2.2.2 Programming Constructs

In this lesson, you will learn how the three basic programming constructs of sequence, selection, and iteration control the flow of a program. You will explore how instructions are carried out in order, how programs make decisions using IF and SWITCH/CASE selection, and how count-controlled and condition-controlled loops repeat instructions.

Programming Constructs

The flow of almost every program can be built using three basic programming constructs: sequence, selection, and iteration.

Sequence controls the order in which instructions are performed, selection controls which path the program follows, and iteration controls which instructions are repeated.

Sequence

Sequence means executing instructions one after another in the order in which they appear. Consider Code 1 below.

Codepseudocode

x = 5
x = x * 2
x = x + 3
print(x)

Code 1. OCR Exam Reference Language demonstrating sequence, producing an output of 13.

The instructions run from top to bottom, as shown below in Table 1.

Table 1. Step-by-step execution of Code 1, showing how the value of

x

changes as each instruction is carried out in sequence.

Order	Instruction	Operation	Result
1	$x = 5$	$x = 5$	$x = 5$
2	$x = x * 2$	$x = 5 \times 2$	$x = 10$
3	$x = x + 3$	$x = 10 + 3$	$x = 13$
4	$print(x)$	$print(13)$	$13$

Since the final value of $x$ is 13, the program outputs 13. If the same instructions are placed in a different sequence as shown below in Code 2, however, the output will be different.

Codepseudocode

x = 5
x = x + 3
x = x * 2
print(x)

Code 2. OCR Exam Reference Language demonstrating how changing the sequence of instructions changes the result.

As shown in Table 2 below, the output of this reordering of instructions is 16.

Table 2. Step-by-step execution of Code 2, showing how changing the sequence of the same operations produces a different result.

Order	Instruction	Operation	Result
1	$x = 5$	$x = 5$	$x = 5$
2	$x = x + 3$	$x = 5 + 3$	$x = 8$
3	$x = x * 2$	$x = 8 \times 2$	$x = 16$
4	$print(x)$	$print(16)$	$16$

This demonstrates why the sequence of instructions matters: changing their order can change the result of the program.

Selection

Selection allows a program to make a decision and follow different paths depending on whether a Boolean condition is True or False. Figure 1 below shows how selection causes a program to follow one of two paths depending on the result of a Boolean condition.

**Figure 1.** A Boolean condition leading to different paths depending on whether the condition is True or False. True leads to A, whereas False leads to B.

There are two forms of selection that we will explore: IF, ELSEIF, and ELSE selection, and SWITCH/CASE selection.

IF, ELSEIF, and ELSE Selection

The most common form of selection uses IF, ELSEIF, and ELSE, as shown in Code 3 below. An IF statement can contain just an IF, an IF with an ELSE, or an IF followed by one or more ELSEIF conditions and an optional ELSE.

Codepseudocode

age = input("Enter your age")

if age >= 18 then
    print("Adult")
elseif age >= 13 then
    print("Teenager")
else
    print("Child")
endif

Code 3. OCR Exam Reference Language using IF, ELSEIF, and ELSE for selection.

The conditions are checked from top to bottom. First, $age >= 18$ is evaluated. If it is True, "Adult" is output and the remaining alternatives are skipped. If it is False, the program evaluates $age >= 13$ . If this is also false, the ELSE block is executed.

ELSE does not require a condition because it acts as the final alternative. As soon as one condition evaluates to True, its block is executed and the remaining ELSEIF and ELSE alternatives are skipped.

The equivalent Python of Code 3 is shown below in Code 4. IF becomes $if$ , ELSEIF becomes $elif$ , and ELSE becomes $else$ .

Codepython

age = int(input("Enter your age: "))

if age >= 18:
    print("Adult")
elif age >= 13:
    print("Teenager")
else:
    print("Child")

Code 4. Python using if, elif, and else for selection.

The order of selection conditions matters. If a broad condition is checked too early, it may prevent a later condition from ever being reached. For example, if $age >= 13$ were checked before $age >= 18$ , an age of 20 would satisfy $age >= 13$ first and "Teenager" would be output. This is why Code 3 and Code 4 check $age >= 18$ before $age >= 13$ .

SWITCH/CASE Selection

Another way of representing selection in the OCR Exam Reference Language is a SWITCH/CASE statement. This is useful when one value is being compared against several specific possibilities. In a SWITCH/CASE structure, one expression is checked against several possible values.

Codepseudocode

switch menuChoice:
    case 1:
        print("Start game")
    case 2:
        print("Settings")
    case 3:
        print("Exit")
    default:
        print("Invalid choice")
endswitch

Code 5. OCR Exam Reference Language using SWITCH/CASE to select an action from several specific choices.

In Code 5 above, the value of $menuChoice$ is evaluated once. The program then compares this value with each CASE in turn. If $menuChoice$ is 1, the instructions under case 1 are carried out; if it is 2, the instructions under case 2 are carried out; and so on.

Once a matching CASE is found, the corresponding instructions are executed and the SWITCH/CASE structure is complete. If none of the listed values match, the default section is executed instead. If a default section is not included, the program simply continues with the instructions after the SWITCH/CASE structure.

Unlike an IF/ELSEIF structure, which can test conditions such as $scor e >= 70$ , each CASE normally represents a specific value. This makes SWITCH/CASE particularly useful for situations such as menu choices, commands, or other fixed options.

Iteration

Iteration means repeating a section of code. It is also commonly called looping. There are two main types of iteration: count-controlled iteration (which repeats a known number of times), and condition-controlled iteration (which repeats according to a Boolean condition).

Count-Controlled Iteration

A count-controlled loop is used when the number of repetitions is known in advance. The OCR Exam Reference Language uses a FOR loop as shown in Code 6 below.

Codepseudocode

for i = 1 to 5
    print(i)
next i

Code 6. OCR Exam Reference Language using a count-controlled FOR loop to output the values 1 to 5.

The variable $i$ acts as the counter. On the first repetition, $i$ contains 1, so 1 is printed. Unless another STEP value (the size of each change) is specified, the counter is then automatically incremented (increased) by 1 after each repetition. The loop continues with $i$ taking the values 2, 3, 4, and 5. Once the final value of 5 has been processed, the loop ends. It therefore executes five times and outputs:

1, 2, 3, 4, 5

Codepython

for i in range(1, 6):
    print(i)

Code 7. Python using a count-controlled for loop to output the values 1 to 5.

In Python, the equivalent loop is shown in Code 7 above. Python’s $range()$ excludes its final value, so $range(1, 6)$ generates the values 1 to 5.

The size of each change, or STEP, can also be controlled. In the OCR Exam Reference Language, this is shown in Code 8 below.

Codepseudocode

for i = 2 to 10 step 2
    print(i)
next i

Code 8. OCR Exam Reference Language using STEP to increment a FOR loop counter by 2.

This FOR loop in Code 8 outputs:

2, 4, 6, 8, 10

The optional STEP value controls how much the counter changes after each repetition. A positive STEP counts upwards (as shown in Code 8 above), while a negative STEP can be used to count backwards (as shown in Code 9 below).

Codepseudocode

for i = 5 to 1 step -1
    print(i)
next i

Code 9. OCR Exam Reference Language using a negative STEP value to count backwards.

This FOR loop in Code 9 outputs:

5, 4, 3, 2, 1

Condition-Controlled Iteration

A condition-controlled loop repeats according to the result of a Boolean condition rather than for a predetermined number of repetitions. This means the number of repetitions depends on what happens while the program is running and may not be known in advance.

Two forms used in the OCR Exam Reference Language are WHILE and DO UNTIL loops. The key difference between WHILE and DO UNTIL is when the condition is checked.

A WHILE loop checks its condition before each repetition. It continues while the condition remains True. This is shown in Code 10 below, and its Python equivalent in Code 11.

Codepseudocode

total = 0

while total < 20
    number = input("Enter a number")
    total = total + number
endwhile

print("Total reached 20")

Code 10. OCR Exam Reference Language using a condition-controlled WHILE loop.

The loop continues while $total < 20$ . Once $total$ reaches or exceeds 20, the condition becomes False and execution continues after the loop.

Codepython

total = 0

while total < 20:
    number = int(input("Enter a number: "))
    total = total + number

print("Total reached 20")

Code 11. Python using an equivalent condition-controlled while loop.

Because the condition is checked before the loop body begins, a WHILE loop can execute zero times if its condition is false from the start.

On the other hand, a DO UNTIL loop checks its condition after the loop body has executed. The same task as above could be represented as shown below in Code 12.

Codepseudocode

total = 0

do
    number = input("Enter a number")
    total = total + number
until total >= 20

print("Total reached 20")

Code 12. OCR Exam Reference Language using a DO UNTIL loop to perform the same task.

The loop repeats until $total >= 20$ becomes true. Because the condition is checked at the end, the instructions inside a DO UNTIL loop are always executed at least once. This loop assumes the user enters values that eventually cause $total$ to reach or exceed 20. If $total$ never reaches or exceeds 20, the loop will continue indefinitely.

A condition-controlled loop must eventually reach a state where it can stop. If the variables involved in its condition are never changed appropriately, the loop may run forever. This is called an infinite loop.

Python does not provide a direct DO UNTIL statement. The same behaviour can be recreated using other programming structures, but the important concept here is to understand how a DO UNTIL loop works and how it differs from a WHILE loop.

Although both WHILE and DO UNTIL are condition-controlled loops, the position of the condition changes how they behave. This is particularly important when the condition is already satisfied before the loop begins, as summarised below in Table 3.

Table 3. Comparison of WHILE and DO UNTIL condition-controlled loops.

Characteristic	WHILE Loop	DO UNTIL Loop
Condition Checked	Before the loop body	After the loop body
Repeats While/Until	Repeats while the condition is True	Repeats until the condition is True
Minimum Executions	0	1
Available Directly in Python	Yes	No

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2.2.2 Programming Constructs

Programming Constructs

Sequence

Selection

Iteration

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