造成无限循环的支架

Question

The brackets associated with following the three For loops cause the function to loop infinitely,

void Bubblesort (char ulist[27], char slist[27], int n)
{
int i,j;
char temp;
for (i=1;i<=n;i++)
{
    slist[i] = ulist[i];
    for (j=1; j <= n-1 ;j++)
    {
        for (i = 1; i <=n-j;i++)
        {     
            if (slist[i] < slist[i+1])
            {

                temp = slist[i];
                slist[i] = slist[i+1];
                slist[i+1] = temp;
            }

            }
        }
    }
}

While without the brackets; the function reaches the end.

void Bubblesort (char ulist[27], char slist[27], int n)
{
int i,j;
char temp;
for (i=1;i<=n;i++)
    slist[i] = ulist[i];
    for (j=1; j <= n-1 ;j++)
        for (i = 1; i <=n-j;i++)
            if (slist[i] < slist[i+1])
            {
                cout << "Step 4";
                temp = slist[i];
                slist[i] = slist[i+1];
                slist[i+1] = temp;
            }



}

Would someone be able to explain the reason for this?

Answer 1

You have changed the semantics of the loop and your indents are misleading.

void Bubblesort (char ulist[27], char slist[27], int n)
{
    int i,j;
    char temp;
    for (i=1;i<=n;i++)
        slist[i] = ulist[i];
        for (j=1; j <= n-1 ;j++)
            for (i = 1; i <=n-j;i++)
                if (slist[i] < slist[i+1])
                {
                    cout << "Step 4";
                    temp = slist[i];
                    slist[i] = slist[i+1];
                    slist[i+1] = temp;
                }
}

should be indented as

void Bubblesort (char ulist[27], char slist[27], int n)
{
    int i,j;
    char temp;
    for (i=1;i<=n;i++)
        slist[i] = ulist[i];

    for (j=1; j <= n-1 ;j++)
        for (i = 1; i <=n-j;i++)
            if (slist[i] < slist[i+1])
            {
                cout << "Step 4";
                temp = slist[i];
                slist[i] = slist[i+1];
                slist[i+1] = temp;
            }
}

Answer 2

The problem is that without the brackets you don't actually have nested for loops. consider

for (i=1;i<=n;i++)
    slist[i] = ulist[i];
    for (j=1; j <= n-1 ;j++)

Without brackets only the slist[i] = ... line executes as part of the first for loop. The second for loop executes as a completely separate entity

Once the loops are grouped together though the outer most and inner most loops are using the same index variable. Their mutual changes to the value conspire to create an infinite loop. Try using a different index variable for each for loop and it will fix the problem. This works without brackets because the initial error I described causes the first and last loop to not be nested hence they're not manipulating the same value at the same time

Answer 3

The two functions have different algorithms. In the first function you have three nested loops. And the third loop uses the same control variablle 'i' as the first loop. So the loops are infinite because after the third loop i is always equal to 2.

In the second function the first loop is a separate loop that is the other two loops are not nested inside the first loop. So changing control variable 'i' in the third loop does not influence to the first loop.