3.17. Pseudocode

We can write our control structures such as loops in a form using pseudocode which allows us to write out a program without worrying about syntax. Sometimes the symbols and keywords used in pseudocode will vary depending on the convention you are using, but the idea is that pseudocode should be easy to humans to read.

3.17.1. While loops

We write while loops using pseudocode as follows

WHILE condition is true
    process
ENDWHILE

Here is an example

Python code

i = 0
while i < 3:
    print(i)
    i = i + 1  # increment i by 1

Pseudocode

i = 0
WHILE i < 3
    Display i
    i = i + 1
ENDWHILE

Here is another example

Python code

colour = input("Enter a colour: ")

while colour != "red":
    print("That is not red!")
    colour = input("Enter a colour: ")

Pseudocode

Display 'Enter a colour: '
Get colour
WHILE guess <> 'red'
    Display 'That is not red!'
    Display 'Enter a colour: '
    Get colour
ENDWHILE

Note that <> can be used to indicate not equal and is equivalent to using !=.

3.17.2. For loops

We write for loops using pseudocode as follows

FOR variable = start TO finish STEP increment
    process
NEXT variable

Here is an example

Python code

for i in range(3):
    print(i)

Pseudocode

FOR i = 0 TO 2 STEP 1
    Display i
NEXT i

Not that for For loops in pseudocode, the loop includes the start and end values. This means that FOR i = 0 TO 2 STEP 1 means that i takes the values 0, 1 and 2.

Question 1

Which of the following programs corresponds to the pseudocode shown below?

FOR i = 10 TO 0 STEP -2
    Display i
NEXT i

1. for i in range(10):
       print(i)
   

2. for i in range(10, 0):
       print(i)
   

3. for i in range(10, 0, -2):
       print(i)
   

4. for i in range(10, -1, -2):
       print(i)
   

Solution

D.

In pseudocode, the start and end values are inclusive. This means i will take the values 10, 8, 6, 4, 2 and 0 for FOR i = 10 TO 0 STEP -2. The corresponding for loop in Python requires i starting at 0, going to but not including -1 in steps of 2 (up to and including -2) also works. The other options in the for loops presented here only go up to but do not include 0.

Question 2

Construct the pseudocode that corresponds to the following Python program.

colours = ['red', 'yellow', 'pink', 'green', 'purple', 'orange', 'blue']

i = 0
while i < len(colours):
    print(colours[i])
    i = i + 1

Solution

Solution is locked

Question 3

Which of the following best describes the purpose of the pseudocode algorithm given below.

Create list a
Create list b
Create emplty list c

FOR i = 0 TO Length(a) - 1 STEP 1
    Append a[i] to c
    NEXT i

FOR i = 0 TO Length(b) -1 STEP 1
    Append b[i] to c
    NEXT i

Display c

1. It adds all the elements in the list a to the corresponding elements in the list b and stores the result in c. E.g. if a is the list [7, 1, 3] and b is the list [-1, 5, 8] then c will be the list [6, 6, 11].

2. It adds all the elements in the lists a and b and stores the result in c. E.g. if a is the list [7, 1, 3] and b is the list [-1, 5, 8] then c will be the integer 23.

3. It concatenates (joins) the lists a and b and stores the result in c. E.g. if a is the list [7, 1, 3] and b is the list [-1, 5, 8] then c will be the list [7, 1, 3, -1, 5, 8].

Solution

Solution is locked