My functions are not working properly in C?

Question

I am trying to do a simple calculator using C. I have created 4 functions which are doing addition, multiplication, subtraction and divison. However, when I call the functions in the main function, they are not working correctly. The results are always multiplied even if I type "+", "-", "/". What may be the problem here?

Translation of some words to be clear: arti = plus, eksi = minus, carpi = cross, bolum = divide, carpim = multiplication, toplama = addition, cikartma = subtraction, bolme = divison

Thanks :)

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

int carpim(int numOne, int numTwo, int result){
    
    result = numOne * numTwo;
    printf("Sonuc: %d", result);
    return 0;
    
}

int toplama(int numOne, int numTwo, int result){
    
    result = numOne + numTwo;
    printf("Sonuc: %d", result);
    return 0;
    
}

int cikartma(int numOne, int numTwo, int result){
    
    result = numOne - numTwo;
    printf("Sonuc: %d", result);
    return 0;

}

int bolme(int numOne, int numTwo, int result){
    
    result = numOne / numTwo;
    printf("Sonuc: %d", result);
    return 0;

}

int main(){
    
    char islem[0];
    char arti[] = "+";
    char eksi[] = "-";
    char carpi[] = "x";
    char bolum[] = "/";
    int sayiBir;
    int sayiIki;
    int sonuc = 0;
    
    printf("(+)\n(-)\n(x)\n(/)\nIstediginiz islemin sembolunu girin:");
    scanf("%c", islem);
    
    printf("Ilk sayiyi girin: ");
    scanf("%d", &sayiBir);
    printf("Ikinci sayiyi girin: ");
    scanf("%d", &sayiIki);
    
    if (strcmp(islem, arti)) {
        toplama(sayiBir, sayiIki, sonuc);
    }
    else if (strcmp(islem, carpi)){
        carpim(sayiBir, sayiIki, sonuc);
    }
    else if (strcmp(islem, eksi)){
        cikartma(sayiBir, sayiIki, sonuc);
    }
    else if (strcmp(islem, bolum)){
        bolme(sayiBir, sayiIki, sonuc);
    }
    
}

Answer 1

You have two major problems:

• The first is that the array islem is an array of zero elements, it can't hold anything much less a null-terminated string

• The second problem is that you read a single character, and attempt to use it as a null-terminated string when you pass it to the strcmp functions (the str prefix means it handles null-terminated strings).

To solve these problems, make islem a single character:

char islem;

and read it using scanf passing a pointer:

scanf(" %c", &islem);

Then compare using normal comparison operator instead:

if (islem == '+') {
    toplama(sayiBir, sayiIki, sonuc);
}

---

On another note, you don't need the sonuc variable, and should not pass it to the functions. Instead the function can define the result variable locally:

void carpim(int numOne, int numTwo){
    int result = numOne * numTwo;
    printf("Sonuc: %d", result);
}

And you don't really need the result variable either:

void carpim(int numOne, int numTwo){
    printf("Sonuc: %d", numOne * numTwo);
}