Why is it that when I have 2 scanf statements within a while loop in my C program, after 1 loop only one of them would run?

Question

#include "stdafx.h"
#include <stdio.h>
#include <Windows.h>

double FindPopulation(char);

void main()
{
    char state;
    double population;
    int flag = 1;

    while(flag == 1) 
{

    printf("This program will display the population for any New England state. \n");

    printf("e=ME, v=VT, n=NH, m=MA, c=CT, r=RI\n");

    printf("Enter the state using the above symbols: ");

    scanf("%c", &state);

    printf("You entered %c \n", state);

    population = FindPopulation(state);

    if (population == 0)
    {
        printf("You have entered a invalid state!\n");
    }

    else

        printf("This state has a population of %.2f million!!\n", population);

    printf("To quit please enter the number 0, to run this program again please enter the number 1: ");

    scanf("%d", &flag);

    }

}

double FindPopulation(char s) 
{

    double pop;

    switch (s)
    {
    case 'e': pop = 1.34;
        break;

    case 'v': pop = 0.63;
        break;

    case 'n': pop = 1.36;
        break;

    case 'm': pop = 6.90;
        break;

    case 'c': pop = 3.57;
        break;

    case 'r': pop = 1.06;
        break;

    default: pop = 0;
    }

    return pop;

}

I am new to C, never did while loops in C before. Anyways the idea is, there are 2 scanfs, one of them asks for user input to bring back the correct state population, the other scanf will be a flag that breaks out the while loop. The flag scanf works perfectly fine. If I type in a 0 it will break out of the loop and end the program. If I type in a 1 it will continue to loop. However, the first scanf after the first loop will stop working. It will not ask for another user input. Instead it will skip itself and print the output:

"Enter the state using the above symbols: You entered "

For the second scanf I can press 0 to quit any time even after multiple iterations.

Can someone tell me why is one scanf is working correctly while the other one fails?

Why would the second scanf take \n as input before all the other print statement are printed during the second loop? It skips all those print statements? — Inkplay_, Mar 29 '19 at 02:39
take a look here: https://stackoverflow.com/questions/13542055/how-to-do-scanf-for-single-char-in-c . `printf` sends data to `stdout`, `scanf` reads from `stdin`,, 2 different file descriptors — yano, Mar 29 '19 at 02:47

Nevus · Accepted Answer · 2019-03-29T04:12:14.937

When you get first get a character input to variable state using scanf it stores it to state. Then you get input to flag using scanf. If you press 0 and Enter key, the 0 (int) is stored in flag while \n is left as it is in file stdin.

The next scanf takes character(%c) from stdin(standard input file) and since \n is first character in stdin it puts \n in state. Since your function FindPopulation() does not take \n as valid input it displays the error.

You can solve it by puttinggetchar(); after scanf("%d", &flag);.
getchar(); gets a character from stdin ie removes \n from stdin.
You could also use while((c = getchar()) != '\n' && c != EOF); to remove all characters in stdin.

Why is it that when I have 2 scanf statements within a while loop in my C program, after 1 loop only one of them would run?

1 Answers1