Text file with C in terminal

Question

So I calculated the mean and standard deviation from double values read within a file.

My file data has 1 number per line: My data in the file is the following

1

2

3

4

5

6

7

8

9

10

My code is below:

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

int main(){

    FILE *inputfile;
    char name[100];

    printf("Enter the file you want to use to calculate standard deviation:\n ");
    gets(name); 
    inputfile = fopen(name, "r");

    if (inputfile == NULL)
    {
        printf("Failed to open text file.\n");
        exit(1);
    }

    double i; 
    double j=1;
    double average; 
    double stdish=0;
    double stdreal=0; 
    double x=0;
    double sum=0;
    double stdfinal;

    while(fscanf(inputfile, "%lf", &i) != EOF){
        x=x+1;
        sum = sum + i;
        j =pow(i,2);
        stdreal +=j;
    }
        average = sum/x;
        stdish = (stdreal/x)-(pow(average,2));
        stdfinal = sqrt(stdish);

    printf("The average is %.4lf\n", average);
    printf("The standard deviation is %.4lf\n", stdfinal);


return 0;
}

My standard deviation is incorrect, and I am not sure why. In my program, I use fopen to get the text from the input file. Also, I am trying to make it so that I input the text file from the terminal instead of in the actual program itself. How to do that?

Answer 1

First you need to find out the average (mean) and then iterate through the loop to find out the variance. The SQRT(variance) will give you standard deviation.

double CalculateMean() 
{ 
    double sum = 0;  
    for(int i = 0; i < max; i++) 
        sum += value[i]; 
    return (sum / max); 
} 

double CalculateVariane() 
{ 
    mean = CalculateMean(); 
    double temp = 0;  

    for(int i = 0; i < max; i++) 
    { 
         temp += (value[i] - mean) * (value[i] - mean) ; 
    } 
    return temp / max; 
} 

Reference: http://www.softwareandfinance.com/CPP/MeanVarianceStdDevi.html

Answer 2

Declare main as int main(int argc, char *argv[]) or as int main(int argc, char **argv) . They mean the same thing. In this version, argc is the number of command line arguments (+1 because the program name is argument 0). argv is an array of char * s, each of which is a command line argument.

You can do something like this:

int main(int argc, char *argv[])
{

    FILE *inputfile;
    char name[100];

    if(argc == 1) {
        printf("Enter the file you want to use to calculate standard deviation:\n ");
        gets(name);
    } else {
        strcpy(name, argv[1]);
    }
    inputfile = fopen(name, "r");

    if (inputfile == NULL)
    {
        printf("Failed to open text file.\n");
        exit(1);
    }

    double i; 
    double j=1;
    double average; 
    double stdish=0;
    double stdreal=0; 
    double x=0;
    double sum=0;
    double stdfinal;

    while(fscanf(inputfile, "%lf", &i) != EOF){
        x=x+1;
        sum = sum + i;
        j =pow(i,2);
        stdreal +=j;
    }
    average = sum/x;
    stdish = (stdreal/x)-(pow(average,2));
    stdfinal = sqrt(stdish);

    printf("The average is %.4lf\n", average);
    printf("The standard deviation is %.4lf\n", stdfinal);


    return 0;

}