Codes, puzzles and neat output

What you will be able to do

After this lesson you can

• switch between letters and numbers with ord and chr,
• write and decode a Caesar cipher,
• test for palindromes and anagrams, and
• format numbers and align columns with an f-string.

Letters are numbers

Inside the computer every letter is stored as a number. Two functions switch between the two:

• ord("a") gives the number of a letter. It is 97 for a, 98 for b, and 122 for z.
• chr(97) gives the letter that has the number 97.

Because the letters of the alphabet follow each other, ord("c") - ord("a") is 2: the place of c in the alphabet, counting from 0. This is the key to the codes.

A secret code

Julius Caesar is said to have hidden his messages by moving every letter a few places along the alphabet. This is a Caesar cipher. With a shift of 3, a becomes d, and hello becomes khoor.

Predict first. What does the shift of 3 turn hello, world into? What will decoding it give? Then run it.

Decoding needs no new function. Moving by -3 goes back. And the % of Python never gives a negative answer for a positive divisor, so -3 works correctly.

The cipher is easy to break, as you will see in the project of the next course. But it shows the idea of a key (the shift) and a code (the rule).

A shift of 26 brings every letter back home, so shifting by 26 − s undoes a shift of s.

Palindromes and anagrams

• A palindrome reads the same backwards: level, noon, and even the sentence "A man, a plan, a canal: Panama" when we ignore the spaces, the commas and the capital letters.
• Two words are anagrams if they use the same letters: listen and silent.

To test them, clean the text first, and then compare: clean == clean[::-1] for a palindrome, and sorted(a) == sorted(b) for anagrams. The slice [::-1] reads the text backwards.

Neat output with f-strings

An f-string can format a number or align a column:

• {price:.2f} shows two digits after the point.
• {name:<8} puts the text at the left in a space of 8 characters.
• {marks:>6} puts it at the right in a space of 6.
• {n:,} puts a comma between the thousands.
• {x:.1%} shows a fraction as a percentage.

Common mistakes

• Forgetting % 26. z shifted by 3 becomes a symbol and not c.
• Shifting capital letters with the small-letter rule. Make the letter small first, or handle A to Z separately.
• Shifting spaces and marks. Copy them as they are.
• Comparing a palindrome without cleaning. "Level" is not equal to its reverse because of the capital L.
• sorted(a) == sorted(b) for texts with spaces. Clean first.
• A missing : in the format or the wrong side for < and >.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the problem Shift by three and the problem A palindrome sentence.
2. Build. Open the lab Secret messages and the problem Anagrams.
3. Stretch. Open the problem Neat numbers and format a small table of prices.

Remember these

• ord() gives the number of a letter, and chr() gives the letter back.
• A Caesar cipher: (place + shift) % 26. The % 26 wraps the alphabet round.
• Clean the text first, then compare: a palindrome with [::-1], anagrams with sorted.
• f-string formats: .2f, <8, >6, , and %.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What does the shift of 3 turn hello into?

2. Why is % 26 needed?

3. How do you test whether two texts are anagrams?

4. What does {x:>6} do?