c - c程序中的for循环混淆

Question

为什么循环从 2 运行到 7？

int i;    
for(i=1;i<=6;printf("\n%d\n",i))    
i++;

这个的输出是

2
3
4
5
6
7

但是 i 的限制是 6。

Answer 1

for 循环的语法是

for ( clause-1 ; expression-2 ; expression-3 ) statement

执行如下，引用自 C11, chapter §6.8.5.3, (emphasis mine)

The expression expression-2 is the controlling expression that is evaluated before each execution of the loop body. The expression expression-3 is evaluated as a void expression after each execution of the loop body. [....]

这里，i++ 是正文，printf("\n%d\n",i) 是 expression-3。

所以，执行的顺序是这样的

 i = 1;
 start loop

    i < = 6          //==> TRUE
    i++;            //i == 2
    printf         // Will print 2    ///iteration 1  done

    i < = 6         //==> TRUE
    i++;           //i == 3
    printf        // Will print 3   ///iteration 2  done
    .
    .
    .
    i < = 6         //==> TRUE
    i++;           //i == 6
    printf        // Will print 6   ///iteration 5  done

    i < = 6         //==> TRUE
    i++;           //i == 7
    printf        // Will print 7   ///iteration 6  done

    i < = 6 ==> FALSE

 end loop.