Course 2 · Python BasicsLesson 5 of 14Free to read

Indexing and slicing · Python from Zero

How do you get a single letter, or a piece, out of a string?

Why you need it

Text is everywhere: names, messages, codes, file names. Very often you need part of it: the first letter for initials, the last 4 digits of a card, the year from a date. Python lets you reach any position with an index, and any piece with a slice.

The intuition

A string is a row of numbered boxes, and numbering starts at 0:

P	y	t	h	o	n
0	1	2	3	4	5
-6	-5	-4	-3	-2	-1

word[0] is "P", word[5] is "n". Negative numbers count from the end: word[-1] is "n".
len(word) is 6, the number of characters. The last index is len(word) - 1.
A slice word[start:stop] takes from start up to but not including stop: word[0:2] is "Py". Leave out a side to go to the edge: word[2:] is "thon".

Definition

Index

An index is the position of an item in a sequence; Python indexes start at 0.

Slice

A slice s[start:stop] is a new string containing the characters from index start up to, but not including, index stop.

Examples

Taking pieces

Try it yourself

Try other numbers in the brackets.

word = "Python"
print(word[0], word[-1])
print(word[0:2], word[2:])
print(len(word))

Step by step

L2Index 0 is the first character; -1 is the last.P n
L3[0:2] takes indexes 0 and 1 (not 2). [2:] takes from 2 to the end.Py thon
L4len counts the characters.6

Joining strings

Try it yourself

+ glues strings together; it doesn't add spaces for you.

first = input()
last = input()
print(first + " " + last)
print(first[0] + last[0])

Input (one line per input() call)

Grace
Hopper

Step by step

L3Three strings joined: first, a space, last.Grace Hopper
L4The first character of each: the initials.GH

Common mistakes

Counting from 1

The first character is s[0], not s[1]. s[len(s)] is an IndexError; the last one is s[len(s) - 1] or simply s[-1].

The stop index is not included

s[1:3] has 2 characters (indexes 1 and 2). Length of a slice = stop − start.

Quick quiz

1. What does this print?

1
2

s = "hello"
print(s[1])

A
```
h
```
B
```
e
```
C
```
l
```

Show the answer

Answer: B

Index 1 is the second character because counting starts at 0.

2. What does this print?

print("abcdef"[2:4] + "!")

A
```
cd!
```
B
```
bc!
```
C
```
cde!
```

Show the answer

Answer: A

Indexes 2 and 3 are c and d; 4 is not included.

Practice

3 exercises in three steps: predict what a program prints, change a program to meet a new requirement, then write one from scratch. Your code runs in the browser and is checked the moment you submit.

PredictSlice the banana
Type exactly what this program prints.
ModifyFirst, last, length
The program prints the first letter of a word. Make it print three lines: the first letter, the last letter, and the length.
WriteInitials
Read a first name and then a last name (two lines). Print the initials with dots, like A.L. for Ada and Lovelace.

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Terms in this lesson

Term	Chinese	Definition
Index	下标	The position of an item in a sequence, starting from 0.
Slice	切片	A part of a sequence taken with [start:stop], excluding stop.
len()	len() 长度函数	A function that returns the number of items in a sequence.
Concatenation	拼接	Joining strings end to end with +.