Issues finding average of elements - c

Question

Working with Repl.it and trying to use a function in C to average the elements in a variable length array. My program works fine in every other i/o area other than the average which returns:
The average for that day is: -nan. Any insight on what the issue may be?
The goal is to receive user input as a double(for example, how many pints of blood were taken per hour over a 7 hr period and then use a function call to calculate the average amount for that seven hour period.
New code is as follows:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
double average(int size, float ary[*]);
int main(void)
{
char dayOne[8], dayTwo[8];
int size;
float ave;
printf("Over how many hours do you want to view donation amounts?: ");
scanf("%d", &size);
if (size < 7 || size > 7)
size = 7;
printf("Enter day that you want to see average donation amount for: ");
scanf("%s", dayOne);
{
float ary[size];
for (int i = 0; i < size; i++)
{
printf("\nEnter amount taken in hour %d:", i + 1);
scanf("%f", &ary[i]);
}
ave = average(size, ary);
printf("\nThe average donated for %s is: %2.1f", dayOne, ave);
}
printf("\n\nEnter day that you want to see average donation amount for: ");
scanf("%s", dayTwo);
if(strcmp(dayOne, dayTwo)== 0)
printf("\nEnter a different day please: ");
scanf("%s", dayTwo);
{
float ary[size];
for (int i = 0; i < size; i++)
{
printf("\nEnter amount taken in hour %d:", i + 1);
scanf("%f", &ary[i]);
}
ave = average(size, ary);
printf("\nThe average donated for %s is: %2.1f", dayTwo, ave);
}
return 0;
}
double average(int size, float ary[])
{
double sum = 0;
double ave;
for (int i = 0; i < size; i++)
sum += ary[i];
ave = (double)sum / size;
return ave;
}

Answer 1

This is wrong:
int ary[7];
…
scanf("%f", &ary[i]);
%f is for scanning a float, but ary[i] is an int. The resulting behavior is not defined.
This is wrong:
double size;
…
ave = average(size, ary);
Nothing in the “…” assigns a value to size, so it has no defined value when average is called.

Answer 2

Quite a few syntactical and logical changes to be made buddy. As Eric Postpischil has mentioned in his answer, your size variable isn't initialized to anything before use. And using %f for accepting integers is incorrect too. Along with these, there are a few other glitches which I will list below, with the corresponding fixes. I have also attached the full working code along with the output.
Error: size variable not initialized.
Fix:: Since you have a fixed number of hours, you can either use the 7 directly or through size. Hence initialize the variable while declaring it. And just a suggestion buddy, use int for size. ==> int size = 7;
Error: Accepting input for array values. The array ary is of type int, but you have used %f which is a format specifier for float.
Fix: Use %d which is the format specifier for int data type. ==> scanf("%d", &ary[i]);
Error: Format specifier used for printing the average value. You have used %f again which is for float while the variable ave for average if of type double.
Fix: Use %lf which is the format specifier for double. ==> printf("\nThe average for the 2nd day is: %.3lf", ave); The .3 before lf is just to limit the number of decimal points to 3. It is optional.
Error: You accept the second day and display a message if it is the same as the first day, but the scanf for this is out of the if loop. hence it prompts the user to input another name of the day regardless of whether the second input day is same as the first or not.
Fix: just move that scanf into the if loop where you check if that day is same as the previous one.
I wouldn't really call this one an error, because it's just an improvement which I find that needs to be done. You accept the second day which should be different from the first with the message that average would be found. But you are not calculating the average for this day. Hence I have included that part in the code.
CODE:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
double average(int, int [7]);
int main(void)
{
char dayOne[8], dayTwo[8];
int size = 7;
double ave;
printf("Enter day that you want to see average donation amount for: ");
scanf("%s", dayOne);
int ary[7];
for (int i = 0; i < 7; i++)
{
printf("Enter amount taken in hour %d:", i + 1);
scanf("%d", &ary[i]);
}
ave = average(size, ary);
printf("\nThe average for the 1st day is: %.3lf", ave);
printf("\n\nEnter day that you want to see average donation amount for:");
scanf("%s", dayTwo);
if(strcmp(dayOne, dayTwo)== 0) {
printf("\nEnter a different day please: ");
scanf("%s", dayTwo);
}
for (int i = 0; i < 7; i++)
{
printf("Enter amount taken in hour %d:", i + 1);
scanf("%d", &ary[i]);
}
ave = average(size, ary);
printf("\nThe average for the 2nd day is: %.3lf", ave);
return 0;
}
double average(int size, int ary[7])
{
int sum = 0;
double ave;
for (int i = 0; i < size; i++) {
sum = sum + ary[i];
}
ave = sum / size;
return ave;
}
OUTPUT:
Enter day that you want to see average donation amount for: Day1
Enter amount taken in hour 1: 1
Enter amount taken in hour 2: 2
Enter amount taken in hour 3: 3
Enter amount taken in hour 4: 4
Enter amount taken in hour 5: 5
Enter amount taken in hour 6: 6
Enter amount taken in hour 7: 7
The average for the 1st day is: 4.000
Enter day that you want to see average donation amount for: Day2
Enter amount taken in hour 1: 8
Enter amount taken in hour 2: 8
Enter amount taken in hour 3: 8
Enter amount taken in hour 4: 8
Enter amount taken in hour 5: 8
Enter amount taken in hour 6: 8
Enter amount taken in hour 7: 8
The average for the 2nd day is: 8.000
Hope this helps.

Answer 3

double size;
The size variable has not been set before calling the average function.
ave = average(size, ary);
May be size can be initialized to 0 and incremented in the for loop as given below
double size = 0;
double ave;
printf("Enter day that you want to see average donation amount for: ");
scanf("%s", dayOne);
{
int ary[7];
for (int i = 0; i < 7; i++)
{
printf("Enter amount taken in hour %d:", i + 1);
scanf("%f", &ary[i]);
size++;
}
ave = average(size, ary);
printf("\nThe average for the day is: %f", ave);
}
If the goal is to receive user input as a double, the array used to hold those values should be double.
double ary[7];
instead of
int ary[7];