Ascending order arrangement of numbers of a file - c

Question

How to read a file which contains two columns and sort the first column numbers in ascending order and to print them with their corresponnding 2nd column values using C ?

Answer 1

fopen opens a file.
fscanf reads from a file and splits what is read into bits according to a format specification (e.g. "%d %s" means an integer followed by whitespace followed by a string of non-whitespace characters).
qsort is a standard library function that will sort an array. It sorts the array by comparing one item to another item. You give it the name of a function (which you write) that does this comparison.
I encourage you to read the manual pages for these functions if you are not familiar with them.
The program below uses all this to:
Open a file test.txt
Read lines from the file into an array arr
Sort the array using qsort, using the rowcmp function (rowcmp looks at the numerical value in the first column to determine whether one element is greater than, equal to, or less than another element)
Print out the elements of the array.
The code...
#include <stdio.h>
#include <stdlib.h>
#define MAXLEN 100
#define MAXITEMS 100
// A row has two columns, the first is a number and
// the second is any string of up to MAXLEN chars
struct row {
int col1;
char col2[MAXLEN];
};
// Function to do comparison of rows
// Sorts numerically on the value in the first column
int rowcmp(struct row * r1, struct row * r2) {
if (r1->col1 < r2->col1) return -1;
if (r1->col1 == r2->col1) return 0;
return 1;
}
int main(int argc, char ** argv) {
struct row arr[MAXITEMS]; // up to MAXITEMS rows
int rows = 0, i;
FILE * stream = fopen("test.txt", "r");
// Read in rows and put first and second columns into struct,
// building up an array
while (fscanf(stream, "%d %s", &(arr[rows].col1), arr[rows].col2) !=EOF) {
rows++;
}
// Sort the array using the rowcmp function to compare items
qsort(&arr[0], rows, sizeof(struct row), (__compar_fn_t)rowcmp);
fclose(stream);
// Print the sorted array
for (i=0; i<rows; i++) {
printf("%d\t%s\n", arr[i].col1, arr[i].col2);
}
}
With input file:
1 apple
3 cucumbers
21 dates
7 figs
4 grapes
output is
1 apple
3 cucumbers
4 grapes
7 figs
21 dates