP WHILE number<0 OR number>10 DO WHILE Loop A DOâ&#x20AC;Ś..WHILE loop structure is similar to the DO loop but it depends on the condition being true before the loop is executed.

Whereas the DO loop must be executed at least once, a DO WHILE loop may never be executed as the program is able to by-pass it, if the initial condition is false.

The general structure of a DO WHILE loop is shown below: DO WHILE some condition is true The program performs sequence of instructions LOOP Example 4.8 This algorithm makes use of a sentinel value. The sentinel value is used to tell the loop when to stop. In this example the sentinel value is the string 'STOP'. This program will keep asking and storing names until STOP is entered.

Algorithm

1.

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4.

5.

get password loop as long as name is not banana display error message get password end loop

Program

INPUT password$ DO WHILE password$<>"Banana" PRINT"Wrong password try again" INPUT password$ LOOP

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Infosheet 5.4

Manipulating Data Structures Arrays allow programs to store lots of data. Loops give programs the power to manipulate this data.

There are a range of common operations that may be performed on array data types. These include: • • • • •

accessing a data item held in an array searching for a particular item in an array finding the maximum or minimum item in an array counting the occurrences of an item in an array sorting an array

Example 5.1 To find the maximum value in an array Algorithm

1.

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3.

get information of array find maximum number in array display maximum number

Refine 1

2.1 2.2 2.3 2.4 2.5 2.6 2.7

set largest to first value start loop increment array index by 1 if value of next array element is greater than largest value then set largest value to value of next array element end if until all array values examined

Program DIM mark(10), name$(10) CALL get_info(mark(),name$()) CALL Find_largest(largest,position,mark()) CALL print_largest(largest,name$(), position) END SUB get_info(mark(),name$()) FOR number= 1 TO 10 STEP 1 READ name$(number, mark(number) NEXT number DATA "john",23,"susan",34,"alice",45,"ben",87,"peter",96,"Andrew",45,"Linda",34, "louise",61,"Claire",72,"Mike",50 END SUB SUB Find_largest(largest,position,mark()) LET largest=mark(1) FOR numbers= 2 TO 10 STEP 1 IF mark(numbers) > largest THEN LET largest = mark(numbers) LET position=numbers END IF NEXT numbers END SUB SUB print_largest(largest,name$(), position) PRINT" The largest mark is"; largest;" by ";name$(position) END SUB

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Infosheet 6.1

Manipulating Data Structures Example 5.2 To find the minimum value in an array Algorithm

3.

1. get information of array 2. find minimum number in array display minimum number

Refine 1 2.1 2.2 2.3 2.4 2.5 2.6 2.7

set smallest to first value start loop increment array index by 1 if value of next array element is smaller than smallest value then set smallest value to value of next array element end if until all array values examined

Program DIM mark(10), name$(10) CALL get_info(mark(),name$()) CALL Find_smallest(smallest,position,mark()) CALL print_ smallest (smallest,name$(), position) END SUB get_info(mark(),name$()) FOR number= 1 TO 10 STEP 1 READ name$(number) READ mark(number) NEXT number DATA "john",23,"susan",34,"alice",45,"ben",87,"peter",96,"Andrew",45,"Linda",34, "louise",61,"Claire",72,"Mike",50 END SUB SUB Find_ smallest (smallest,position,mark()) LET smallest =mark(1) FOR numbers= 2 TO 10 STEP 1 IF mark(numbers) < smallest THEN LET smallest = mark(numbers) LET position=numbers END IF NEXT numbers END SUB SUB print_ smallest (smallest,name$(), position) PRINT" The smallest mark is"; smallest;" by ";name$(position) END SUB

Higher Computing - Soft Dev â&#x20AC;&#x201C; High Level Language Constructs Produced by S Lambert, R Simpson and HSDU for The City of Edinburgh Council, 2004

Infosheet 6.2

Manipulating Data Structures Example 5.3 Counting Occurrences Three numbers are used to represent votes for different candidates. This program will count how often they occur.

Algorithm 1 2 3

get votes count occurrences display number of occurrence found

1.1 1.2 1.3 1.4 1.5

set counter to 1 start loop get and store value increase counter by 1 end loop while value =0

2.1 2.2 2.3 2.4 2.5 2.6 2.7

loop number of votes if value of array element = 1then add 1 to occurrences of 1 else add 1 to occurrences of 2 end if end loop

out - noofvotes in - noovotes in - vote1, vote2

out - vote1, vote2

Refine 1

Refine 2

Program DIM vote(100) CALL get_info(vote(),counter) CALL addvotes(vote(),counter,bush,blair) CALL print_totals(bush,blair) END SUB get_info(vote(),counter) LET counter=1 DO READ vote(counter) LET counter=counter+1 LOOP WHILE vote(counter-1)<>0 DATA 1,2,1,1,2,1,2,1,2,1,2,2,2,1,2,1,2,1,2,1,2,1,2,2,2,1,1,2,1,1,2,1,2,1,1,1,1,2,1,2,0 END SUB SUB addvotes(vote(),counter,bush,blair) FOR totalvotes=1 TO counter-1 STEP 1 IF vote(totalvotes)=1 THEN LET bush=bush + 1 ELSE LET blair=blair + 1 END IF NEXT totalvotes END SUB SUB print_totals(bush,blair) PRINT "Bush has ";bush;" votes" PRINT "Blair has ";blair;" votes" END SUB

Higher Computing - Soft Dev â&#x20AC;&#x201C; High Level Language Constructs Produced by S Lambert, R Simpson and HSDU for The City of Edinburgh Council, 2004

Infosheet 6.3

Manipulating Data Structures Example 5.4 Linear Search To find out if a number is in an array.

Algorithm 1.

2.

3.

search array for value display position in array

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 1.11 1.12

set found to be false set array index to 0 ask user for value to be found in list get and store value start loop add 1 to index ask user for next value in list get and store array value if array element = number_to_be_found set found to true end if until found = true or all elements are compared

Refine 1

Program DIM Name$(10), Mark(10) CALL get_array-values(name$(),mark()) CALL get_name(soughtname$) CALL search(soughtname$,Name$()) SUB get_array_values(name$(),mark()) FOR noofnames=1 TO 10 STEP 1 READ name$(noofnames), mark(noofnames) NEXT noofnames DATA "john",23,"susan",34,"alice",45,"ben",87,"peter",96,"Andrew",45,"Linda",34, "louise",61,"Claire",72,"Mike",50 SUB get_name(soughtname$) PRINT"What name are you after?"

INPUT soughtname$ END SUB SUB search(soughtname$,Name$()) LET found$="NO" LET namesearch=1 DO IF Name$(namesearch)=soughtname$ THEN Found$="YES" Indexmark=namesearch END IF LET namesearch=namesearch+1 LOOP WHILE namesearch<>11 OR found$="NO" IF found$="YES" THEN PRINT soughtname$; " has been found at position ";indexmark ELSE PRINT"This name is not in the list" END IF END SUB

Higher Computing - Soft Dev â&#x20AC;&#x201C; High Level Language Constructs Produced by S Lambert, R Simpson and HSDU for The City of Edinburgh Council, 2004

Infosheet 6.4

String Operations Variables that have been declared as a string data type are seen as a sequence of characters by programming languages. Hence the string data type is structured. Strings can be manipulated using different techniques within programming languages.

Programming languages also provide the functions for handling and manipulating strings. In True Basic the following can be used: CONCATENATION - & – Joins two or more strings.

Let number$ = ‘one’, test $= ‘computing’, and date$ = ‘24/12’ number$ & test$ & date$= ‘onecomputing24/12’ Example 6.1 Concatenation Of Two Strings This program will ask the user for a name and age and then construct a code using the name and age.

Algorithm 1. ask user for first name 2. ask user for age 3. make code of first name and age 4. display code to user Program PRINT "What is your name" INPUT name$ PRINT "What is your age" INPUT age$ LET password$=name$ & age$ PRINT "Your password is ";password$ LENGTH - LEN (String) - returns an integer which represents the number of characters of a string.

If exam$ = ‘higher’ then len (exam$) = 6 Example 6.2 Length of String This program will check that a users password has at least 6 characters.

Algorithm 1. get and store password 2. calculate length of password 3. valid password Program Print "What is your new password" INPUT password$ LET passlength=LEN(password$) IF passlength < 6 THEN PRINT "Password must be at least 6 characters" ELSE PRINT "password accepted" END IF

Higher Computing - Soft dev – High Level Language Constructs Produced by S Lambert, R Simpson and HSDU for The City of 2004

Infosheet 7.1

String Operations SELECTING CHARACTERS It is possible to select a number of characters in a string in the following manner.

LET x$="abcdefghijk" LET y$=x$[2:5] PRINT y$ output - "bcde"

Example 6.3 Input Validation Of Each Character Of A String This program will ask the user for a name and validate that each character is either a to z.

Algorithm

1.

2.

3.

get and store name check characters of name are valid display name

{out : name} {in/out : name, in/out : ch} {in : name}

Refine 2

3.1 repeat 3.2 set error to NO 3.3 ask for name 3.4 calculate length of name 3.5 Repeat for each character 3.6 if name is less than a or greater than z 3.7 set error to YES 3.8 end if 3.9 end loop 3.10 if error is set to YES 3.11 print invalid message 3.12 end if 3.13 until error is NO

Program DO error$="NO" PRINT"what is your name" INPUT name$ LET lengthofword=LEN(name$) FOR noofcharacter=1 TO lengthofword STEP 1 IF name$[noofcharacter:noofcharacter]<”a” OR name$[noofcharacter:noofcharacter]>”z” THEN LET error$="YES" END IF NEXT noofcharacter IF error$="YES" THEN PRINT" This name is invalid please type it again !"

END IF LOOP WHILE error$="YES"

Higher Computing - Soft dev – High Level Language Constructs Produced by S Lambert, R Simpson and HSDU for The City of 2004

Infosheet 7.2