/********
This program reads a series of random numbers from a file and calculates and 
prints the mean and variance of the numbers. The program uses three functions. 

The first one prompts the user for a file name and then reads the numbers from 
the file. It expects the first number in the file to be the number of data in 
the file. It checks for errors and returns to the main program if there was a 
problem. Note the use of pointers for arguments to this function.

The second function calculates, prints and returns the average of the numbers.

The third calculates, prints and returns the variance of the numbers.

Written by Erik Cheever, 1/94

Update by Bruce maxwell, 1/00
********/

#include <stdio.h>

#define MaxNum 1000        // Number of random numbers that can be handled
#define ERROR -1           // Constants used to signify errors
#define NO_ERROR 0

/* 
  This program reads a file of numbers and returns them in an array.
*/
int ReadFile(int *nums, int *maxNums) {
    char FileName[80];            // Note that since maxNum is passed as a pointer we need to use 
    FILE *InFile;                 // the "*" operator to access the actual value of the variable. 
    int i;

    /* Get name of file */
    printf("Enter file name: ");
    scanf("%s",FileName);

    /* Open the file */
    InFile = fopen(FileName,"r"); 
    if (InFile == NULL) { /*If error, return and report*/
        printf("\nCould not open file '%s'.\n",FileName);
        return (ERROR);
    }

    /* 
      Note since maxNums is a pointer that we don't need to use the
      "&" operator in the fscanf statement.  
    */
    fscanf(InFile,"%d",maxNums);

    /* 
      Note that since maxNums is a pointer that we need to use the "*" operator when
      we want to use the value stored where the pointer is pointing. 
    */
    if (*maxNums > MaxNum) {    //Check if too many to be held in array
        printf("\nToo much data in file (%d data).\n",maxNums);
        fclose(InFile);
        return (ERROR);
    }
 
    /* Read data into array and return */
    for (i=0;i<*maxNums;i++) { 
        fscanf(InFile,"%d",&(nums[i]));
    }

    fclose(InFile);

    return(NO_ERROR);
}

/* 
   This function calculates and prints the average of 
   an array of numbers. It also returns the average 
   value back to the main program.
*/
float CalcPrintMean(int *nums, int maxNums) {
    int i; 
    float avg;                                   /* Use a float for the average */

    avg = 0.0;
    for (i=0;i<maxNums;i++) {                    /* Calculate average... */
        avg = avg + nums[i];
    }
    avg = avg/maxNums;

    printf("Average = %7.2f.\n",avg);            /* ...print it... */

    return(avg);                                 /* ...and return it. */
}

/* 
    This function calculate and prints the variance 
    an array of numbers. It also returns the variance
    back to the calling program.
*/
float CalcPrintVar(int *nums, int maxNums, float avg) {
    int i;
    float SumSqr, temp, var;

    SumSqr = 0.0;                                /* Calculate variance... */
    for (i=0;i<maxNums;i++) {
        temp = nums[i] - avg;
        SumSqr = SumSqr + temp * temp;
    }
    var = SumSqr/maxNums;
    printf("Variance = %7.2f.\n",var);           /* ...print it... */

    return(var);                                 /* ...and return it. */
}

main() {
    int randNums[MaxNum], maxNums;
    float avg, var;

    printf("This program will read a file written by RandomWrite and\n");
    printf("calculate the mean and variance of the numbers in the file.\n");
    
    /* 
       Note that in the call to the function ReadFile that both arguments RandNums and 
       maxnums are passed as pointers because we want their values to change in the 
       function. See the function above to see how it is handled there.
    */
    if(ReadFile(randNums, &maxNums)==NO_ERROR) {     /* If no error proceed...*/
        avg = CalcPrintMean(randNums, maxNums);
        var = CalcPrintVar(randNums, maxNums, avg);
    }
    else {                                           /*...else report error*/
        printf("Program ended because of error.\n");
    } 
}