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Free GCSE Computer Science guide
Use conditions to choose a branch, then use iteration to repeat a block of code. Trace the workshop's if statements and a loop that adds the integers from 1 to 10.
Content owner: Michael Print · Written for GCSE learners · Checked against official specifications
Selection chooses which statements to execute based on a condition. Python uses if, optionally followed by elif and else.
Iteration repeats statements. A Python for loop visits items in an iterable, such as a range or list. A while loop repeats while its condition remains true.
This follows the workshop's three-way number example. For the trace, assume the user enters a valid integer from 1 to 10. The comparison > means strictly greater than, so 5 > 5 is false.
counter = int(input("Enter a value between 1 and 10: "))
if counter > 5:
print("You entered a number greater than 5")
elif counter > 2:
print("You entered a number between 3 and 5")
else:
print("You entered a number less than 3")Python checks conditions from the top. Once a condition is true, it executes that branch and skips the remaining branches in this if/elif/else chain. The indented lines belong to their branch; the colon introduces each block.
| Input | counter > 5 | counter > 2 | Chosen message |
|---|---|---|---|
| 1 | False | False | less than 3 |
| 2 | False | False | less than 3 |
| 3 | False | True | between 3 and 5 |
| 5 | False | True | between 3 and 5 |
| 6 | True | not checked | greater than 5 |
| 10 | True | not checked | greater than 5 |
The table abbreviates the printed messages; the code shows their full wording. elif counter > 2 covers integers 3 to 5 here because the earlier counter > 5 test has already failed.
Worked example
For counter = 6, counter > 5 is true. The program prints You entered a number greater than 5 and skips the elif and else. Although 6 is also greater than 2, that later comparison is not evaluated in this chain.
range(1, 11) supplies the integers 1, 2, 3, …, 10. The start is included and the stop value, 11, is excluded. The loop therefore repeats ten times.
total = 0
for i in range(1, 11):
total = total + i
print(total) # 55total is an accumulator: it keeps the running sum. Initialise it once before the loop, add the current i inside the loop and print the final result after the loop.
| i | total before addition | total after addition |
|---|---|---|
| 1 | 0 | 1 |
| 2 | 1 | 3 |
| 3 | 3 | 6 |
| 4 | 6 | 10 |
| 5 | 10 | 15 |
| 6 | 15 | 21 |
| 7 | 21 | 28 |
| 8 | 28 | 36 |
| 9 | 36 | 45 |
| 10 | 45 | 55 |
The single output is 55. Moving print(total) into the loop would print every running total instead. Moving total = 0 into the loop would reset the accumulator on every iteration.
counter = 1
total = 0
while counter <= 10:
total = total + counter
counter = counter + 1
print(total) # 55This loop checks counter <= 10 before each iteration. It adds the current counter, then increases it. After adding 10, the counter becomes 11, the condition is false and the program prints 55.
A for loop over a known range makes this fixed ten-iteration task concise. A while loop is useful when repetition should continue until a condition changes, such as receiving an accepted input. Its body may run zero times if the condition is false at the first check.
A short captioned explanation from Michael's Python fundamentals workshop. Use the examples on this page to practise the idea afterwards.
Test whether the number is greater than 5 first. If it is not, test whether it is greater than 2. An integer input of 3 reaches the middle branch; an input of 6 reaches the first branch and skips the others.
A short captioned explanation from Michael's Python fundamentals workshop. Use the examples on this page to practise the idea afterwards.
range(1, 11) includes 1 through 10. The loop adds each value to a running total, starting at zero, so the final total is 55.
These questions are independently written. The answers and marks are indicative rather than an official mark scheme.
For the selection program, state the full output message for inputs 2, 3, 5 and 6. Explain why input 6 does not print two messages.
Indicative answer: Input 2: You entered a number less than 3; input 3: You entered a number between 3 and 5; input 5: the same middle message; input 6: You entered a number greater than 5. Award one mark for each correct output.
For input 6, the first condition is true (1), so the rest of the chain is skipped and only one message prints (1).
The for-loop program is changed to use range(1, 10) and now prints 45. Repair the range so the program adds every integer from 1 to 10 inclusive, explain the change and state the new output.
Indicative answer: Change the range to range(1, 11) (1). The stop value is excluded, so the original range omitted 10 (1). The corrected final output is 55 (1).
A learner removes counter = counter + 1 from the while-loop example. Explain why the program does not finish and state the line to restore, including where it belongs.
Indicative answer: Without an update, counter stays at 1 and the condition remains true (1). Insert counter = counter + 1 inside the loop (1), after adding the current counter (1). The corrected program is the while-loop example above.
Write a Python for loop that prints the integers 1 to 5 inclusive, each on a separate line.
for number in range(1, 6):
print(number)Indicative marking: a for loop with an iteration variable (1); start value 1 (1); excluded stop value 6 (1); an indented print of the iteration variable (1). The output is 1, 2, 3, 4 and 5 on separate lines.
Selection and iteration are shared programming concepts, but Python syntax and range limits differ from some exam-board pseudocode. Read the response format and bounds in each question.
These are independently written explanations and practice questions. CompSciTutoring.co.uk is not affiliated with or endorsed by an examination board. The marking guidance is indicative; always check the syllabus for your examination year.
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Use the free examples first, then explore one-to-one tuition if guided practice would help.