Python的“ if”和“ while”条件不起作用 [英] Python 'if' and 'while' conditions not working

问题描述

我正在编写一个简单的Python程序。应该从制表符描述的文件中读取两个已排序列表，然后将它们合并为一个已排序列表。算法虽然不太严格，但是Python似乎忽略了循环和if语句中的条件！

I am writing a simple Python program. It's supposed to read two sorted lists from tab-delineated file and merge them into a single sorted list. The algorithm isn't too tough but Python seems to be ignoring the conditions in my loops and if statements!

这是我的输入文件：

1   2   3   10
7   9   100

以下是打印命令的相关代码，用于调试：

Here's the relevant bit of code with print commands for debugging:

print 'list1 len =' + str(len(list1)) + ', list2 len = ' + str(len(list2))
while (i < len(list1)) or (j < len(list2)):
    print 'i = ' + str(i)
    print 'list1[i] = ' + str(list1[i])
    if (list1[i] < list2[j]):
        print str(list1[i]) + ' < ' + str(list2[j])
        output.append(list1[i])
        i += 1
    else:
        output.append(list2[j])
        j += 1

程序读取正确的值，但似乎总是读取if条件

The program reads in the correct values but seems to always read the if-condition as true at every iteration.

list1 len =4, list2 len = 3
i = 0
list1[i] = 1
1 < 7
i = 1
list1[i] = 2
2 < 7
i = 2
list1[i] = 3
3 < 7
i = 3
list1[i] = 10
10 < 7
i = 4
Traceback (most recent call last):
  File "q2.py", line 22, in <module>
     print 'list1[i] = ' + str(list1[i])
IndexError: list index out of range

if语句不仅不起作用（ 10< 7 是不正确的！），而且在 while 循环，因为'i'变为4，即 list1 。

Not only is the if-statement not working (10 < 7 isn't right!), it's also failing at the while loop, since 'i' gets to 4, the size of list1. What is happening?!

推荐答案

您要和，而不是或，在您的 while 循环测试中：

You want and, not or, in your while loop test:

while i < len(list1) and j < len(list2):

（i 将是真的，如果其中一个测试是真的。因此 i 不会小于 len（list1） j 小于 len（list2）。 False或True 仍然是 True 。

(i < len(list1)) or (j < len(list2)) is going to be true if one of those tests is true. So i doesn't have to be smaller than len(list1) as long as j is smaller than len(list2). False or True is still True.

下一步，您的 if 测试很有可能是比较字符串，而不是整数。字符串按字典顺序进行比较：

Next, your if test is most likely comparing strings, not integers. Strings are compared lexicographically:

>>> 'abc' < 'abd'
True
>>> 'ab' < 'b'
True
>>> '10' < '2'
True

在测试其他字符之前先比较第一个字符，然后'1'在'2'之前排序。

The first characters are compared before other characters are tested, and '1' sorts before '2'.

比较而不是整数：

if int(list1[i]) < int(list2[j]):

您可能希望在输入文件时将文件输入转换为整数但是，请阅读它们。

You probably want to convert your file inputs to integers the moment you read them, however.

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