为什么定义指令在这里不起作用？

Question

This is the code I'm trying to run and this is just taking one input and giving me the standard deviation result. Also the 'SIZE' is highlighted as red in GNU nano. I wanna know why, so please help.

This is the code-

#include<stdio.h>
#include<math.h> 
#define SIZE 5  
float std_dev(float a[], int n);
float mean(float a[], int n);

int main(){
    float value[SIZE];
    int i;
    printf("Enter float values\n",SIZE);
    for(i=0; i<SIZE; i++){
        scanf("%f",&value[i]);
        printf("Std.deviation is %f\n", std_dev(value, SIZE));
    }
}

float std_dev(float a[], int n){
    int i;
    float x, sum=0.0;
    x=mean(a,n);
    for(i=0; i<n; i++){
        sum+=(x-a[i])*(x-a[i]);
        return(sqrt(sum/(float)n));
    }
}

float mean(float a[], int n){
    int i;
    float sum=0.0;
    for(i=0; i<n; i++){
        sum = sum + a[i];
        return(sum/(float)n);
    }
}

Commands I gave for the output are-

gcc StandardDeviation.c -o compilfile -lm
./compilfile

One more thing, I'm using Kali Linux.

I'm trying to get the result as the standard deviation after giving the five float values.

Answer 1

Edit: I did the basic mistake here that I was using return statement inside the loop. It should be something like this-

float std_dev(float a[], int n){
int i;
float x, sum=0.0;
x=mean(a,n);
for(i=0; i<n; i++){
sum+=(x-a[i])*(x-a[i]);
}
return(sqrt(sum/(float)n));
}

-in every function I created here. Thank you for this solution @axiac. Few more points-

1. I am using Visual Studio Code now.

2. I am learning how to format the code and ask question properly in this platform.

3. I implemented all the suggestions you guys gave me. Thank you!