Character strings

The string as an indexed sequence of characters: length, access, slicing, concatenation, immutability, traversal, lexicographic order and common operations (case, search, replacement, splitting, joining).

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String: what is it?
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An ordered sequence of characters, identified by their position like the cells of an array, used to represent text.
len('hello') is 5, 'hello'[0] is 'h' and 'hello'[-1] is 'o'. A single character is a string of length 1 in Python.
What does this program print?

```python
s = "informatique"
print(s[2:7:2])
`
- fra
- forma
- omt
- nom
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fra
The indices taken are 2, 4 and 6: f, r, a (7 is excluded). forma ignores the step, omt starts at index 3, nom at index 1.
Joining two strings end to end is called …; in Python, it is written with the … operator.
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Joining two strings end to end is called concatenation; in Python, it is written with the + operator.
'ab' + 'cd' gives 'abcd', but 'ab' + 1 raises a TypeError: convert with str(1). The * operator repeats: 'ab' * 3 gives 'ababab'.
Slice: what is it?
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A contiguous part of a string, given by a start index, an excluded end index and, optionally, a step.
s[2:5] takes indices 2, 3 and 4. An omitted bound goes all the way: s[:3] for the first three characters, s[-3:] for the last three, s[::-1] for the reversed string.

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