Sequence (Cambridge (CIE) O Level Computer Science)

Revision Note

Author

Robert Hampton

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Computer Science Content Creator

Sequence

What is 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 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 1

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	`OUTPUT "Enter the first number"`
02	`INPUT Num1`
03	`OUTPUT "Enter the second number"`
04	`INPUT Num2`
05	`Result ← Num1 - Num2`
06	`OUTPUT Result`

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

Example 2

Python example
`def calculate_area(length, width):` """ This function calculates the area of a rectangle Inputs: length: The length of the rectangle width: The width of the rectangle Returns: The area of the rectangle """ # Calculate area `return area` `area = length * width` # ------------------------------------------------------------------------# Main program # ------------------------------------------------------------------------ `length = 5` `width = 3` `correct_area = calculate_area(length, width)` `print(f"Correct area (length * width): {correct_area}")`

Python example

def calculate_area(length, width):

"""

This function calculates the area of a rectangle

Inputs:

length: The length of the rectangle

width: The width of the rectangle

Returns:

The area of the rectangle

"""

# Calculate area

return area

area = length * width

# ------------------------------------------------------------------------# Main program

# ------------------------------------------------------------------------

length = 5

width = 3

correct_area = calculate_area(length, width)

print(f"Correct area (length * width): {correct_area}")

In the example, the sequence of instructions is wrong and would cause a runtime error
In the calculate_area() function a value is returned before it is assigned
The correct sequence is:

# Calculate area
area = length * width
return area

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