CODEWITHSUNDEEP
c
Rivf
Rivf

Reputation: 125

C - Returning a Structure Member from Function

I am writing this code that calculates Miles per Gallon where the number of miles and gallons are read from the keyboard. The program will perform a simple calculation to get the miles per gallon. I wrote a function in order to do this this calculation but it is not returning the result. Inside the function everything is working as it should. I even wrote a printf to ensure the calculation was done correctly. But then when I try to return it to the main function, the member of the structure appears to have a value of 0 instead of the result of the calculation. Here is my code:

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

struct gas {
 float distance;
 float gals;
 float mpg;
};

float CalculateMpg(struct gas idk){
    idk.mpg = idk.distance / idk.gals;
    printf("%f \n", idk.mpg);
    return idk.mpg;
}

int main()
{
    struct gas idk;

    printf("Enter distance (in miles): ");
    scanf("%f", &idk.distance);
    printf("Enter gallons fueled: ");
    scanf("%f", &idk.gals);

    CalculateMpg(idk);
    printf("%f \n", idk.mpg);
    printf("Miles driven gallons used with %.2f miles and %.2f gallons: %.2f mpg \n", idk.distance, idk.gals, idk.mpg);

    return 0;
}

Upvotes: 0

Views: 39

Answers (1)

Jiho Lee
Jiho Lee

Reputation: 987

Reason: Because C uses call-by-value

When you call function CalculateMpg(), idk is copied to argument variable by value.

In this function, only copied argument is edited.

You should make function uses pointer and call it to CalculateMpg(&idk)

Sample Solution:

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

struct gas {
 float distance;
 float gals;
 float mpg;
};

float CalculateMpg(struct gas* idk){
    idk->mpg = idk->distance / idk->gals; // arrow is just syntactic sugar of (*idk).field
    printf("%f \n", idk->mpg);
    return idk->mpg;
}

int main()
{
    struct gas idk;

    printf("Enter distance (in miles): ");
    scanf("%f", &idk.distance);
    printf("Enter gallons fueled: ");
    scanf("%f", &idk.gals);

    CalculateMpg(&idk);
    printf("%f \n", idk.mpg);
    printf("Miles driven gallons used with %.2f miles and %.2f gallons: %.2f mpg \n", idk.distance, idk.gals, idk.mpg);

    return 0;
}

Upvotes: 1

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