Sum, Mean and Standard Deviation Using Pointers in C

Algorithm

1. Read n and the n real numbers into an array.
2. Pass the array to a function as a pointer together with n, and two output pointers for the sum and the mean.
3. Inside the function, walk the array with pointer arithmetic to compute the sum and the mean.
4. Walk the array again to add up the squared differences from the mean.
5. Standard deviation = sqrt(sum of squared differences / n); print sum, mean and standard deviation.

C program

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

#define MAX 100

void statistics(const double *arr, int n, double *sum, double *mean, double *std_dev) {
    const double *p;
    double total = 0.0;
    double squared = 0.0;
    double diff;

    for (p = arr; p < arr + n; p++) {
        total += *p;
    }
    *sum = total;
    *mean = total / n;

    for (p = arr; p < arr + n; p++) {
        diff = *p - *mean;
        squared += diff * diff;
    }
    *std_dev = sqrt(squared / n);
}

int main(void) {
    double arr[MAX];
    double sum, mean, std_dev;
    int n, i;

    printf("Enter number of values (1-%d):\n", MAX);
    if (scanf("%d", &n) != 1 || n < 1 || n > MAX) {
        printf("Error: invalid count\n");
        return 1;
    }

    printf("Enter %d real numbers:\n", n);
    for (i = 0; i < n; i++) {
        scanf("%lf", &arr[i]);
    }

    statistics(arr, n, &sum, &mean, &std_dev);
    printf("Sum               : %.2f\n", sum);
    printf("Mean              : %.2f\n", mean);
    printf("Standard deviation: %.2f\n", std_dev);
    return 0;
}

Sample input and output

8
2 4 4 4 5 5 7 9

Enter number of values (1-100):
Enter 8 real numbers:
Sum               : 40.00
Mean              : 5.00
Standard deviation: 2.00

1
3.5

Enter number of values (1-100):
Enter 1 real numbers:
Sum               : 3.50
Mean              : 3.50
Standard deviation: 0.00

How the program works

The statistics function never uses array indexing; p starts at arr and advances one element at a time until it reaches arr + n. Since p is a pointer to double, p++ moves to the next element automatically.

The standard deviation here is the population form, dividing by n. A sample standard deviation would divide by n - 1 instead, which the lab statement does not ask for.

Viva questions

Common mistakes

• Dividing the squared differences by n when the sample form was expected, or the other way round, without stating which.
• Passing the sum and mean by value, so the caller never sees them.
• Walking past the end of the array because the loop bound uses <= instead of <.