How to load data from file into float* in C - c

Question

I have an ASCII file in which the entries of a vector are stored. I do not know the length (number of rows) of the file, nor do I have an estimation about its size as it may strongly vary from a few lines to some tens of thousands. I need an efficient way to read the data stored in that file and load them to a float* variable. The code should be in C.
My question is how to allocate memory for the vector which I need to create given that I do not know its size beforehand? Can you please give an example?
Finally, what is in your opinion the most appropriate prototype for such a function? Should it be something like:
load_data(const char* filename, float* data, int* vector_size);
?
Update 1.: While doing some initial tests, I wrote the following code:
void create_random_matrix(float* matrix, const int nrows) {
matrix = (float *) malloc(sizeof (float) * nrows);
short i;
for (i = 0; i < nrows; i++) {
matrix[i] = 7.0f;
}
}
which should return an array with all its elements equal to 7.0f. Instead, when I call it from my main.c:
float *a;
create_random_matrix(a, 10);
printf("%f",a[0]);
it prints 0.0f. How is that possible?!
Update 2. Was it not for your help, the following (working) code wouldn't have been written:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#define LINE_SIZE 10
#define ALLOCATION_CHUNK 50
int load_vector_data(const char* filename, float** vector, int* length) {
*vector = malloc(sizeof (float) * ALLOCATION_CHUNK);
int allocated_rows = ALLOCATION_CHUNK;
u_short i = 0;
FILE* fr = fopen(filename, "r");
if (fr == NULL) {
exit(FILE_NOT_FOUND);
}
char line[LINE_SIZE];
while (fgets(line, LINE_SIZE, fr) != NULL) {
if (i >= allocated_rows){
allocated_rows += ALLOCATION_CHUNK;
*vector = realloc(*vector, sizeof (float) * allocated_rows);
}
strip_newline(&line, LINE_SIZE);
(*vector)[i] = strtod(line, (char **) NULL);
i++;
}
*length = i;
*vector = realloc(*vector, sizeof (float) * i);
fclose(fr);
}
void strip_newline(char *str, int size) {
u_short i;
for (i = 0; i < size; ++i) {
if (str[i] == '\n') {
str[i] = '\0';
return;
}
}
}
I tried it with an 8000-lines file and seems to be working just fine! Please, feel free to comment.

Answer 1

fgets is you friend for reading the data from the file (If my assumption that each bit of data is on a new-line is correct). Read each line in 1 by 1 and use an strtof on the text you read. Reading text and converting to floats is inherently a slow process so I reckon the above is perfectly good enough.
As for your second question there are a couple of ways to do it. You could pass a float** in and make the malloc inside the function. Though this has the disadvantage of you needing to free it outside the function which is not exactly obvious. The only other way, i can think of, would be to scan through the file and count the number of new-lines then pre-allocate the array length for that.
Its hard to say whether doing a malloc and a bunch of reallocs would be more efficient than the scan through to count the number of lines, it would probably be worth trying both methods (neither is particularly hard) and seeing which one is faster for you.