Thread: Basic problem in C program im trying to run

Code:

    if (ch='y') {

Look up the difference between "=" and "==".
The first assigns a value; the second compares two values.

Tim S.

For starters, you should indent your code properly, e.g.,

Code:

#include <stdio.h>

void main()
{
    int x;
    char ch;
    printf("Please enter your number: ");
    scanf("%d", &x);
    if (x > 0)
        printf("Your number is positive!\n");
    else if (x < 0)
        printf("Your number is negative!\n");

    while (x > 0 || x < 0) {
        printf("Enter another number?\n");
        ch = getchar();
        getchar();
    }

    if (ch = 'y') {
        printf("Please enter your number: ");
        scanf("%d", &x);
        if (x > 0)
            printf("Your number is positive!\n");
        else if (x < 0)
            printf("Your number is negative!\n");
    }
    else
        printf("Thank u for using my software, good bye!\n");

    flushall();
    getchar();
}

Notice that I indent the body of the function, each if/else block, and the while loop. I also use blank lines to separate parts of the code, but I don't use too many at one go.

Next, notice that your while loop's condition depends on x, but x does not change in the body of the while loop. This explains why you get an infinite loop. Also, note that there is a difference between = and == that will matter in the code after the while loop.

By the way, void main should be int main, and I don't recall flushall being in the standard library. You probably want fflush(stdout) instead.

Try this slight change to your program, and note the changes and comments.

Code:

#include <stdio.h>
int main(void)
{
    int x;
    char ch;
    printf("Please enter your number: ");
    scanf("%d" , &x);
    if (x > 0)
        printf("Your number is positive!\n");
 
 
    else if (x < 0)
        printf("Your number is negative!\n");
 
    ch = getchar();
    printf("%d\n",ch); //here, ch will always equal 10 - a newline
    //that is left over from entering x. We can use it, however.
  
    while (ch)  //ch will always be true on the first loop
    {
        printf("Enter another number?\n");
        scanf(" %c", &ch); //note the space before the %c! That space causes scanf
        //to "eat" whitespace in the keyboard (stdin) buffer.
     
        if (ch=='y')  //so now ch is correct
        {
            printf("Please enter your number: ");
            scanf("%d" , &x);
            if (x > 0)
                printf("Your number is positive!\n");
            else if (x < 0)
                printf("Your number is negative!\n");
        }
        else
            break; //ch not equal to 'y' 
   }
   printf("Thank u for using my software, good bye!\n");
   getchar();
 
   return 0;
}

Your problem is a very common one. getchar() or scanf("%c"...) without a space before the %c, will ALWAYS take a waiting newline char from the stdin, and use it for a char input - because a newline is what it's looking for.

Integers don't have that problem, since they have a different size than char's have.

Originally Posted by Adak

Try this slight change to your program, and note the changes and comments.

Code:

#include <stdio.h>
int main(void)
{
    int x;
    char ch;
    printf("Please enter your number: ");
    scanf("%d" , &x);
    if (x > 0)
        printf("Your number is positive!\n");
 
 
    else if (x < 0)
        printf("Your number is negative!\n");
 
    ch = getchar();
    printf("%d\n",ch); //here, ch will always equal 10 - a newline
    //that is left over from entering x. We can use it, however.
  
    while (ch)  //ch will always be true on the first loop
    {
        printf("Enter another number?\n");
        scanf(" %c", &ch); //note the space before the %c! That space causes scanf
        //to "eat" whitespace in the keyboard (stdin) buffer.
     
        if (ch=='y')  //so now ch is correct
        {
            printf("Please enter your number: ");
            scanf("%d" , &x);
            if (x > 0)
                printf("Your number is positive!\n");
            else if (x < 0)
                printf("Your number is negative!\n");
        }
        else
            break; //ch not equal to 'y' 
   }
   printf("Thank u for using my software, good bye!\n");
   getchar();
 
   return 0;
}

Your problem is a very common one. getchar() or scanf("%c"...) without a space before the %c, will ALWAYS take a waiting newline char from the stdin, and use it for a char input - because a newline is what it's looking for.

Integers don't have that problem, since they have a different size than char's have.

Thank you Adak!
Finally I see the program running perfectly beside one thing...
after the user inputs 'Y' to the ch variable the program prints 10, now, I do realize that it has something to do with the "leftover" u mantioned in the notes but I can't figure out what "left over" is... if you can give me deeper details about it i'll really appreciate it.
Moreover, there are 3 more things that i'd like to know, if I may;
1. Why did you used printf("%d\n", ch ); before starting the loop? Is it because you want it to be true on the first time the loop starts?
2. I didn't understand the stdin buffer part, about "eating" white space (what's white space [newbie qotd ] in the program. if you can repeat on the explanation deeper i'll really appreciate it.
3. Why did you added return 0; in the end of the program? Does it help in any way?

Thanks Adak!