C program for stddev/mean - c

Question

#include<stdio.h>
#include<stdlib.h>
#include<math.h>
#include<string.h>
#define MAX 100
void calc(float**, int, float*, float*);
int main( int argc, char* argv[] )
{
float *array[MAX];
float *MEAN;
float *SD;
int array_size, i;
array_size = argc;
for( i = 0; i < argc ; ++i )
{
*array = (float *) malloc( argc*(sizeof(float)));
*array[i] = atof(argv[i]);
}
calc(array, array_size, MEAN, SD);
return 0;
}
void calc(float* arr[] , int arr_size, float* mean, float* stdev)
{
int sum, i;
for ( i = 0; i < arr_size ; ++i )
{
sum += *arr[i];
}
*mean = sum / arr_size;
printf("%.2f", *mean);
for ( i = 0; i < arr_size ; ++i )
{
sum += pow((*arr[i]-*mean), 2);
}
*stdev = sqrt((sum/arr_size));
printf("%.2f", *stdev);
}
The code is supposed to determine the mean and standard deviation of a number of user-inputted floats. I keep getting a segmentation fault when trying to compile it. I tried freeing malloc but it produced more problems for the code, so I just left it as is - although it's probably a problem with memory distribution.
Thank you

Answer 1

There are several problems:
float *array[MAX]; is a pointer to an array of floats. You should
use either an array of floats or a pointer. Since you are using
malloc, you can use a pointer like this: float *array;
Do not call malloc for each float, but call it once for the entire array
If you define MEAN and SD as pointers, you will have to use
malloc on them. It is better to declare them as float only and use
their addresses (via &MEAN and &SD).
You sum variable should be a float, and you should initialize it
to 0. You should also set it to 0 before calculating stdev.
Remember that argv[0] contains the name of your program, so
you should ignore it.
Also remember to call free on anything you allocate with malloc
when you are done with it.
A modified version of your code:
#include<stdio.h>
#include<stdlib.h>
#include<math.h>
#include<string.h>
void calc(float*, int, float*, float*);
int main( int argc, char* argv[] ) {
float *array;
float MEAN;
float SD;
int array_size, i;
array_size = argc-1;
array = (float *) malloc( array_size*sizeof(float));
for( i = 0; i < array_size ; ++i ) {
array[i] = atof(argv[i+1]);
}
calc(array, array_size, &MEAN, &SD);
free (array);
return 0;
}
void calc(float* arr , int arr_size, float* mean, float* stdev) {
float sum=0;
int i;
for ( i = 0; i < arr_size ; ++i ) {
sum += arr[i];
}
*mean = sum / arr_size;
printf("%.2f ", *mean);
sum=0;
for ( i = 0; i < arr_size ; ++i ) {
sum += pow((arr[i]-*mean), 2);
}
*stdev = sqrt((sum/arr_size));
printf("%.2f", *stdev);
}

Answer 2

You are allocating and accessing array wrongly. Should be like this:
for( i = 0; i < argc ; ++i )
{
array = malloc( argc * sizeof(*array));
array[i] = atof(argv[i]);
}
Also inside calc() function change *arr[i] to arr[i] everywhere.

Answer 3

This is definitely wrong :
*array = (float *) malloc( argc*(sizeof(float)));
You are allocating an array of floats after declaring the array. This does not make sense. Then you are dereferencing the values wrongly. In addition, in your code your are trying to transform the name of the program (argv[0]) to an int type.
This may be what you wanted to do :
float * array = NULL;
array = malloc( (argc-1)*(sizeof(float)));
for (i=1; i<argc; ++i)
array[i]=atof(argv[i]);
This code reserves a memory location in the heap of size (argc-1)*float, and initialises it with the values specified as command line parameters. A check should be inserted when argc is 1.

Answer 4

You have several problems,
check that you have at least one argument
you declared array as an array of pointers to float, either use array[], or *array
your array need only be allocated once
allocate an array of number of samples*sizeof(float), once, before loop
store values read into array[ndx] locations (atof is fine)
argv starts at program name, store into array[n] the value converted from argv[n+1]
MEAN and SD need space, you declared pointers, change to value, and pass address (&)
Here is the revised main(),
int main( int argc, char* argv[] )
{
//float array[MAX];
float *array;
float MEAN;
float SD;
int array_size, ndx;
if(!(argc>0)) { printf("please give 1 or more arguments\n"); exit(1); }
array_size = argc-1; //don't count the program name
array = (float*) malloc(array_size*(sizeof(float)));
for( ndx = 1; ndx<argc ; ++ndx )
{
array[ndx-1] = atof(argv[ndx]);
printf("[%d] %f\n",ndx-1,array[ndx-1]);
}
calc(array, array_size, &MEAN, &SD);
free(array); array=NULL; //always clean up after yourself
return 0;
}
Your statistics calculations need work, but this at least compiles and runs,
you need to link against libmath (see below),
your sum was not initialized, before either loop,
pow and sqrt return double - read about C math functions and passing double,
your array_size needs to be set carefully
Here,
void calc(float arr[] , int arr_size, float* _mean, float* _stdev)
{
double sum, mean, stddev;
int i;
sum=0.0;
for ( i = 0; i < arr_size ; ++i )
{
sum += arr[i];
//printf("[%d] %f\n",i,arr[i]);
}
printf("sum %f\n",sum);
*_mean = mean = sum / arr_size;
printf("mean %f\n",mean);
printf("%.2f\n", *_mean);
sum=0.0;
for ( i = 0; i < arr_size ; ++i )
{
sum += pow((arr[i]-mean), 2);
//printf("[%d] %f\n",i,arr[i]);
}
printf("sum^2 %f\n",sum);
*_stdev = stddev = sqrt((sum/arr_size));
printf("stddev %f\n",stddev);
printf("%.2f\n", *_stdev);
}
Oh, you want to link to libmath,
cc stddev.c -lm -o stddev
And when running it,
$ ./stddev 1 2 3 4 5
[0] 1.000000
[1] 2.000000
[2] 3.000000
[3] 4.000000
[4] 5.000000
sum 15.000000
mean 3.000000
3.00
sum^2 10.000000
stddev 1.414214
1.41
Also, you need to free the memory you malloc'ed (array). This is good habit, though not needed for this tiny program.