The difference between i++ and ++i in the loop in C language

foreword

I encountered it when I was brushing the questions today, so I will briefly record it (maybe I don’t know). In C language, both i++ and ++i mean self-increment. The difference is that ++i is first increased and then assigned, while i++ is first assigned and then increased. I think there are beginners like me who have always had doubts before: they are both similar, so when to use i++ and when to use ++i? Today I learned that the judgment mechanism of i++ and ++i in the loop is different.

for loop

In the for loop, i++ and ++i are the same, they are judged first and then added.

	for (int i = 0; i < 5; i++)
	{
		cout << i << " ";
	}

	for (int i = 0; i < 5; ++i)
	{
		cout << i << " ";
	}

The output is the same.

while loop

In the while loop, i++ and ++i are different: i++ first judges and then increases before entering the loop body:

	int i = -5;
	while (i++ < 0)
	{
		cout << i << " ";
	}

In the above code, first judge that i == -5 is less than zero, then increment i = i + 1, and finally enter the loop;

And ++i is to increase first and then judge and then enter the loop body:

	i = -5;
    while (++i < 0)
	{
		cout << i << " ";
	}

 In the above code, first increment i = i + 1, then judge that i == -4 is less than zero, and finally enter the loop;

The test results are as follows:

 do-while loop  

The i++ and ++i in the do-while loop are the same as in the while loop, except that the do-while executes the loop body first:

    cout << "do-while循环i++：";
	i = -5;
	do
	{
		cout << i << " ";
	} while (i++ < 0);


	cout << "do-while循环++i：";
	i = -5;
	do
	{
		cout << i << " ";
	} while (++i < 0);