CODEWITHSUNDEEP
cwhile-loop
O.Kaan
O.Kaan

Reputation: 13

Wondering about while loop

I started learning C 3 weeks ago, and while learning while loops I tried to build an addition program, basically you keep adding numbers it additions them and after 2nd number it gives you a subtotal for every addition, and if you press 0 to quit It gives you a final sum then quits. Now I have one main, one additional question. The main question is, I had to use sum = 0 before the while functions, if I use it after "the while" it gives me the number I entered as the result. Now I really wonder what is the idea behind it. When I write it like below does it equates "sum with 0" for the start and changes the value as I enter another number, or there is some other idea behind it.

And the additional question is , why do I need to use 2 getchar(); to make my program stay on the screen, why not one?

#include <stdio.h>
int main(void)
{
    float num;
    float sum;
    printf(" please enter a number +0 to start the program (0 to quit): \n");
    scanf(" %f", &num);

    sum =0; //THIS HERE**********************

    while (num  > 0)
    {   
        printf("please enter integer:\n");
        scanf("%f", &num);
        sum = sum + num;
        printf("current sum is = %f\n", sum);
    }
    printf("final sum is = %f\n", sum);
    getchar();
    getchar();
    return 0;
}

Upvotes: 1

Views: 67

Answers (3)

Aasmund Eldhuset
Aasmund Eldhuset

Reputation: 37980

= in most programming languages is different from the mathematical =. It does not mean that the two sides will permanently be equal to each other; rather, it means that the right-hand side should be computed and that the result should be assigned to the variable on the left-hand side. Later, another line might change the variable value to something else.

Thus, sum = sum + num; means that the current values of sum and and num are to be added, and the result is to be put back into sum. In order for this to work the way you wish, sum must be 0 the first time this line is executed; hence, you need sum = 0; somewhere. However, if this line is inside the loop, it is repeatedly executed, so that the result of the previous summation disappears and is replaced with 0 before each new number.

Upvotes: 1

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726987

If I use it after the while it gives me the number I entered as the result.

This is because when you do this

while (num > 0) {
    sum = 0;
    ...
    sum = sum + num;
}

the value that has been accumulated by sum on prior iterations of the while loop gets erased each time the loop iterates, meaning that only the last value would be added to sum (and kept as the result of the additions).

why do I need to use 2 getchar();

Because when scanf consumes the last number the end-user has entered, it reads everything up to, but not including, the '\n' character, which corresponds to the Enter key. This '\n' character remains in the buffer, waiting to be consumed by your program. The first call of getchar() consumes that "lingering" '\n', while the second one makes your program stay on screen until you press enter again.

Upvotes: 1

Richard
Richard

Reputation: 61479

If you put sum=0 inside the while loop it will be called each time the while loop loops.

This means when you reach sum=sum+num, you will actually be calculating sum=0+num.

You have two use two getchar() calls because the first one is sucking up an additional character that was not absorbed by your scanf. Probably this character is a newline, so you cannot see it. The second getchar() then keeps your terminal open because it is waiting for a character.

To figure out if my hypothesis is correct about the first getchar() you could try this:

char temp = getchar();
printf("%d",(int)temp); //Print out the character number from the first getchar
getchar(); //This keeps the window open

Upvotes: 5

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