python之字符串的简单操作：连接，重复，索引，切片，长度，遍历

字符串的简单操作

连接，重复，索引，切片，长度，遍历

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连接

<str> + <str>

例子1

a = 'I'
b = 'love'
c = 'you'
print(a + b + c)
# Iloveyou

例子2

a = 'I'
b = 'love'
c = 'you'
print(a + ' ' + b + ' ' + c)
# I love you

错误例子

a = 2
b = '3'
c = a + b
# TypeError: unsupported operand type(s) for +: 'int' and 'str'
# int型 和 str型不能连接

重复

<str>*<int>

例子1

a = '2'
b = '3'
print(a + b*3)
# 2333

例子2

a = '2'
b = '3'
c = 3
print(a + b*c)
# 2333

例子3

a = [2, 3]
print(a *3)
# [2, 3, 2, 3, 2, 3]

错误例子

a = '2'
b = '3'
print(a + b*3.0)
# ypeError: can't multiply sequence by non-int of type 'float'
#<int> 不能改为<float>

索引

<str>[index]，返回一个字符

正索引的索引值从左到右，从0开始

负索引的索引值从右到左，从-1开始

例子1

a = 'abcdefg'
print(a[0])
# a

例子2

a = 'abcdefg'
print(a[2])
# c

例子3

a = 'abcdefg'
print(a[-1])
# g

例子4

a = 'abcdefg'
print(a[-5])
# c

错误例子

a = 'abcdefg'
print(a[100])  #print(a[-100])
# IndexError: string index out of range
# 索引值越界报错

切片

<str>[<int1>:<int2>]，返回从<int1>到<int2>的字符串

<int1>和<int2>可以为空，为空时，<int1>默认值为0，<int2>默认值为n+1（n表示字符串最后一个字符的索引值）

例子1

a = 'abcdefg'
print(a[0:3])
# abc

例子2，<int1>为空

a = 'abcdefg'
print(a[:3])
# abc

例子3<int2>为空

a = 'abcdefg'
print(a[1:])
# bcdefg

例子4，<int1>和<int1>都为空

a = 'abcdefg'
print(a[:])
# abcdefg

例子5，当<int1>大于<int2>时<str>[<int1>:<int2>]返回空值

a = 'abcdefg'
print(1)
print(a[3:2])
print(2)
# 1
# 
# 2

例子6，利用切片去掉字符串末尾的换行符

# 不去掉换行符\n
a = 'abc\ndefg\n'
print(a[:])
#abc
#defg
#

# 去掉换行符\n
a = 'abc\ndefg\n'
print(a[:-1])
#abc
#defg

错误例子

a = 'abcdefg'
print(a[0:3.0])
# TypeError: slice indices must be integers or None or have an __index__ method
# <int1> 和 <int1>必须都为int型

遍历

例子

for i in '我爱你':
    print(i)
# 我
# 爱
# 你