Students should know that character codes are grouped and that they run in…

The idea

Character encoding tables do not assign codes at random: codes are arranged in blocks, and within each block the codes increase by one as you move along the alphabet or the digits. In ASCII the capital letters form one such block, starting with ‘A’ at 65, so ‘B’ is 66, ‘C’ is 67 and so on up to ‘Z’ at 90. Because the block is sequential, you only need one anchor value to work out any other letter in that block: add the letter's position offset to the anchor. For example, ‘E’ is the fifth capital letter, so its code is 65 + 4 = 69. The same grouping-and-sequence idea applies to the lower case letters (a separate block) and to the digits ‘0’ to ‘9’ (another block), so once you know the first code in a block you can calculate the rest by counting forward.

What earns the marks

• States that character codes are organised into groups (blocks) such as capital letters, lower case letters and digits.
• States that within a group the codes run in sequence, increasing by one for each successive character.
• Uses the anchor that ‘A’ is 65 in ASCII and that ‘B’ is 66, so later capitals can be calculated by counting forward.
• Calculates a specific capital letter code correctly, e.g. ‘E’ = 69 or ‘Z’ = 90, by adding the offset to 65.
• Recognises that the same sequential pattern applies to other groupings such as lower case letters and digits, not only capitals.

Common mistakes and how to put them right

• Assuming every character has an unrelated, arbitrary code.

  Put it right by explaining that codes are grouped and sequential, so one anchor value plus an offset gives the rest.

• Counting the anchor letter as offset 1.

  Put it right by noting ‘A’ is offset 0, so ‘E’ is 65 + 4 = 69, not 65 + 5.

• Believing capital and lower case letters share the same codes.

  Put it right by explaining they are separate groupings, each with its own starting code.

Exam tips

• Identify which group the character belongs to (capital, lower case or digit) before calculating.
• Use the given anchor value and count the offset carefully, remembering the first character is offset 0.
• Show your addition, e.g. 65 + 4 = 69, so a small slip is easy to spot and correct.