C中有趣的增量运算符（在while循环中）

Question

I want to implement the factorial algorithm, and the following code gives me wrong result for factorial(5)

int factorial(int n)
{
    int i = 1, ret = 1;
    while(i++<=n)
        ret *= i;
    return ret;
}

Looks like the code will continue to run even when i = 6, I don't understand why the while loop didn't stop.

Answer 1

You are using a post-increment operator which means the final check for i <= n happens before i is incremented for the final time. In other words, when i is 5 your code:

check if i <= 5 -> true

increment i -> i is now 6

execute loop -> executes for i = 6

check if i <= 5 -> false (i is now 6)

You should change to:

while(++i <= n)

Answer 2

Try to do one thing per statement.

int factorial(int n)
{
    int i = 1, ret = 1;
    while(i<=n)
    {
        i++;
        ret *= i;
    }
    return ret;
}

Check the differences between the versions above and below this text.

int factorial(int n)
{
    int i = 1, ret = 1;
    while(i<=n)
    {
        ret *= i;
        i++;
    }
    return ret;
}

Answer 3

i++ returns the value of i and only then increments i . (As opposed to ++i which first incerements i and returns incremented value)

Answer 4

That's how i++ works. you need ++i

Answer 5

You are doing one extra iteration of the loop.

Instead of (i++<=n) , what you really want is making it (i<=n) and then add the i++; after the ret*=1 .

This is actually much more cleanly done using a for-loop IMHO.