Program for 1-D Statistics - c

Question

So I'm having some issues with a program I'm writing to calculate statistics of an arbitrary amount of numbers. As you probably know, C does not have a native way of growing an array based on user input, so I made one. I'm not too familiar with pointers as I have not learned them yet. However, the main issue I'm having is that my program will not allow me to input anymore than 16 numbers. Any ideas on why?
#include <stdio.h>
#include <math.h>
float getMin(float arr[], int size)
{
int i;
float minimum = arr[0];
for (i=1; i < size; i++) {
if (arr[i] < minimum) {
minimum = arr[i];
}
}
return(minimum);
}
float getMax(float arr[], int size)
{
int i;
float maximum;
for(i=0; i<size; i++) {
if (arr[i] > maximum) {
maximum = arr[i];
}
}
return(maximum);
}
float getAverage(float arr[],int size)
{
int i;
float avg, sum;
for(i=0; i<size; i++) {
sum += arr[i];
}
avg = sum/size;
return avg;
}
float getVar(float arr[], int size, float average)
{
int i;
float var, diff[size], sq_diff[size], sum;
for(i=0; i<size; i++) {
diff[i] = arr[i] - average;
}
for(i=0; i<size; i++)
{
sq_diff[i] = pow(diff[i], 2);
}
var=getAverage(sq_diff, size);
return(var);
}
float getStdDev(float var)
{
float std_dev = sqrt(var);
return(std_dev);
}
int main(void)
{
int n=0, i=0;
float array[n], x;
printf("This program computes 1-D statistics.");
printf("\nEnter a negative number to stop input.\n");
do {
printf("Number %d: ", i+1);
scanf("%f", &x);
n++;
array[i] = x;
i++;
}while(x >= 0);
float avg = getAverage(array, n-1);
float var = getVar(array, n-1, avg);
printf("\nMinimum: %f", getMin(array, n-1));
printf("\nMaximum: %f", getMax(array, n));
printf("\nAverage: %f", getAverage(array, n-1));
printf("\nVariance: %f", getVar(array, n-1, avg));
printf("\nStandard Deviation: %f", getStdDev(var));
return(0);
}

Answer 1

I am afraid you have some general miss-understanding of how C and programming languages work in general. Programming is not exactly the same as math. If you declare a variable n and an array myarr[n], your array won't grow as n grows. The array will take the value n had at the point you declared it and will use that. Also declaring arrays like that is probably not a good idea, I don't think many C compilers will compile what one would expect. In your case you are declaring an array of size 0, the fact that it seems to work up to 16 numbers is completely system dependent. You can not expect this kind of behavior to be consistent over different systems/compilers.
Instead what you should do if you really need this is use dynamic memory. I have written a simple example really fast you could use. This is by no means a complete but for your needs it should be good enough. Use the initArr(&arr) whenever you create a new growing array. If for example you have an array of type MyArr with name arr you can get the data of the array like this arr.data[i] while you can get the length of the array (the meaningful data) like this arr.len. Finally you can get the current max size of your array like this arr.size. Please not that you should only add new elements using addElement(&arr, x) as this function will take care of the array growth. When you are done using your array you can use destrArr(&arr) to free the memory. This is good practice even though when your program exits the memory is freed anyway.
Finally please review your functions. I haven't been through your logic but remember that you should never exceed boundaries. For example you should not try to access array[-1] or array[size_of_array] where size_of_array is the total size of your array. Remember that an array declared as float arr[100] can only safely be used in the region arr[0] to arr[99]. Also doing sum += x is equivalent to doing sum = sum + x. Consider what the initial value of sum is if you do not initialize it. In fact it depends but could contain garbage values.
Try looking at your compiler warnings and fix them. It is worth understanding them and are rarely worth neglecting. Try the -Wall flag if you are using gcc, it might give you some more warnings worth looking at.
I hope this was helpful and not too confusing. In any case have a look at the following code and add your functions and libraries back. This should work out of the box.
Good Luck! :)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define INIT_SIZE 2
typedef struct MyArr {
float *data;
int size;
int len;
} MyArr;
int initArr(MyArr *arr) {
arr->data = (float *)malloc(INIT_SIZE*sizeof(float));
if (arr->data == NULL) {
printf("OUT OF MEMORY\n");
return -1;
}
arr->size = INIT_SIZE;
arr->len = 0;
return 1;
}
int addElement(MyArr *arr, float x) {
if (arr->len == arr->size) {
int i;
float *tmp = (float *)malloc(2*arr->size);
if (tmp == NULL) {
printf("OUT OF MEMORY\n");
return -1;
}
for (i = 0; i < arr->size; ++i) {
tmp[i] = arr->data[i];
}
free(arr->data);
arr->data = tmp;
arr->size = 2*arr->size;
}
arr->data[arr->len] = x;
arr->len++;
return 1;
}
void destrArr(MyArr *arr) {
free(arr->data);
arr->size = 0;
arr->len = 0;
}
int main(void)
{
int n=0, i=0;
float x;
MyArr array;
initArr(&array);
printf("This program computes 1-D statistics.");
printf("\nEnter a negative number to stop input.\n");
do {
printf("Number %d: ", i+1);
scanf("%f", &x);
n++;
addElement(&array, x);
i++;
}while(x >= 0);
float avg = getAverage(array.data, n-1);
float var = getVar(array.data, n-1, avg);
printf("\nMinimum: %f", getMin(array.data, n-1));
printf("\nMaximum: %f", getMax(array.data, n));
printf("\nAverage: %f", getAverage(array.data, n-1));
printf("\nVariance: %f", getVar(array.data, n-1, avg));
printf("\nStandard Deviation: %f", getStdDev(var));
destrArr(&array);
return(0);
}