Convert character to character code

The idea

Converting a character to its character code means finding the integer that the computer uses to represent that single character. Every character stored by a computer is encoded as a number, for example using ASCII or Unicode. In many languages a built-in function returns this number: in Python, ord('A') returns 65, and ord('a') returns 97. The reverse operation, turning a code back into a character, uses a different function such as chr(). To answer a question on this, identify the single character being converted, recall or apply the encoding rule, and give the resulting integer code. Remember that upper-case and lower-case letters have different codes, and that digits such as '7' have their own codes distinct from the number 7. The operation works on one character at a time, so converting a whole string means converting each character in turn.

What earns the marks

• States that converting a character to a character code returns the integer used by the computer's character encoding to represent that character.
• Identifies the correct code for a given character, for example that 'A' is 65 in ASCII.
• Recognises that upper-case and lower-case versions of the same letter have different codes, for example 'A' is 65 and 'a' is 97.
• Distinguishes a digit character such as '7' from the numeric value 7, noting they have different representations.
• Names a typical built-in function that performs the conversion, such as ord() in Python.
• Explains that the conversion applies to a single character, so a string must be processed one character at a time.

Common mistakes and how to put them right

• Assuming that the character '7' and the number 7 are the same.

  Put it right: The character has its own code (55 in ASCII) which differs from the numeric value.

• Believing that 'A' and 'a' share the same code.

  Put it right: Upper-case and lower-case letters have different codes, 65 and 97 in ASCII.

• Confusing the two directions of conversion.

  Put it right: Character-to-code uses a function such as ord(), while code-to-character uses a function such as chr().

Exam tips

• Learn the ASCII codes for a few anchor characters such as 'A' (65) and 'a' (97) so you can work out nearby codes.
• Check whether the question gives an upper-case or lower-case character, because the code differs.
• Read the question carefully to confirm whether it asks for character-to-code or code-to-character before choosing an answer.