Strings and positions

What you will be able to do

After this lesson you can

• say what a string is and count its characters,
• pick any character by its position, from the front or from the back,
• explain the error that comes from a position that is too big, and
• change a string by making a new one.

Text is a row of characters

You already used text in print("Hello"). In Python, text has a name: a string. A string is a row of characters: letters, digits, spaces and signs, one after the other, inside quotation marks.

The word computer is a row of eight characters. The function len() counts them.

The picture shows both ways. Use whichever is easier. To get the last character, word[-1] is easier than word[len(word) - 1].

Why does counting start from 0?

It looks strange at first. The reason is that a position tells how far you are from the start. The first character is 0 steps from the start. Almost every programming language does this, so it is worth learning once, well. A good rule: in a string of length n, the positions are 0 to n - 1.

The positive positions run from 0 to 5, and the negative ones from -6 to -1. Both name the same 6 letters.

A position that does not exist

What if you ask for a position that is not there?

A string cannot be changed in place

Can you change a letter of a string? Try to change cat to bat.

What do we do instead? We make a new string and store it in the same name.

Strings and numbers: the digits are characters too

len("2026") is 4, because "2026" is a string of four characters. len(2026) is an error, because a number has no length. Remember the lesson on types: the quotation marks decide.

Common mistakes

• Counting positions from 1. The first character is word[0].
• Asking for word[len(word)]. The last position is len(word) - 1, or word[-1].
• Trying to change a letter in place. word[0] = "b" is an error. Make a new string.
• Forgetting that a space is a character. len("a b") is 3.
• Using len() on a number. len(12) is an error. Use len("12").

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 lab Pick letters out. Pick the first, the last and the middle letter of a word with the checker's help.
2. Build. Open the problem First and last letter, and the problem Positions and slices.
3. Stretch. Open the problem Negative positions. Work out each answer on paper first.

Remember these

• A string is a row of characters. len() counts them, spaces too.
• The positions start from 0. A negative position counts from the end: -1 is the last.
• A position that is too big gives an IndexError.
• A string cannot be changed in place. Make a new string.

Check yourself

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

1. What are len("Hi there"), "Hi there"[0] and "Hi there"[-1]?

2. Which positions does the word python have?

3. Why does "cat"[3] give an error?

4. How do you change "cat" into "cap"?