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我们每天都面临着选择，程序设计时也有这样的事，Python提供了选择结构来处理这样的事情。 选择结构由3部分组成：关键字(if, elif, else), 判断结果真假的条件表达式，代码块。 上下相邻且具有相同缩进长度的语句是一个代码块。 |
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判断一个数为正数，负数或零 |
x = 9
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if是关键字, “x<0”是判断真假的条件表达式， print(“x = “, x)与print(“It is the negative number.”)组成代码块 |
if x < 0:
print("x = ", x)
print("It is the negative number.")
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elif是关键字, “x>0”是判断真假的条件表达式， print(“x = “, x)与print(“It is the positive number.”)组成代码块 |
elif x > 0:
print("x = ", x)
print("It is the positive number.")
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else是关键字, else后面没有条件表达式， print(“x = “, x)与print(“It is zero”)组成代码块 |
else:
print("x = ", x)
print("It is zero.")
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判断一个数是奇数或偶数 |
x = 7
print("x = ", x)
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if是关键字, “x%2 == 1”是判断结果真假的条件表达式, |
if x%2 == 1:
print("It is the odd number")
else:
print("It is the odd number")
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判断一个数是否为3位数, 如是，打印出来 “x >= 100 and x <= 999”是判断结果真假的条件表达式， |
x = 345
if x >= 100 and x <= 999:
print("x = ", x)
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运行if-1.py |
$ python if-1.py
x = 9
It is the positive number.
x = 7
It is the odd number
x = 345
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下一个例子: 三元操作符.