Sequence (Cambridge (CIE) O Level Computer Science): Revision Note

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

• 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

• 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

FUNCTION CalculateArea(length, width)
    RETURN area
    area ← length * width
END FUNCTION

// Main program
length ← 5
width ← 3
correct_area ← CalculateArea(length, width)
OUTPUT "Correct area (length * width): ", correct_area

def calculate_area(length, width):
  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:

area ← length * width
RETURN area

OR

area = length * width
return area