average.c

#include <stdio.h>
#include <limits.h>

// Important note: When reading input, something like 123abc is considered
// a valid value, and will be read as 123. The non-numeric input will not be
// detected immediately.
// This is because of how the scanf function works, and can't be solved with
// the current limitations (unless we read as a string and attempt to convert
// it to a number manually, either through strtol or with our own function
// (the atoi function can't be used, it's prone to the same problem)).

// Reads until newline (or end of file), discarding all it reads
void skip_line(void)
{
    int c;
    while ((c = getchar()) != '\n' && c != EOF)
    {
    }
}

// Get the expected number of values to read
int get_expected(void)
{
    while (1)
    {
        printf("Please input the number of values to read: ");
        fflush(stdout);  // Make sure the output is flushed and written to the console

        int expected;
        int result = scanf("%d", &expected);

        if (result == EOF)
        {
            return -1;
        }
        else if (result == 1 && expected >= 0)
        {
            // Got a valid number
            return expected;
        }
        else
        {
            // Either not a valid input, or a negative value

            skip_line();  // Skip everything until the end of the line (or end of input)
    
            printf("Not a valid number, it must be a positive number.\n");
        }
    }
}

// Get a single value
int get_value(void)
{
    while (1)
    {
        printf("Enter a value: ");
        fflush(stdout);  // Make sure the output is flushed and written to the console
        
        int value;
        int result = scanf("%d", &value);
        
        if (result == EOF)
        {
            return INT_MIN;  // Lets hope the user doesn't input the minimum integer value
        }
        else if (result == 1)
        {
            // Got a valid value
            return value;
        }
        else
        {
            // Not a valid value

            skip_line();  // Skip everything until the end of the line (or end of input)
    
            printf("Not a valid value, please try again.\n");
        }
    }
}

int main(void)
{
    int expected = get_expected();
    
    if (expected <= 0)
    {
        printf("No expected values, exiting\n");
        return 0;
    }

    double sum = 0;
    int value = 0;
    int counter;
    
    for (counter = 0; counter < expected && value != INT_MIN; ++counter)
    {
        value = get_value();
        if (value != INT_MIN)
        {
            sum += value;
        }
    }
    
    double average = sum / counter;
    if (value == INT_MIN)
    {
        printf("Premature end of values, average = %f\n", average);
    }
    else
    {
        printf("Average = %.02f\n", average);
    }

    return 0;
}

With severe limitations on the functions that can be called, shows a way to read numbers and calculate an average of that.