为什么循环比循环体多执行一次？

Question

引自算法教科书：

"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for j=1 to 3开头的for循环将执行4次而不是3次！

问题：为什么这样的循环会执行4次而不是3次？

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.

我数的是 3，不是 4。

Answer 1

我想你对书中的说法感到困惑

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.

这意味着循环条件将比循环体多测试一次，因此根据您的示例：

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

Answer 2

这是 for...loop

的伪机器代码

// int result = 0;
// for(int x = 0; x < 10; x++) {
//   result += x;
// }
MOV edx, 0 // result = 0
MOV eax, 0 // x = 0
.ForLoopLabel:
CMP eax, 10 // FillFlags(x - 10)
JGE .ForLoopFinishedLabel // IF x >= 10 THEN GoTo ForLoopFinishedLabel
// for loop's body
ADD edx, eax // result += x
// end of body
ADD eax, 1 // x++
JMP .ForLoopLabel // GoTo ForLoopLabel
.ForLoopFinishedLabel:

为什么循环比循环体多执行一次？

Why are loops executed one more time than the loop body?

algorithm

computer-science