Deshuffle text with C - c

Question

I wanted to get an opinion here. Would it be possible to write a C program that deshuffles a text file? What do I mean by that? Say I have the following data in a textfile:
1 X
4 T
3 Z
2 L
And I wanted to deshuffle it and output another file as so:
1 X
2 L
3 Z
4 T
Such that all of the data following the number is preserved with the actual index number. Do know that I have 40,500 shuffled entities, so that should probably be taken into account since that could take a long time if the program needs to loop through all of the entities for each entity...And I only used letters for representation. The actual data files don't have letters, but rather have floats. Sorry if this causes any confusion
So, bottom line, would this be possible with C? And if so, could I get a hint at where to start? I could obviously input all of the textfile data into an array dat[][], but how should I deshuffle it then?
Thanks!
Amit

Answer 1

Look up qsort, its part of stdlib.

Answer 2

I'd just use sort -n instead of writing a program.

Answer 3

If you know the largest index in the file and there are no holes (i.e. all indices are present), you can create an array of that size, then as you read each line of the file, put the data into the correct location in the array. When you're done reading the file, your array will have all the elements in the correct order.

Answer 4

You can build a vector< pair<int,float[4]> > and just sort it using STL.
Just noticed a c tag.... But C idea is about the same:
Build an array with the data:
struct pair
{
int idx;
float val[4];
}
pair *data;
And then just sort it.

Answer 5

For those interested, I ended up using a series of loops to write this program, the code is show below. The only downside of it is that if my nrow variable is equal to 40,500, this program can take as long as 30 mins to run on a 3GHz dual core computer. I'm sure there are ways to optimize it, though at least it does what I want...for now.
Here's the code:
#include "stdlib.h"
#include "stdio.h"
// Deshuffle the output files of the bond/swap mode
int main(int argc, char* argv[])
{
if (argc < 4) {
printf("\ndeshuffle usage: [number of chains] [number of atoms] [.dat file] \n");
exit(1);
}
int i,j,k,l;
int nch = atoi( argv[1] );
int ns = atoi( argv[2] );
double **in_dat, **s_dat, dm1;
int nrow = ns*nch;
in_dat = (double**) calloc(nrow, sizeof(double*));
s_dat = (double**) calloc(nrow, sizeof(double*));
for (i=0; i<nrow; i++) {
in_dat[i] = (double*) calloc(6, sizeof(double));
s_dat[i] = (double*) calloc(6, sizeof(double));
for (j=0; j<6; j++)
in_dat[i][j] = 0.0;
s_dat[i][j] = 0.0;
}
// store input data into 2D array in_dat
FILE *inp;
inp = fopen( argv[3], "r" );
for (i=0; i<nrow; i++) {
for (j=0; j<6; j++) {
fscanf(inp, "%lf", &dm1);
in_dat[i][j] = dm1;
}
}
fclose(inp);
// Sort data in s_dat based on comparison with in_dat
k=0;
while (k < (nrow)) {
for (i=0; i<nrow; i++) {
for (l=0; l<nrow; l++) {
if (in_dat[l][0] == (k+1)) {
for (j=0; j<6; j++) {
s_dat[i][j] = in_dat[l][j];
}
k++;
break;
}
}
}
}
// Write sorted data to file
FILE *otp;
otp = fopen("results.out", "w");
for (i=0; i<nrow; i++) {
for (j=0; j<6; j++) {
fprintf(otp, "%lf \t", s_dat[i][j]);
if (j==5)
fprintf(otp, "\n");
}
}
fclose(otp);
printf("\n Done. \n\n");
return 0;
}