Reading and writing from files with a new form - c

Question

I'm trying to read from a file and I have to use a new form of it I'm not really certain how to use. I've posted the code below of what function I have and I'm not sure what to do about this error and how to fix it?
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
double* read_file(FILE* file, int length);
int main(int argc, char* argv[])
{
double* array = malloc(10 * sizeof(double));
int length = atoi(*(argv + 1));
FILE* file = *(argv + 2);
if (argc < 4 || argc > 4)
{
printf("Insufficient arguments. Check your command line.\n");
return 0;
}
array = read_file(file, length);
printf("%p", array);
return 0;
}
double* read_file (FILE* file, int length)
{
FILE* ptr;
double* array = malloc(length * sizeof(double));
int i = 0;
if ((ptr = fopen(file, "r")) == NULL)
{
return 0;
}
else
{
for (i = 0; i < length; i++)
{
fscanf(ptr, "%lf", (array + i));
}
}
fclose(ptr);
return array;
}

Answer 1

First of all, you're trying to assign a char string to a variable of type pointer to FILE. The compiler won't let you do that.
// not allowed
FILE* file = *(argv + 2);
Secondly, you're passing a pointer to FILE to fopen(), but fopen() expects it's first argument to be a char string so that won't work either.
// file is FILE*, not allowed
ptr = fopen(file, "r"));
If you fix those two lines the code should compile.

Answer 2

fix like this
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
//double* read_file(FILE* file, int length);
//You do not need to pass a file pointer, you need a file name.
//It opens the file within this function
double *read_file(const char *filename, int length);
int main(int argc, char* argv[]){
//if (argc < 4 || argc > 4)
//argc < 4 || argc > 4 same as argc != 4
//It is thought that it is 3 because only argv[1] and argv[2] are used.
//It is necessary to check arguments before using them.
if (argc != 3) {
printf("Usage : %s number_of_elements file_name\n", argv[0]);
printf("Insufficient arguments. Check your command line.\n");
return 0;
}
//double* array = malloc(10 * sizeof(double));//It is not necessary as it is secured by read_file. Make memory leak.
int length = atoi(argv[1]);
const char *file = argv[2];
double *array = read_file(file, length);
if(array != NULL){
for(int i = 0; i < length; ++i)
printf("%f\n", array[i]);
free(array);
}
return 0;
}
double* read_file (const char *file, int length){
FILE *ptr;
if ((ptr = fopen(file, "r")) == NULL){
return NULL;
}
//It causes a memory leak unless you first confirm that the file can be opened
double *array = malloc(length * sizeof(double));
for (int i = 0; i < length; i++){
if(1 != fscanf(ptr, "%lf", array + i)){
printf("Failed to read the %ith element.\n", i+1);
break;
}
}
fclose(ptr);
return array;
}