while loop won't terminate with a logical conditional

问题

This loop won't terminate if I add an OR conditional statement. If one is false, then it should terminate.

//global var

int x = 100;
    char *n= malloc (64);

void add(void)
{       

    do      
    {
        printf("Would you like to add 1? (y/n) ");
        fgets(n, 64, stdin);
            //removes newline
        n[strlen(n)-1] = '\0';
            x++;
    }       
//if I add || (x!=100) it keeps looping me even if I press "n"
//otherwise without it it works fine

        while((strncmp(n, "n", 1) != 0) || x!=100 );

        free(n);
}

回答1:

At the bottom of your loop, you're doing x++. Once you hit the while condition, x == 101, so your loop never terminates, x never equals 100 when the condition is being checked.

Perhaps you wanted:

while((strncmp(n, "n", 1) != 0) && x != 100);

Which would terminated the loop if either of the two conditions is false.

&& is the 'logical and' operator, || is the 'logical or' operator. To help you keep track, you can use a Truth Table to help you sort out the details. Logical and is the same as logical conjunction and logical or is the same as logical disjunction.

回答2:

The while loop will execute as long as the entire condition evaluates to 1, or true. If x is never 100, then the logical OR in your condition will always be true, because one of it's operands is true.

x begins at 100, and each time through the loop, you increment x. So before you ever test x in the loop condition, it is greater than 100 - it's 101 - and always increases.

回答3:

You picked the wrong boolean operator. If you type n, and x is equal to 10, the while condition simplifies like this:

while((strncmp(n, "n", 1) != 0) || x!=100 ); while((0 != 0) || (10!=100) ); while(0 || 1) while(1)

Since each comparison is already a NOT, you want && instead of ||

回答4:

Checking numeric equality to terminate a loop is usually a bad idea. You want to check for a range of values, e.g., x < 100.

Hence, the classic for loop test is i < n and not i != n.

I only terminate loops with equality when the variable is a boolean type.

回答5:

the condition x!=100 is never going to be false, since x is already incremented to 101 when the first do while is executed so to make the statement false u should use && in place of ||

来源：https://stackoverflow.com/questions/12114453/while-loop-wont-terminate-with-a-logical-conditional

标签

loops

comparison

while-loop

conditional