Calculate Standard Deviation in C

Standard deviation is used to measure deviation of data from its mean. The mathematical formula to calculate the standard deviation is as follows −

$$s=\sqrt{Variance}$$

where

Variance$$=\frac{1}{n}\:\:\displaystyle\sum\limits_{i=1}^n (x_{i}-m)^{2}$$

and

$$m=mean=\frac{1}{n}\:\displaystyle\sum\limits_{i=1}^n x_{i}$$

Algorithm

Refer an algorithm given below to calculate the standard deviation for the given numbers.

Step 1 − Read n items.

Step 2 − Calculate sum and mean of the items.

Step 3 − Calculate variance.

Step 4 − Calculate standard deviation.

The logic used in the program for calculating standard deviation is as follows −

for (i = 1 ; i<= n; i++){
   deviation = value[i] - mean;
   sumsqr += deviation * deviation;
}
variance = sumsqr/(float)n ;
stddeviation = sqrt(variance) ;

Example

Following is the C program to calculate the standard deviation for the given numbers −

#include <math.h>
#define MAXSIZE 100
main( ) {
   int i,n;
   float value [MAXSIZE], deviation,
   sum,sumsqr,mean,variance,stddeviation;
   sum = sumsqr = n = 0 ;
   printf("Input values: input -1 to end 
");
   for (i=1; i< MAXSIZE ; i++) {
      scanf("%f", &value[i]);
      if (value[i] == -1)
      break;
      sum += value[i];
      n += 1;
   }
   mean = sum/(float)n;
   for (i = 1 ; i<= n; i++) {
      deviation = value[i] - mean;
      sumsqr += deviation * deviation;
   }
   variance = sumsqr/(float)n ;
   stddeviation = sqrt(variance) ;
   printf("
Number of items : %d
",n);
   printf("Mean : %f
", mean);
   printf("Standard deviation : %f
", stddeviation);
}

Output

When the above program is executed, it produces the following output −

Input values: input -1 to end
2 4 6 8 12 4.5 6.7 0.3 2.4 -1
Number of items: 9
Mean: 5.100000
Standard deviation: 3.348300