Function is returning garbage value - c

Question

I have the following code in C :
#include <stdio.h>
// Global variables :
int i;
int x[10];
int Nmax;
int sum;
// Functions used :
void Summation();
int Average(float avg); // This function will return avg
int main()
{
float avg;
printf("Mention how many numbers to be added\n");
scanf("%d",&Nmax);
printf("Enter %d numbers\n",Nmax);
for (i=0; i<Nmax; i++){
scanf("%d",&x[i]);
}
Summation();
Average(avg);
printf("%d %d %f\n",Nmax,sum,avg);
printf("Average = %.2f\n",avg);
return 0;
}
// Summation :
void Summation()
{
sum=0;
for (i=0; i<Nmax; i++) {
sum=sum+x[i];
}
printf("Sum of them = %d\n",sum);
}
// Average
int Average(float avg)
{
avg=(float)sum/(float)Nmax;
return avg;
}
Somehow the function Average is not returning the expected average value.
Instead it's showing a garbage value.
A typical input/output :
Mention how many numbers to be added
4
Enter 4 numbers
1 2 3 4
Sum of them = 10
4 10 1637156136366093893632.000000
Average = 1637156136366093893632.00
What's going wrong here? Hope I don't require a pointer here or do I?

Answer 1

For the average function you are passing the argument as pass by value. Once you call the function your are not assigning the return value back to the avg variable inside the main function. So what you are printing is the garbage value inside the avg variable declared in main function. Change your code to
avg = Average(avg);
Also this is a very bad way of writing c programs. Please dont you use global variables if you dont need them.

Answer 2

You are mixing int and float and that can cause problems.
You are never initializing avg in main(), use parameters instead of global variables to avoid the confusion. You pass avg's value to Average() it has absolutely no use in this case.
In c parameters to functions are always passed by value so the value of avg is not being modified in Average(), you may think it is because you seem confused about how you can modify global variables everywhere. The truth is global variables are mean, of course they have their uses for example, a global state in a library could be stored in a global variable1 but generally they are not needed.
Try like this
#include <stdio.h>
// Functions used :
float Summation(float *data, int size);
float Average(float *data, int size); // This function will return avg
int
main(void)
{
int size;
printf("Mention how many numbers to be added\n");
if (scanf("%d", &size) == 1)
{
float data[size];
float sum;
float average;
int i;
printf("Enter %d numbers\n", size);
for (i = 0 ; ((i < size) && (scanf("%f", &data[i]) == 1)) ; i++)
;
size = i;
sum = Summation(data, size);
average = Average(data, size);
printf("%d %f %f\n", size, sum, average);
printf("Average = %.2f\n", average);
}
return 0;
}
// Summation :
float
Summation(float *data, int size)
{
float sum;
sum = 0;
for (int i = 0 ; i < size ; i++)
sum = sum + data[i];
return sum;
}
// Average
float
Average(float *data, int size)
{
return Summation(data, size) / (float) size;
}
Note that you "don't really need an Average() function.", although it could be useful to make the API clean.
1Although I don't like that either, it makes the library very limited in multithreaded environments. Still sometimes it makes senes, but you can always have a structure with all the needed data and pass it in every call to the library API. So global variables are very rarely good.