Calculate the mean of an array of n numbers in C

Question

I'm new to C and I'm trying to master the basics so I've written a program which gives you the mean of tree numbers and it works, but when I try to write a more general version of it, as output I receive: Insert a series of numbers: "The mean of the numbers is -nan". Which means that I can not even insert the numbers. I don't now how to write the scanf part in a better way.

Here's the code:

#include <stdio.h>

int main() {

    int p;
    double n[p];
    int i;
    double sum = 0;
    double m;

    printf("Insert a series of numbers:\n");

    for(int r = 0; r < p; r++) {
        scanf("%lf", &n[r]);
    }

    for(i = 0; i < p; i++) {
        sum += n[i];
    }

    m = sum / p;
    printf("The mean of the numbers is %lf", m);

    return 0;
}

Undefined behaviour. `p` is uninitialized, so defining `double n[p];` is undefined. — P.P, Apr 18 '20 at 16:43
There are a few issues here. To start with you haven't defined `p`! How does the `for` loop know when to stop? When you declare variable `int p` it has a random value. Make sure you initialise your variables like `int p=10'` — Rohan, Apr 18 '20 at 16:45
Can I even make the user decide the value of p? Maybe by putting another printf and scanf before the first for — Schiele, Apr 18 '20 at 16:47
@Schiele Yes, then is `n` what is called a VLA or variable length array -> https://en.wikipedia.org/wiki/Variable-length_array. — RobertS supports Monica Cellio, Apr 18 '20 at 17:06

RobertS supports Monica Cellio · Accepted Answer · 2020-04-18T17:45:21.627

3

int p;

You forgot to initialize p. Since p is an object of the automatic storage class, it has an indeterminate value.

Thus,

double n[p];
....

for(int r = 0; r < p; r++){
    scanf("%lf", &n[r]);
}

for(i = 0; i < p; i++){
    sum += n[i];
}
m = sum / p;

invokes undefined behavior.

Initialize p like f.e.

int p = 5;

Also don't forget to check the return value of scanf() if all items were assigned successfully, like for example:

if(scanf("%lf",&n[r]) != 1) 
{
     fprintf(stderr,"Error at scanning!");
     return 1;
}

edited Apr 18 '20 at 17:45

answered Apr 18 '20 at 16:48

RobertS supports Monica Cellio

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I've another doubt, despite having initizialized I get the following warning: 'p' is used uninitialized is this function, referring to double n[p]. Why? – Schiele Apr 18 '20 at 16:56
@Schiele Is `int p = N;` before `double n[p];` in your compile code (as is it in the example provided in the question)? – RobertS supports Monica Cellio Apr 18 '20 at 17:02
Yes, it is as int he example in the question. – Schiele Apr 18 '20 at 17:06
@Schiele Hmm, what did you changed exactly in comparison to the provided code in the question? How do you initialize `p`? – RobertS supports Monica Cellio Apr 18 '20 at 17:08
By letting the user decide the value of p, so I've inserted `printf("Insert a value for p:\n"); scanf("%d", &p);` before `printf("Insert a series of numbers");` – Schiele Apr 18 '20 at 17:12
@Schiele Well, in this case `double n[p];` needs to be placed between `printf("Insert a value for p:\n"); scanf("%d", &p);` and `printf("Insert a series of numbers");` to make the assignment effect `n`. – RobertS supports Monica Cellio Apr 18 '20 at 17:14
Oh okay, I didn't know, thank you but, just for asking, why? – Schiele Apr 18 '20 at 17:15
@Schiele The compiler reads the file from top to bottom. It encounters `double n[p];` before the user entered a value and this value is assigned to `p`. Note, you should better initialize `p` with at least `0` at its declaration -> `int p = 0;` for security reasons. If the scan fails it still got a value and you do not have undefined behavior. – RobertS supports Monica Cellio Apr 18 '20 at 17:17
Thank you again for your help! I've understood everything. – Schiele Apr 18 '20 at 17:24
1

@Schiele I've added something important to the answer - Always check the return of `scanf()`. Don't forget to check it out. – RobertS supports Monica Cellio Apr 18 '20 at 17:40

Calculate the mean of an array of n numbers in C

1 Answers1