学习一下 for 循环的几种不同方式，了解一下原理及差异

完整的测试代码参考 GitHub ：for 循环测试代码

1 常用形态

对于 for 循环来说，最常用的形态如下

for (表达式1; 表达式2; 表达式3)
{
	// code
}

流程图如下：

编写测试代码如下

void forMethod1()
{
    cout << "\nThis is i++ test start" << endl;

#if 0
    for (int i = 0; i < MAX; i++)
    {
        cout << "for start i = " << i << endl;

        cout << "for test" << endl;

        cout << "for end i = " << i << endl;
    }
#else
    int i; // 循环变量声明放for循环之前，便于查看循环结束后的循环变量值

    for (i = 0; i < MAX; i++)
    {
        cout << "for start i = " << i << endl;

        cout << "for test" << endl;

        cout << "for end i = " << i << endl;
    }

	// 此时循环变量已是最后一次修改后的，但不满足循环条件的值
    cout << "for exit, i = " << i << endl;
#endif

    cout << "This is i++ test end\n" << endl;
}

上述代码执行结果如下

This is i++ test start
for start i = 0
for test
for end i = 0
for start i = 1
for test
for end i = 1
for start i = 2
for test
for end i = 2
for start i = 3
for test
for end i = 3
for start i = 4
for test
for end i = 4
for exit, i = 5
This is i++ test end

从执行结果来看，在退出 for 循环代码块后，i 的值是 5 ，按流程图也就是在经过了最后一次循环变量自增，再去判断循环条件是否满足，此时不满足 i < 5 ，随后退出 for 循环。

2 代码块中修改循环变量

另一种比较常用的方式，就是在代码块中修改循环变量，以实现不同的循环方式。

测试代码如下

void forMethod3()
{
    cout << "\nThis is i++ test V2 start" << endl;

    int i;
    for (i = 0; i < MAX;)
    {
        cout << "for start i = " << i << endl;

        cout << "for test, start i++" << endl;

		// 循环体内执行改变循环变量操作，实现不同循环效果
        i++;

        cout << "for end i = " << i << endl;
    }

    cout << "for exit, i = " << i << endl;

    cout << "This is i++ test V2 end\n" << endl;
}

代码实际运行结果如下

This is i++ test V2 start
for start i = 0
for test, start i++
for end i = 1
for start i = 1
for test, start i++
for end i = 2
for start i = 2
for test, start i++
for end i = 3
for start i = 3
for test, start i++
for end i = 4
for start i = 4
for test, start i++
for end i = 5
for exit, i = 5
This is i++ test V2 end

此中方式，区别在于将改变循环变量操作放进代码块内执行，以实现不同循环方式，比如

// 步长为1
i = i + 1;
i += 1;
i++;
++i;

// 步长为2
i = i + 2;
i += 2;

...