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python - Does it make any difference if we change the position of operands in a logical statement?

问题描述

I wrote the following function in python which returns a sublist (i.e. the part of the list that it had been iterating through until it finds a particular value (in my case 5) without including it in the sublist and also stops iterating further).

def sublist(a):
    b=[]
    i=0
    while(a[i]!=5 and i<=len(a)-1):
        b.append(a[i])
        i=i+1
    return b

the above function worked fine with all test cases except this one a=[1, 6, 2, 3, 9]. It gave the following error: IndexError: list index out of range.

But when I interchanged the operands of the while loop condition and wrote the same code in the following way I got output for all the test cases:

def sublist(a):
    b=[]
    i=0
    while(i<=len(a)-1 and a[i]!=5):
        b.append(a[i])
        i=i+1
    return b

Why didn't I get output in the first case?

标签： pythonpython-3.x

解决方案

这被称为“逻辑短路”，当一个逻辑表达式必须被评估时，编程语言观察第一个操作数并说：“这是一个 OR，如果第一个操作数对我来说没问题，那么整个表达式也没问题！” 所以它忽略了第二个。

对于 AND 是一样的，但是：“这是一个 AND，如果第一个操作数没问题，我必须检查第二个，否则整个表达式不正确”

在这种情况下，您首先检查 a[i]

可能

抛出异常，然后为 i<=len(a)-1。

我做了一点ELI5。

编辑：我忘了如果你不使用括号也看这个表https://en.wikipedia.org/wiki/Logical_connective#Order_of_precedence

同样作为一名计算机科学家，您必须对短路有很好的了解，它可以帮助您进行很多表演

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