Order pair of ints inserted by input - c

Question

Im getting input from user and then order a order of pairs if its possible
Case 1 - 4 pairs input [1,2][2,3][3,4][4,5] This a possible input so i should order this and show 1,2,3,4,5
Case 2 -4 pairs input [1,2][2,3][3,2][4,5] this is inculent so just show a message that doesnt make sense
Im trying doing this by using a adjacency matrix but now im stuck!
I create a adjacency matrix marking user input
Case 1 Matrix would be:

Answer 1

Let's say you have a 2D array to collect your pairs like:
#define MAXPAIRS 20 /* if you need a constant, #define one (or more) */
#define MAXC 1024
...
char line[MAXC]; /* buffer to hold each line of input */
int pairs[MAXPAIRS][2] = {{0}}, /* 2D array to hold pairs */
npairs = 0; /* number of pairs entered */
You can read each pair into your 2D array as follows:
while (npairs < MAXPAIRS) { /* loop while array not full */
printf ("enter pair [%2d] : ", npairs); /* prompt */
if (!fgets (line, MAXC, stdin)) { /* read/validate line */
puts ("(user canceled input)");
break;
}
else if (*line == '\n') /* end input on ENTER alone */
break;
/* parse integers from line into pair */
if (sscanf (line, "%d %d", &pairs[npairs][0], &pairs[npairs][1]) != 2) {
fputs (" error: invalid integer input.\n", stderr);
continue;
}
npairs++; /* increment counter */
}
(note: if you want your prompt to ask for "pair[1]... pair[2] ..." instead of "pair[0]... pair[1]...", you can use printf ("enter pair [%2d] : ", npairs + 1); adding 1 to the npairs value.)
Which simply collects pairs from the user until the array is full, or the user signals done entering input by pressing Enter alone on the line after the last input.
With your pairs stored, you can loop over the array validating that the array is incluent, between rows (if required) and between pair values, creating your matrix as you go:
int matrix[npairs+1][npairs+1]; /* using a VLA */
for (int i = 0; i < npairs + 1; i++) { /* loop over pairs + 1 */
/* 2nd row on, check value is 1-greater than previous row */
if (i && i < npairs && pairs[i][0] - pairs[i-1][0] != 1) {
fputs (" error: not incluent - between rows.\n", stderr);
return 1;
}
/* check that pair values are adjacent, increasing */
if (i < npairs && pairs[i][1] - pairs[i][0] != 1) {
fputs (" error not incluent - between pair values.\n", stderr);
return 1;
}
for (int j = 0; j < npairs + 1; j++) /* fill row in matrix */
matrix[i][j] = i < npairs && j == i + 1 ? 1 : 0;
}
If you have made it this far, you matrix is filled and your pairs incluent and you simply need to output the matrix, e.g.
puts ("\nmatrix:"); /* output matrix */
print_matrix (npairs+1, npairs+1, matrix);
Putting it altogether along with the print_matrix() function, you would have:
#include <stdio.h>
#define MAXPAIRS 20 /* if you need a constant, #define one (or more) */
#define MAXC 1024
/* simple print matrix function */
void print_matrix (int rows, int cols, int (*arr)[cols])
{
for (int i = 0; i < rows; i++) {
for (int j = 0; j < cols; j++)
printf (j ? " %d" : "%d", arr[i][j]);
putchar ('\n');
}
}
int main (void) {
char line[MAXC]; /* buffer to hold each line of input */
int pairs[MAXPAIRS][2] = {{0}}, /* 2D array to hold pairs */
npairs = 0; /* number of pairs entered */
while (npairs < MAXPAIRS) { /* loop while array not full */
printf ("enter pair [%2d] : ", npairs); /* prompt */
if (!fgets (line, MAXC, stdin)) { /* read/validate line */
puts ("(user canceled input)");
break;
}
else if (*line == '\n') /* end input on ENTER alone */
break;
/* parse integers from line into pair */
if (sscanf (line, "%d %d", &pairs[npairs][0], &pairs[npairs][1]) != 2) {
fputs (" error: invalid integer input.\n", stderr);
continue;
}
npairs++; /* increment counter */
}
int matrix[npairs+1][npairs+1]; /* using a VLA */
for (int i = 0; i < npairs + 1; i++) { /* loop over pairs + 1 */
/* 2nd row on, check value is 1-greater than previous row */
if (i && i < npairs && pairs[i][0] - pairs[i-1][0] != 1) {
fputs (" error: not incluent - between rows.\n", stderr);
return 1;
}
/* check that pair values are adjacent, increasing */
if (i < npairs && pairs[i][1] - pairs[i][0] != 1) {
fputs (" error not incluent - between pair values.\n", stderr);
return 1;
}
for (int j = 0; j < npairs + 1; j++) /* fill row in matrix */
matrix[i][j] = i < npairs && j == i + 1 ? 1 : 0;
}
puts ("\nmatrix:"); /* output matrix */
print_matrix (npairs+1, npairs+1, matrix);
}
(note: the use of a ternary in several places above which has the form test ? if_true : if_false (a shorthand if (test) ... else ...) It can be used virtually anywhere, function parameters, etc.., anywhere a choice is needed.)
Example Use/Output
$ ./bin/incluentpairs
enter pair [ 0] : 1 2
enter pair [ 1] : 2 3
enter pair [ 2] : 3 4
enter pair [ 3] : 4 5
enter pair [ 4] :
matrix:
0 1 0 0 0
0 0 1 0 0
0 0 0 1 0
0 0 0 0 1
0 0 0 0 0
Edit Per-Comment
To output the message on error you can do:
for (int i = 0; i < npairs + 1; i++) { /* loop over pairs + 1 */
/* 2nd row on, check value is 1-greater than previous row */
if (i && i < npairs && pairs[i][0] - pairs[i-1][0] != 1) {
fputs (" error: not incluent - between rows.\n", stderr);
return 1;
}
/* check that pair values are adjacent, increasing */
if (i < npairs && pairs[i][1] - pairs[i][0] != 1) {
if (i) {
char msg[64];
sprintf (msg, "contradictory %d->%d %d->%d\n",
pairs[i-1][0], pairs[i-1][1], pairs[i][0], pairs[i][1]);
fputs (msg, stderr);
}
else
fputs (" error not incluent - between pair values.\n", stderr);
return 1;
}
for (int j = 0; j < npairs + 1; j++) /* fill row in matrix */
matrix[i][j] = i < npairs && j == i + 1 ? 1 : 0;
}
Example Use/Output
$ ./bin/incluentpairs
enter pair [ 0] : 1 2
enter pair [ 1] : 2 3
enter pair [ 2] : 3 2
enter pair [ 3] : 4 5
enter pair [ 4] :
contradictory 2->3 3->2
Look things over and let me know if you have questions (just leave a comment below).