为什么循环比循环主体多执行一次？ [英] Why are loops executed one more time than the loop body?

问题描述

算法教科书中的引言：

当for或while循环以通常方式退出时（即，由于测试在循环标题中执行），比循环主体多执行一次测试。

"When a for or while loop exits in the usual way (i.e., due to the test in the loop header), the test is executed one time more than the loop body."

因此，例如，以为j = 1到3 开头的循环将不执行3次，而是执行4次！

So, for example, a for loop that begins with for j=1 to 3 will be executed not 3 times, but 4 times!

问题：为什么这样的循环执行4次而不是3次？

Question: Why would such a loop be executed 4 times and not 3 times?

根据我的推理：

When j = 1, the loop is executed. 
When j = 2, the loop is executed.
When j = 3, the loop is executed. 
When j = 4, the loop is NOT executed.

我算3，而不是4。

推荐答案

我认为您对书中的陈述感到困惑

I think you are confused about what the statement in the book states

当for或虽然循环以通常的方式退出（即，由于循环头中的测试），所以测试比循环主体执行的时间多一遍。

When a for or while loop exits in the usual way (i.e., due to the test in the loop header), the test is executed one time more than the loop body.

这意味着循环条件的测试时间将比循环体多一倍，因此，以您的示例为例：

This means that the loop condition will be tested one more time than the loop body therefore by your example:

for j = 1:3
        j = 1, pass and looped
        j = 2, pass and looped
        j = 3, pass and looped
        j = 4, failed and code executes as written

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