getting inf when calculating standard devation in c

Question

Attempting to write a program in c that intakes in an amount of numbers. ie 1, 2, 3, 4, and returns size, max, min, average and standard deviation.

While trying to calculate standard deviation when calculating sumx ( line 51) it begins returng inf. not sure why I would be getting inf with an addition function.

I have all the print methods to try identify where it breaks.

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

int main(void) {

    setvbuf(stdout, NULL, _IONBF, 0); // Turn off output buffering. Required for automated testing.     return EXIT_SUCCESS;

    int temp;
    float grade, alteredGrade,standardD, sdPartOne, sdPartTwo, sdPartThree, sum;
    float sum;
    float sumx = 0.0;
    int counter = 0;
    int max;
    int min;
    float average;
    char buffer[BUFSIZ+1];

    printf("Enter scores, one per line.  Press <ENTER> on a blank line to end.\n"); // NOTE: The \n is required for automated testing.

    do {
        grade = atoi(fgets(buffer, BUFSIZ, stdin));

        //sets max and min to grade
        max = grade;
        min = grade;

        //lets loop iterate once to establish initial comparable value - breaks
        if(counter > 0){
            temp = grade;
            if(max > temp){
                min = grade;
            }

            else{
                max = grade;
            }
        }
        //average
        sum = grade + sum;
        average = (float)(sum/counter);
        counter++;
        //standard deviation
        sdPartOne = (grade - average);
        sdPartTwo = sdPartOne * sdPartOne;
        //breaks
        sumx = sumx + sdPartTwo;
        sdPartThree = sumx / counter;
        standardD = sqrt(sdPartThree);

    }

    while(buffer[0] != '\n'); 

    printf("\t%f", sdPartOne);
    printf("\t%f", sdPartTwo);
    printf("\t%f", sumx);
    printf("\t%f", sdPartThree);
    printf ("\n%d", counter - 1);
    printf("\t%d", max);
    printf("\t%d", min);
    printf("\t%f", average);
    printf("\t%f", standardD);
}

input 1, 2, 3, 4 returns -2.500000 6.250000 inf inf 4 0 0 2.500000 inf — asdf, Feb 14 '18 at 04:26
Your loop is flawed. If the user inputs an empty line (the loop ending condition) then `grade` will be equal to zero and you will use that value as well instead of breaking out of the loop. — Some programmer dude, Feb 14 '18 at 04:27
`average = (float)(sum / counter)` happens when `counter == 0` — Ry-, Feb 14 '18 at 04:27

score 2 · Answer 1 · answered Feb 14 '18 at 06:22

You initialize

 int counter = 0;

and in your first loop do

 average = (float)(sum/counter);    // In the first loop count is zero, i.e. divide by zero
 counter++;

so you divide by zero which leads to inf

Just switch the lines like:

 counter++;
 average = (float)(sum/counter);

getting inf when calculating standard devation in c

1 Answers1