Converting pseudocode to a high-level language

Generally, it is quite straightforward to convert from pseudocode to a high-level language, but you do need to be careful and look out for areas where the syntax is 'similar but different' and may confuse you. In particular, think carefully about count-controlled loops.

With count-controlled loops, many languages use syntax that is quite different from pseudocode. Consider the following segment of pseudocode:

FOR i = 1 TO 5
   PRINT(i)
NEXT i

Which, if implemented would produce the output:

1
2
3
4
5

In its pseudocode form, a count-controlled loop has three components:

A counter

This is the variable that is used to control the number of iterations of the loop. In the pseudocode example, the variable i is specified as the counter. It is common practice to name the loop counter i when it has no separate meaning within the context of the program. In nested for loops, i and j are commonly used.

An important thing to consider when converting the pseudocode to real code, is whether the counter variable needs to be declared before it can be used in the loop header. Some languages are strongly typed; this means that each variable must be declared with an identifier and data type before it can be used.

A specified range of values for the counter

This defines the range of values that the counter will take. In the pseudocode example, i will take the values from 1 up to and including 5. The body of the loop will be executed five times.

In many languages, the range is specified by stating a condition within the loop header, such as i < 6. This condition will be evaluated at the start of each iteration of the loop. When the condition evaluates to False, the loop will terminate.

The initial start value for the counter is very important if the counter is being used to index an array within the body of the loop. The first element of an array has an index of 0 so the counter must have a start value of 0.

An implied or explicit step value

The step value defines how the value of the counter will change on each iteration of the loop. It is usually not specified and you should assume that the step value is +1.

The table below shows how the pseudocode example can be implemented, in a range of programming languages.

LanguageEquivalent program codeNotes
Python
for i in range (1,6):
    print (i)

In Python, variables do not need to be declared before they are used so the counter i is created in the loop header.

The range of values is usually specified using the built-in function range which creates a sequence of numbers. The limit of the range must be specified and the counter will take the values up to but not including that value. If you specify:

for i in range (6)

i will take values from 0 to 5 inclusive as 0 is the default start value.

The default step value is 1 so in this example does not need to be specified.

Java
for(int i = 1; i < 6; i++){
    System.out.println(i);
}

Java requires you to define a variable and specify its data type before it is used. This can be done within the loop header as shown.

The range of values for the counter and the step value are specified in the same way as explained for PHP.

i < 6 specifies that the loop will run while the value of i is less than 6. When the value of i reaches 6, the body of the loop will not be executed.

i++ specifies that the value of the counter i will be incremented by 1 on each iteration of the loop. The step value can be changed.

AQA pseudocode conventions

ConstructSyntax/explanationPseudocode example
Subroutines

Parameters are specified within parentheses (brackets) following the name of the subroutine.

Any return value(s) follow the keyword RETURN

FUNCTION/ENDFUNCTION
and
PROCEDURE/ENDPROCEDURE
are sometimes used

SUBROUTINE
calculate_pay(hours, rate)
   pay ←  hours x rate
   RETURN pay
ENDSUBROUTINE
Calling a subroutineThe name of the subroutine being called is specified with the values of the parameters in brackets.

calculate_pay(40, 10.75)

calculate_pay(hours_worked, basic_rate)

Variable assignmentAssignment is indicated with a left pointing arrow.x ← 3
ConstantsConstant names (identifiers) are written in uppercase and prefixed with the word constant. They will be given a value when first declared.constant VAT ← 0.2
InputUser input is collected and assigned to a variable.user_name ← USERINPUT
OutputThe keyword OUTPUT is followed by a text string or variable.

OUTPUT 'Enter a number'

OUTPUT user_name

Selection
IF logical-condition THEN
    statement-block
ENDIF
IF logical-condition THEN
    statement-block
ELSE
    statement-block
ENDIF
IF logical-condition THEN
    statement-block
ELIF logical-condition
    statement-block
ELSE
    statement-block
ENDIF
IF found = True THEN
   msg ← 'Game over'
ENDIF
IF mark > 25 THEN
   result ← 'pass'
ELSE
   result ← 'fail'
ENDIF
IF age <= 18 THEN
   OUTPUT 'Youth rate'
ELIF age >= 66 THEN
   OUTPUT 'Pensioner rate'
ELSE
   OUTPUT 'Adult rate'
ENDIF
Iteration (count controlled)
FOR identifier ← intexp TO intexp
    statement-block
ENDFOR

The FOR keyword is followed by the name of a variable that will take successive values from a defined range. In the example shown it will take the values from 1 to 5 inclusive.

FOR i ← 1 TO 5
   OUTPUT i                 
ENDFOR

will output:
1
2
3
4
5

Iteration (condition controlled – condition at end)
REPEAT
    statement-block
UNTIL logical-condition

The statement block is run at least once. The condition is checked at the end of each iteration of the loop.

REPEAT
    OUTPUT 'Guess a number'
    num ← USERINPUT
UNTIL num >= 1 AND num <=100
Iteration (condition controlled – condition at start)
WHILE logical-condition
    statement-block
ENDWHILE

The statement block may never be run. The condition is checked at the start of each iteration of the loop.

total ← net_value
WHILE total < 300
     OUTPUT 'enter a number'
     num ← USERINPUT
     total ← total + num