Reading particular column from a table in a text file using C - arrays

Question

I have a text file with a large matrix (56*10000). But here is a sample.
A B C D
1 3 4 5
3 5 6 10
2 2 6 11
3 2 3 38
3 3 1 19
I have a code which can read the file and also print it. I have two problems.
I can't seem to find a way to print a particular column. For example in MATLAB, if we want to print the second column we indicate it like [:,2].
Is there way where I can access the column by a particular name. For example, since these columns are named A B C D, if I want to print the second column, I just have to say, if case in B, print (B) and it has to print,
3
5
2
2
3
Here is the code that I have.
#include <stdio.h>
#include <stdlib.h>
int main()
{
FILE * fp;
int M = 5, N = 4;
char filter[1024] ;
double *Z_BS = (double *)malloc(M * N * sizeof(double));
fp = fopen("test.txt","r");
if (fp == NULL)
{
exit(1);
}
fgets(filter,1024,fp);
for (int rows = 0; rows < M; rows++)
{
for (int cols = 0; cols < N; cols++)
{
fscanf(fp, "%lf", Z_BS);
printf("%lf\t", *Z_BS);
}
printf("\n");
if (feof(fp))
{
break;
}
}
}

Answer 1

In C there is no way like matlab, but with the help of below idea you can generate the required offset
offset = row_size*i + column_number
lets say there are five 5 rows and 5 columns and you want to access 3 column.
A B C D E
1 2 3 4 5
6 7 8 9 10
11 12 13 14 15
16 17 18 19 20
21 22 23 24 25
when i = 0, 5*0+3 = 3
when i = 1, 5*1+3 = 8
when i = 2, 5*2+3 = 13
when i = 3, 5*3+3 = 18
when i = 4, 5*4+3 = 23
NOTE: check for EOF always or the offset is valid before using, you should not access anything beyond the file.

Answer 2

Unrelated, but if you just want to print the values, there's no need to allocate memory.
Using a local variable is enough
for (int rows = 0; rows < M; rows++) {
for (int cols = 0; cols < N; cols++) {
double Z_BS;
fscanf(fp, "%lf", &Z_BS);
printf("%lf\t", Z_BS);
}
printf("\n");
if (feof(fp)) {
break;
}
}
To print just one column, you can simply use the col variable, say print the second column
for (int cols = 0; cols < N; cols++) {
double Z_BS;
fscanf(fp, "%lf", &Z_BS);
if (cols == 1)
printf("%lf\t", Z_BS);
}
It says cols == 1, because the loop starts at 0 (first column = 0, second column = 1, ...).
To print by column name, don't skip the first row, but compare the name, e.g.
char colname[] = "B";
int colnum;
// Scan header row
for (int cols = 0; cols < N; cols++) {
char current[51];
fscanf(fp, "%.50s", current);
if (strcmp(current, colname) == 0) {
colnum = cols;
break;
}
}
and later while printing
if (cols == colnum)
printf("%lf\t", Z_BS);