Basic Programming Concepts (Edexcel GCSE Computer Science)
Revision Note
Written by: James Woodhouse
Reviewed by: Lucy Kirkham
What is a Programming Concept?
A programming construct determines the order in which lines of code are executed
They control the logic and behaviour of code
There are four core programming constructs:
Sequence
Selection
Iteration
Repetition
Sequence
What is a sequence?
Sequence refers to lines of code which are run one line at a time
The lines of code are run in the order that they are written from the first line of code to the last line of code
Sequence is crucial to the flow of a program, any instructions out of sequence can lead to unexpected behaviour or errors
Example
A simple program to ask a user to input two numbers, number two is subtracted from number one and the result is outputted
Line | Pseudocode |
01 |
|
02 |
|
03 |
|
04 |
|
05 |
|
06 |
|
A simple swap of line 01 and line 02 would lead to an unexpected behaviour, the user would be prompted to input information without knowing what they should enter
Selection
What is selection?
Selection is when the flow of a program is changed, depending on a set of conditions
The outcome of this condition will then determine which lines or blocks of code are run next
Selection is used for validation, calculation and making sense of a user's choices
There are two ways to write selection statements:
if... then... else... statements - this is when you test conditions sequentially
case select or switch... statements - this is when you test an expression against multiple possible constant values (known as cases)
Example
Concept | Python |
IF-THEN-ELSE |
|
Repetition
What is repetition?
Repetition is repeating a line or a block of code using a loop
Repetition can be:
count controlled - this is when the code is repeated a fixed number of times (e.g. using a for loop)
condition controlled - this is when the code is repeated until a condition is met (e.g. using a while loop or a do while loop)
Count controlled
Condition controlled
Examples
Repetition | Python |
FOR loop (Count controlled) |
|
This will print the word "Hello" 10 times (0-9 inclusive) | |
| |
This will print the even numbers from 2 to 10 inclusive | |
| |
This will print the numbers from 10 to 0 inclusive | |
WHILE loop (Condition controlled) |
|
| This will loop until the user inputs the colour "Red". Check condition is carried out before entering loop |
Iteration
What is Iteration?
Iteration is going over every item in a data structure
An example of this would be searching through a list to find an item within the list
If a list was not sorted, a linear search may be used
On line number 7, you can see where iteration takes place as it uses a for loop to iterate through the list of numbers to see if the number 1 is present
Worked Example
Tick (✓) one box in each row to identify whether each programming construct has or has not been used in the program [3]
total = 0
for i in range (1,5):
num = input("Enter a number")
total = total + num
print(total)
Has been used | Has not been used | |
|---|---|---|
Sequence |
|
|
Selection |
|
|
Iteration |
|
|
Repetition |
How to answer this question
Always look for keywords first, if you find any then that construct must have been used
Sequence must always be used if the program works
Answer
Has been used | Has not been used | |
|---|---|---|
Sequence | ✔ |
|
Selection |
| ✔ |
Iteration |
| |
Repetition | ✔ |
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