Columnar Transposition in C - c

Question

I'm trying to make a columnar transposition in C. The user inputs a string and key. The length of the key determines how many columns there are.To encryp the message the key must be sorted alphabetically. Here is an example using the string as hello there and the key as 'KEY'.
I created a function to sort the key alphabetically but I have no clue how to swap the columns.
void gridStr(char *line, char *key);
char encrypt(char *key);
int main(void) {
char key[50];
char line[256];
printf("Enter your string: ");
if (fgets(line, sizeof line, stdin) == NULL) {
fprintf(stderr, "No line read\n");
exit(EXIT_FAILURE);
}
printf("Enter your key: ");
if (fgets(key, sizeof key, stdin) == NULL) {
fprintf(stderr, "No line read\n");
exit(EXIT_FAILURE);
}
int len = strlen(line);
if (len && line[len - 1] == '\n')
line[--len] = '\0';
int len1 = strlen(key);
if (len1 && key[len1 - 1] == '\n')
key[--len]= '\0';
gridStr(line, key);
encrypt(key);
}
void gridStr(char *line, char *key)
{
char mat[10][10] = {0};
int columns = strlen(key)-1;
int rows = 0;
int i=0,j = 0;
while (line[i]) {
if (line[i] == ' ') {
putchar('_');
} else {
putchar(line[i]);
}
mat[rows][i % columns] = line[i];
i++;
if (i > 0 && i % columns == 0) {
putchar('\n');
rows++;
}
}
if (i % columns != 0) putchar('\n');
rows++; // from current row to number of rows
printf("\nMatrix:\n");
for (i = 0; i < rows; i++) {
for (j = 0; j < columns; j++) {
if (mat[i][j] == ' ') {
putchar('_');
} else {
putchar(mat[i][j]);
}
}
printf("\n");
}
}
char encrypt(char *key){
char temp;
int i,j;
int n = strlen(key);
for (i = 0; i < n-1; i++) {
for (j = i+1; j < n; j++) {
if (key[i] > key[j]) {
temp = key[i];
key[i] = key[j];
key[j] = temp;
}
}
}
printf("The sorted string is : %s", key);
return 0;
}

Answer 1

You do not want to swap the columns each time a bubble moves in the sorting algorithm until it reaches its final position. Therefore, the sorting can be performed separately.
Determining which columns to swap is implemented in the following way:
Iterate over all columns from start to end
If a column has not been swapped, find its counterpart and swap them
Mark both swapped columns in order to not swap them again, continue iteration
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdbool.h>
// use bubble sort to sort letters in a string
void sort_letters(char *phrase) {
int n = strlen(phrase);
//printf("The string to sort is: %s\n", phrase);
for (int i = 0; i < n-1; i++) {
for (int j = i+1; j < n; j++) {
if (phrase[i] > phrase[j]) {
char temp = phrase[i];
phrase[i] = phrase[j];
phrase[j] = temp;
}
}
}
//printf("The sorted string is: %s\n", phrase);
}
// allocate a matrix for string and fill it
char** gridStr(char *line, char *key)
{
int n = strlen(line);
int columns = strlen(key);
int rows = (n + columns - 1) / columns;
char **matrix = malloc(rows * sizeof(char *));
for (int j = 0; j < rows; j++) {
matrix[j] = malloc(columns * sizeof(char));
}
int i = 0;
for (int k = 0; k < rows; k++) {
for (int l = 0; l < columns; l++) {
if (i<n) {
matrix[k][l] = line[i++];
} else {
matrix[k][l] = '*'; // fill letter
}
//putchar(matrix[k][l]);
}
//putchar('\n');
}
return matrix;
}
// swap columns i and j of the given matrix, zero-based indices
void swap_matrix_columns(char **matrix, int rows, int columns, int i, int j) {
for (int k = 0; k < rows; k++) {
char tmp = matrix[k][i];
matrix[k][i] = matrix[k][j];
matrix[k][j] = tmp;
}
}
// print matrix to stdout
void print_matrix(char **matrix, int rows, int columns) {
for (int i = 0; i < rows; i++){
printf("Row %2d: ", i);
for (int j = 0; j < columns; j++) {
putchar(matrix[i][j]);
}
putchar('\n');
}
}
// sort key and transpose matrox columns according new order
void encrypt(char **matrix, int rows, int columns, char *key) {
char sorted_key[strlen(key)];
strcpy(sorted_key, key);
sort_letters(sorted_key);
printf("Sorted key: %s (length: %2zu)\n", sorted_key, strlen(sorted_key));
bool is_used[columns];
for (int i = 0; i < columns; i++) is_used[i]= false;
for (int i = 0; i < columns; i++) {
if (!is_used[i]) {
// find new position, must exist
int j;
for (j = 0; j < columns; j++) {
if (key[i] == sorted_key[j] && !is_used[j]) {
break;
}
}
swap_matrix_columns(matrix, rows, columns, i, j);
is_used[i] = true;
is_used[j] = true;
}
}
}
int main(void) {
char key[50];
char line[256];
printf("Enter your string: ");
if (fgets(line, sizeof line, stdin) == NULL) {
fprintf(stderr, "No line read\n");
exit(EXIT_FAILURE);
}
printf("Enter your key: ");
if (fgets(key, sizeof key, stdin) == NULL) {
fprintf(stderr, "No line read\n");
exit(EXIT_FAILURE);
}
int len = strlen(line);
if (len && line[len - 1] == '\n') {
line[--len] = '\0';
}
int len1 = strlen(key);
if (len1 && key[len1 - 1] == '\n') {
key[--len1]= '\0';
}
//printf("string: |%s| (length: %2zu)\n", line, strlen(line));
//printf("key: |%s| (length: %2zu)\n", key, strlen(key));
char **matrix = gridStr(line, key);
int columns = len1;
// Use simple trick (+ len1 - 1) to determine number of rows with one integer divison
int rows = (len + columns - 1) / columns;
print_matrix(matrix, rows, columns);
encrypt(matrix, rows, columns, key);
print_matrix(matrix, rows, columns);
for (int row = 0; row < rows; row++) {
free(matrix[row]);
}
free(matrix);
return EXIT_SUCCESS;
}
$ gcc -Wall transposition.c
$ ./a.out
Enter your string: hello_there
Enter your key: KEY
Row 0: hel
Row 1: lo_
Row 2: the
Row 3: re*
Sorted key: EKY (length: 3)
Row 0: ehl
Row 1: ol_
Row 2: hte
Row 3: er*
$

Answer 2

You have a 2D array. The first axis is the list of strings. The second axis is the characters of a particular string. I assume when you mean swap column, you mean swap characters of a particular string for all strings.
In pseudo code:
for all strings i
temp = mat[i][some column]
mat[i][some column] = mat[i][someother column]
mat[i][someother column] = temp