How to Answer Programming Questions (OCR A Level Computer Science): Revision Note

Octal is a base 8 number system.

To convert a denary number to base 8:

• the denary value is divided by 8 and the remainder is stored

• the integer value after division is divided by 8 repeatedly until 0 is reached

• the remainders are then displayed in reverse order.

Example 1:

Denary 38

38 / 8 = 4 remainder 6 6

4 / 8 = 0 remainder 4 4

Octal = 46

Example 2:

Denary 57

57 / 8 = 7 remainder 1 1

7 / 8 = 0 remainder 7 7

Octal = 71

Write an algorithm to:

• take a denary value as input from the user

• convert the number to octal

• output the octal value.

You do not need to validate the input from the user.

Write your algorithm using pseudocode or program code.

[6 marks]

What are the inputs? What are the outputs? What processes take place?

• take a denary value as input from the user

• the denary value is divided by 8 and the remainder is stored

• the integer value after division is divided by 8 repeatedly until 0 is reached

• the remainders are then displayed in reverse order.

• output the octal value.

What constructs will I need to use?

• Sequence

  • Calculating the remainder after division by 8 (MOD)

  • Calculating integer after division by 8 (DIV)

  • Ensure remainders are displayed in reverse order

• Iteration

  • the number of divisions needed depends on the size of the input

  • You don’t know how many remainders to generate in advance

  • You need a repeat-until-0 process, which is best represented by a WHILE loop

Do I need to use subprograms?

• This program can be written without the need to use a subprogram (function or procedure)

Answer

Pseudocode

number = input("Enter a number")

endResult = ""

while number != 0

remainder = number MOD 8

number = number DIV 8

endResult = str(remainder) + str(endResult)

endwhile

print endResult