In C language, solve inputting integer from alphabet and repeat the loop

Question

Sorry, I did a mistake that I tot im using C++, but Im using C language.Im writing a program code about calculating some formulae. 1st of all, I set up a menu for the user to select. Then I let them choose from 3 formulae. Example pressure=force/area. I want to get the value of force and area as float from the user. I want to program it as if force is more than or equal to 0, the program continue to ask for the value of area. Else, display error and ask the same question. I tested my program, it works if i key in a negative value. But if i key in a word, the program repeat the loop unstoppable.

#include <stdio.h>
#include <stdlib.h>

int main (void)
{
int menu_selection,error_flag,repeater;
float force, area, pressure;

printf("Please select a formula. [0 - 3]\n>>");
scanf("%d",&menu_selection);
if(menu_selection==1)
    {
    error_flag=0;
    system("cls"); 

    printf("Pressure's formula\n\n");
        do{
            error_flag=0;
            printf("Please key in the force applied.(N)\n>>");
            scanf("%f",&force);
            if(force>0)
            {
                printf("Please key in the surface area.(m^2)\n>>");
                scanf("%f",&area);

                pressure = force/area;

                printf("The pressure is %.2fPa.\n\n",pressure);
            }
            else
            {
                error_flag=1;
                printf("Invalid input, please key in a positive number.");
            }
        }while(error_flag==1);
    }

When I key in alphabet, the word "Invalid input, please key in a positive number." is spamming in the console and the system goes wrong. I know its about the data type problem. How to solve it?? Help please!! And thank you if you provide me the full solution. Thank.

Since you are using `C++` instead of `C`, I would recommend you use `` and use `std::cin` to get your input values. — Cory Kramer, Nov 17 '14 at 14:51
I would also recommend not to use `void` for an empty parameter list in c++ since it's redundant. — eerorika, Nov 17 '14 at 15:09

user2970916 · Answer 1 · 2014-11-18T14:39:58.130

#include <stdio.h>
#include <stdlib.h>
#include <iostream>    // Include the iostream for cin

int main(void)
{
    int menu_selection, error_flag;
    float force, area, pressure;

    printf("Please select a formula. [0 - 3]\n>>");
    scanf("%d", &menu_selection);
    if (menu_selection == 1)
    {
        error_flag = 0;
        system("cls");

        printf("Pressure's formula\n\n");
        do{
            error_flag = 0;
            printf("Please key in the force applied.(N)\n>>");
            std::cin >> force;                     // Read value into force
            if (std::cin.good() && force >= 0)                  // Verify the force is of type float. cin will set its failbit if the user does not input the proper data type.
            {
                printf("Please key in the surface area.(m^2)\n>>");
                scanf("%f", &area);

                pressure = force / area;

                printf("The pressure is %.2fPa.\n\n", pressure);
            }
            else
            {
                error_flag = 1;
                printf("Invalid input, please key in a positive number.\n");
                std::cin.clear();                   // Clears the failbit
                std::cin.ignore(INT_MAX, '\n');     // Flushes the cin stream
            }
        } while (error_flag == 1);
    }

    return 0;
}

Im sorry, i made a mistaken, if im using C language, is this code still work for me? — Arren Lai, Nov 18 '14 at 21:51

score 0 · Answer 2 · edited May 23 '17 at 11:49

0

edit: Actual answer is here. This is a duplicate. Your problem is due to scanf not consuming what is in the input buffer when it fails and therefore re-reading it and failing again every time.

edited May 23 '17 at 11:49

Community

1
1

answered Nov 17 '14 at 15:14

jrsala

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In C language, solve inputting integer from alphabet and repeat the loop

2 Answers2