Proteincalculator with algebra

Question

Im trying to make this protein-calculator (in C language) to work but it doesn't work:

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

int main()
{
    double proteinhalt[20];
    double proteinmangdtot[20];
    double kottfiskmengdtot;
    printf("Enter proteinhalt / 100 gram: ");
    fgets(proteinhalt, 20, stdin);
    printf("Enter proteinmangd att konsumera idag (gram): ");
    fgets(proteinmangdtot, 20, stdin);
    kottfiskmengdtot = ((double)proteinmangdtot/(double)proteinhalt)*100;
    printf("Du behöver %f gram.", kottfiskmengdtot);
}

The error is:

Line 13   error: pointer value used where a floating-point was expected

What is wrong?

Edit: In english:

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

int main()
{
    double proteinpercentage[20];
    double proteinamounttot[20];
    double meatfishamounttot;
    printf("Enter proteinpercentage / 100 gram: ");
    fgets(proteinpercentage, 20, stdin);
    printf("Enter protein amount to consume today (gram): ");
    fgets(proteinamounttot, 20, stdin);
    meatfishamounttot = ((double)proteinamounttot/(double)proteinpercentage)*100;
    printf("You need %f gram.", meatfishamounttot);
}

Answer 1

You have two errors.

1. fgets, expects a character array as argument, but you specify an array of doubles

2. at the assignment of meatfishamounttot, you cast an array of doubles to a double

Solution (simplified, no error checking):

char str[20];
double proteinpercentage;
double proteinamounttot;
double meatfishamounttot;

// read at most 20 characters from stdin
fgets(str, 20, stdin);
// convert the string to a double
proteinpercentage = strtod(str, NULL);

fgets(str, 20, stdin);
proteinamounttot = strtod(str, NULL);

meatfishamounttot = (proteinamounttot / proteinpercentage) * 100;