GCSE · Computer Science · AQA · Spec 8525

Character codes grouping/sequencing

Know just one code, 'A' = 65, and you can work out every capital letter's code in your head. Plus: why the character '7' doesn't have the code 7.

Computer Science · Character codes

Which run does each code belong to?

Your table only tells you three things: 'A' is 65, 'a' is 97 and '0' is 48. Pick a code, then put it in the group it belongs to. Before you place it, work out how far along the run it is, and keep an eye on where each run stops.

Still to sort

Capital letters (0)

The run starts at 'A' = 65

Where the line is: 26 capital letters, one code each, no gaps. But the last one isn't at 65 + 26. Hold that thought: you'll see why in a moment.

Lower case letters (0)

The run starts at 'a' = 97

Where the line is: A separate run of 26, with its own first code. 'a' and 'A' are different characters, so they have different codes.

Digit characters (0)

The run starts at '0' = 48

Where the line is: Just ten digit characters, '0' to '9'. '9' is 9 places after '0', and the run ends there.

None of these groups (0)

Not a capital, not lower case, not a digit

Where the line is: A code one below a run's first code, or one above its last, is outside that run. The ends are where counting goes wrong.

8 of 8 still to sort.

65, 97 and 48 are the real ASCII codes for 'A', 'a' and '0'. Every other code on this board was worked out from just those three.

Predict, then check

No full table, just one known code and some careful counting.

Your table says 'A' is 65. 'Z' is the 26th and last capital letter. What is the code for 'Z'?

Digits are characters too

What's the code for '7'?

Someone types the digit 7 into a message. Your table says 'A' is 65 and '0' is 48.

What code does the character '7' have? Pick the idea closest to what you think right now.

It's 7. The digit 7 just means seven.It's 72. Count 7 on from 'A' = 65.Digits don't get codes. Only letters do.It's 55. The digits have their own run, and '7' is 7 places after '0' = 48.

How sure are you?

I'm sureI think soI'm guessing

It's 7. The digit 7 just means seven.

This is a common misconception

This is the idea almost everyone starts with. But a 7 you type is a character, just like 'h' or 'F', and the table gives it a code of its own. The digit characters are their own run starting at '0' = 48, so '7' is 7 places along: 48 + 7 = 55.

It's 72. Count 7 on from 'A' = 65.

This is a common misconception

The counting is right, but the starting point isn't. 65 starts the capital letters, and digits are a separate group with their own run starting at '0' = 48. Count from there: 48 + 7 = 55. (Code 72 is a capital letter, 'H'.)

Digits don't get codes. Only letters do.

This is a common misconception

Every character you can type needs a code, and digits are no exception. They form their own group, '0' to '9', running in sequence just like the letters do.

It's 55. The digits have their own run, and '7' is 7 places after '0' = 48.

This matches the evidence

Exactly. '0' is 48, '1' is 49, '2' is 50, and so on up the run. '7' is 7 places after '0', so its code is 55. It's the same counting as for letters, just from a different first code.

A digit you type is a character, not a number. Count on from the code for '0' to find its code: the character '7' is 55, not 7.

Your turn: code to character

Which character is code 109?

Your table says 'A' = 65, 'a' = 97 and '0' = 48. Which character has the code 109?

Which group? 109 is bigger than 97, and the lower case run only reaches 'z' = 97 + 25 = 122. So 109 is a lower case letter.
missing step

Which line is step 2?

How far along? 109 − 65 = 44, so it's 44 places after 'A'.How far along? 109 − 97 = 12, so it's 12 places after 'a'.How far along? 109 − 48 = 61, so it's 61 places after '0'.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Why one known code is enough to work out a whole alphabet of them, and why the character '7' doesn't have the code 7.

What you need to know

What it means for character codes to be grouped and to run in sequence
How to work out any code in a group from the group's first code
Why the digit character '7' doesn't have the code 7
How to use a given table to turn characters into codes, and codes back into characters

The big picture

Character codes are grouped: capital letters, lower case letters and digits each have their own group, and inside a group the codes run in sequence, going up by one. So a group's first code unlocks the rest: count the places after the first character and add them on. With 'A' = 65, 'Z' is 25 places along, so it's 90. Digits have their own run too: with '0' = 48, the character '7' is 55. Going from code to character, find the run, take away its first code and count on.

Key points

1Character codes are grouped. Capital letters, lower case letters and digits each form their own group.

2Within a group the codes run in sequence, going up by 1 each time: if 'A' is 65, then 'B' is 66, 'C' is 67, and so on.

3So one known code is enough. A character's code = the group's first code + the number of places after the first character.

4Count the places AFTER the first character, not the letter's position: 'Z' is 25 places after 'A', so with 'A' = 65, 'Z' is 90.

5Capital and lower case letters are separate runs with different codes: in the table used here, 'A' is 65 but 'a' is 97.

6Digits are characters with their own run. With '0' = 48, the character '7' is 55, not 7.

7Code to character: find which run the code is in, take away that run's first code, then count on that many places.

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Worked example

Problem

A character table gives 'A' = 65, 'a' = 97 and '0' = 48. Write the character codes for the text Cab2.

⚠ Watch out

Adding the letter's position in the alphabet instead of the places after the first letter, which gives 'Z' as 65 + 26 = 91 instead of 65 + 25 = 90. 'A' is 0 places along its own run, so every letter is one place fewer than its position.

🧠

Memory hook

First code, then count on. 'A' is 0 steps from itself, so the 26th letter is only 25 steps away.

✓

Check yourself

Using only 'A' = 65, 'a' = 97 and '0' = 48, write the codes for Wok4. Then decode your codes by counting on. Get Wok4 back? You've got it.

Flashcards

(7)

What does it mean that character codes 'run in sequence'?

Inside a group, each next character has the next code up. If 'A' is 65, 'B' is 66, 'C' is 67, and so on.

Which three groups of characters each have their own run of codes?

Capital letters, lower case letters and digits. Each group is one unbroken run with its own first code.

You know the code for 'A'. How do you find the code for any other capital letter?

Count how many places after 'A' the letter is, then add that to the code for 'A'.

If 'A' is 65, why isn't 'Z' 65 + 26?

'A' is 0 places along its own run, so 'Z', the 26th letter, is only 25 places after 'A'. Its code is 65 + 25 = 90.

Does the character '7' have the code 7?

No. '7' is a character in the digit group. Count on from the code for '0': if '0' is 48, '7' is 55.

Do 'A' and 'a' share a code?

No. Capital and lower case letters are separate groups with separate runs, so 'A' and 'a' have different codes.

How do you turn a code back into a character?

Find which run the code is in, take away that run's first code, then count on that many places from the first character.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Keep me posted

Which run does each code belong to?

What's the code for '7'?

Which character is code 109?

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