PHP 中的 For 循环之谜

Question

Using PHP 5.6, why does this code outputs

2
5
8
11

instead of

3
6
9

?

for ($i = 0; $i < 10; $i++) {
    echo $i += 2 . "<br />";
}

Answer 1

Because i starts at 0, not 1. Lets see what is happening:

Loop 1

Initiate i, i = 0;
Check for loop condition ($i < 10). i is 0 so yes, run the loop.
Add 2 to i and echo. 0 + 2 = 2, echo 2.
End of loop, add 1 to i. i is 3.

Loop 2

Check for loop condition ($i < 10). i is 3 so yes, run the loop.
Add 2 to i and echo. 3 + 2 = 5, echo 5.
End of loop, add 1 to i. i is 6.

And so on. So you add 2, echo out then add 1.

You may be expecting the i++ in the for loop to run before the code, but it runs at the end of the code. From the PHP website :

At the end of each iteration, expr3 is evaluated (executed).

If you want your output to be 3, 6, 9 then you would need to initiate i to 1 at the start of your loop.

for($i = 1; $i < 10; $i++)

Answer 2

The increment ( i++ ) in the loop header happens after the loop body is executed. So, i is initialized to 0, then you add 2 and print it. Then, the loop header increments it to 3, then you add 2 and print it again...

Answer 3

In the first iteration, you add 0 + 2 , because you declared $i to be 0 , so the output would be 2. then with the $i++ you do the same as you would write $i += 1 . So now the value of $i is 3.

Then you add again 2 which gives you 5 as an output, then you add another 1. Value of $i = 6, add 2... outputs 8... add 1... add 2... outputs 11...