scanf prematurely terminating while loop (I think)

Question

I am making a simple program to calculate the required exam mark based on your current mark, the % total worth of the exam on the course, and your desired mark. I was having no problems with it until I wanted a simple while loop to redo the proses if the user wanted to do so. The problem that arose was that when I have the scanf to get a char response from the user, it terminates the while loop and then the program. When I comment out the scanf line and leave the code as is (an infinite while loop) it works properly looping forever but with it, it will terminate with no errors. Any thoughts?

    #include <stdio.h>

    #define TRUE 1
    #define FALSE 0

    int main()
    {
        int continueProgram = TRUE;
        float currentMark = 0.0, examPercent = 0.0, desiredMark = 0.0, requiredMark = 0.0;
        char response  = 'y';

        while (continueProgram == TRUE)
        {
            printf("What is your current mark in your class: ");
            scanf("%f", &currentMark);
            printf("How much is you exam worth: ");
            scanf("%f", &examPercent);
            examPercent /= 100.0;
            printf("What is your desiered mark: ");
            scanf("%f", &desiredMark);

            // Simple math to get mark
            requiredMark = (desiredMark - currentMark * examPercent) / examPercent;

            if (requiredMark > 100.0)
            {
                printf("\nI'm sorry but you need above 100%% to get to your required mark\nHere is a list of all the marks you can get:\n\n");

                for (requiredMark = 0.0; requiredMark <= 100.0; requiredMark += 5.0)
                {
                    desiredMark = currentMark * examPercent + requiredMark * examPercent;
                    printf("Exam Mark: %.2f, Final Mark: %.2f\n", requiredMark, desiredMark);
                }
            }
            else
            {
                printf("\nThe mark that you need to get on the exam is: %.2f\nHere is a list of all the marks you can get:\n\n", requiredMark);

                for (requiredMark = 0.0; requiredMark <= 100.0; requiredMark += 5.0)
                {
                    desiredMark = currentMark * examPercent + requiredMark * examPercent;
                    printf("Exam Mark: %.2f, Final Mark: %.2f\n", requiredMark, desiredMark);
                }
            }

            printf("\nDid you want to continue with a new grade (y or n): ");

            // THIS IS WHERE I AM HAVING THE ISSUE
            scanf("%c", &response );

            if (response  == 'y')
            {
                printf("\n\nResetting  terminal...\n\n");
                currentMark = 0.0;
                examPercent = 0.0;
                desiredMark = 0.0;
                requiredMark = 0.0;
            }
            else
                continueProgram = FALSE;
        }
    }

Answer 1

The problem that arose was that when I have the scanf to get a char response from the user, it terminates the while loop and then the program.

This is happening because the scanf() is reading the stray \n (newline character) from the input buffer.

To resolve this, add a space before % character in scanf() like this:

scanf(" %c", &responce);

This will skip the leading whitespace characters (including newline character) and read the input given by the user.

Answer 2

before this

scanf("%c", &response );

need to clear out stdin, perhaps by:

int ch;
while( (ch = getchar()) != EOF && '\n' != ch );

because the prior call to scanf() left the newline char in the input stream.

Answer 3

The reason you face this has to do with leading whitespaces

Change your scanf line to this:

scanf(" %c", &responce);

Here's a link for the explanation

How to do scanf for single char in C

Answer 4

Try this, which works as you expected.

Original :

scanf("%c", &responce);

Replace:

responce=fgetc(stdin);

Answer 5

Personally I like to use fflush while under Windows

fflush(stdin);               //-- clear the input buffer
scanf("%c", &response );

or fseek under Linux environments.

fseek(stdin,0,SEEK_END);    //-- prepare the input buffer to allow fresh data from the keyboard
scanf("%c", &response );