Print out all possible strings in C using recursion - c

Question

My assignment is that code takes a string as input, and if there are x's in the string replace them with either a 0 or a 1, and print out all the possible string combinations. We have to use recursion for this as well. For example, if the input string was "1x0X" the output would be:
1000
1001
1100
1101
I'm really struggling with how I'm supposed to find all the permutations of the string without having the complete string yet. I have a series of function that combine to print out all permutations of a list of numbers, but I don't know how to make a function where it only permutes certain elements of a list.
Does anyone have any suggestions on how to accomplish this?

Answer 1

Jonathan's initial suggestion
This code implements what I suggested in a comment essentially verbatim. It accepts either x or X as a valid marker because the examples in the question do too.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
static void map_x(const char *str)
{
size_t xloc = strcspn(str, "xX");
if (str[xloc] == '\0')
printf("%s\n", str);
else
{
char *copy = strdup(str);
copy[xloc] = '0';
map_x(copy);
copy[xloc] = '1';
map_x(copy);
free(copy);
}
}
int main(void)
{
char buffer[4096];
while (fgets(buffer, sizeof(buffer), stdin) != 0)
{
buffer[strcspn(buffer, "\n")] = '\0';
map_x(buffer);
}
return 0;
}
The main() function is essentially the same in all three variants. The use of strcspn() is a standard idiom that trims everything from the first newline onwards, or overwrites the end of the string if there is no newline in it.
Note that this solution is safe even if a read-only string literal is passed to the function; it does not modify the string that it is passed. The following solutions will both crash or otherwise fail if the initial string is in fact a read-only string literal.
It would be possible to determine the string length, allocate a VLA (variable length array) to take the string copy, and copy the string into the VLA. That would dramatically reduce the cost of allocating memory for the string (VLA allocation is much simpler than a general purpose memory allocator).
Gene's Suggestion
This code implements what Gene suggested in a comment. It will be more efficient because it does no extra memory allocation, an expensive operation on most systems.
#include <stdio.h>
#include <string.h>
static void map_x(char *str)
{
size_t xloc = strcspn(str, "xX");
if (str[xloc] == '\0')
printf("%s\n", str);
else
{
char letter = str[xloc];
str[xloc] = '0';
map_x(str);
str[xloc] = '1';
map_x(str);
str[xloc] = letter;
}
}
int main(void)
{
char buffer[4096];
while (fgets(buffer, sizeof(buffer), stdin) != 0)
{
buffer[strcspn(buffer, "\n")] = '\0';
map_x(buffer);
}
return 0;
}
Mildly optimized variant
This optimizes the work by not rescanning the prefix that is already known to be free of x's.
/* SO 4764-4683 */
#include <stdio.h>
#include <string.h>
static void map_x(char *str, size_t offset)
{
size_t xloc = strcspn(&str[offset], "xX") + offset;
if (str[xloc] == '\0')
printf("%s\n", str);
else
{
char letter = str[xloc];
str[xloc] = '0';
map_x(str, xloc);
str[xloc] = '1';
map_x(str, xloc);
str[xloc] = letter;
}
}
int main(void)
{
char buffer[4096];
while (fgets(buffer, sizeof(buffer), stdin) != 0)
{
buffer[strcspn(buffer, "\n")] = '\0';
map_x(buffer, 0);
}
return 0;
}
The difference in performance is probably not measurable on almost any input simply because the I/O time will dominate.
With all due respect to chux, I think that the code in the answer is more complex than necessary. The extra data structure seems like overkill.

Answer 2

Recursion is best used when the recursive depth is limited and not too big.
Yet setting aside that axiom, below is a double recursive solution. Its eats up stack space quickly (that is its biggest constraint) and so is not a robust solution. Yet it gets the job.
For "code takes a string as input,", use fgets() - not shown.
Yet in the spirit of recursion why not recurse the input too? The print_combo() recursion produces a linked list (LL) of characters and keeps track of the number of 'x' read. Once an end-of-line/end-of-file occurs, it is time to print and the linked-list starts with the last character.
The foo() recursion prints the LL in reverse order, passing in a binary mask to direct the x substitution of 0 or 1. The unsigned binary mask is good for typically 32 x's. That is another restriction.
If you must, mouse over for the code.
typedef struct node {
const struct node *prev;
int ch;
} node;
// Print the line
void foo(const node *prev, unsigned mask) {
if (prev) {
if (prev->ch == 'x' || prev->ch == 'X') {
foo(prev->prev, mask >> 1);
putchar("01"[mask & 1]);
} else {
foo(prev->prev, mask);
putchar(prev->ch);
}
}
}
// Read, form the LL and then print
void print_combo(const node *prev, unsigned xcount) {
node n = {.prev = prev, .ch = getchar()};
if (n.ch == '\n' || n.ch == EOF) {
for (unsigned mask = 0; mask < (1u << xcount); mask++) {
foo(prev, mask);
putchar('\n');
}
} else {
print_combo(&n, xcount + (n.ch == 'x' || n.ch == 'X'));
}
}
int main(void) {
print_combo(NULL, 0);
}
Input
00x01x10x11
Output
00001010011
00001010111
00001110011
00001110111
00101010011
00101010111
00101110011
00101110111

Answer 3

I would do something a bit simpler. Just use a position parameter to iterate over the input string. Whenever you hit the 'x' character recurse twice, once for '0' and once for '1'. Make sure to reset the character back to 'x' after you return. Whenever you hit the digit character just recurse once. Increment the position parameter each time you recurse. When you hit the end of the string, print it out. With this idea, you'd get something like this:
#include <stdio.h>
void print_combo(char *str, int pos) {
char c;
c = str[pos];
switch (c) {
case '0':
case '1':
print_combo(str, pos+1);
break;
case 'x':
case 'X':
str[pos] = '0';
print_combo(str, pos+1);
str[pos] = '1';
print_combo(str, pos+1);
str[pos] = c;
break;
case '\0':
printf("%s\n", str);
break;
default:
printf("bad input\n");
break;
}
}
int main() {
char str[10];
strcpy(str, "1x0x");
printf("printing %s\n", str);
print_combo(str, 0);
strcpy(str, "0x01x");
printf("printing %s\n", str);
print_combo(str, 0);
strcpy(str, "0x01x0X1");
printf("printing %s\n", str);
print_combo(str, 0);
return 0;
}
My output looks like this:
printing 1x0x
1000
1001
1100
1101
printing 0x01x
00010
00011
01010
01011
printing 0x01x0X1
00010001
00010011
00011001
00011011
01010001
01010011
01011001
01011011