How can I duplicate a letter in a string? - c

Question

I want to write a program where it duplicates every letter in a given string.
For example, if the input is abc then the output would be aabbcc.
How can I do this?
Here is my code so far. It only copies the string:
#include <stdio.h>
int main () {
char str_in[100];
char str_out[200] = "";
int i;
printf("Enter a word: ");
scanf("%s", str_in);
for (i = 0; i < sizeof(str_in); i++) {
str_out[i] += str_in[i];
}
printf("Duplicated word: %s", str_out);
return 0;
}

Answer 1

For starters the destination character array should be at least two times larger than the source array.
The loop that performs the copying can look the following way
size_t j = 0;
for ( size_t i = 0; str_in[i] != '\0'; i++ )
{
str_out[j++] = str_in[i];
str_out[j++] = str_in[i];
}
str_out[j] = '\0';

Answer 2

This is one way to do what you want - since you know that the two indexes in str_out that you want to access correspond to the ith and i+1th positions of str_in:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define SIZE 100
int main(void)
{
char str_in[SIZE];
char str_out[SIZE*2];
int i, len;
printf("Enter a word: ");
scanf("%s", str_in);
len = strlen(str_in);
for (i = 0; i < len; i++) {
str_out[i*2] = str_in[i];
str_out[i*2+1] = str_in[i];
}
str_out[i*2] = '\0';
printf("Duplicated word: %s\n", str_out);
return EXIT_SUCCESS;
}
You could also use another variable and update inside the loop as such:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define SIZE 100
int main(void)
{
char str_in[SIZE];
char str_out[SIZE*2];
int i, j, len;
printf("Enter a word: ");
scanf("%s", str_in);
len = strlen(str_in);
for (i = 0, j = 0; i < len; i++, j += 2) {
str_out[j] = str_in[i];
str_out[j+1] = str_in[i];
}
str_out[j] = '\0';
printf("Duplicated word: %s\n", str_out);
return EXIT_SUCCESS;
}
As a side note, it's best to #define your constants instead of having them in your code.

Answer 3

I would suggest only allocating the amount of memory you need:
char *str_out = malloc(strlen(str_in) + 1);
and then using another loop variable that counts by 2 each time so you have 2 times as much space for every character in your loop.
If you do that, you should also put + 1 next to the new loop variable to copy it to the current and next elements of the string, thereby duplicating the characters.
I would also advise you to use size_t for your loop variables to match the return of str_len.
Here I have implemented my suggestions:
size_t str_ctr;
size_t str_ctrx2;
size_t in_str_len = strlen(in_str)
/* ... */
for( str_ctrx2 = str_ctr = 0;
str_ctr < in_str_len;
str_ctr++, str_ctrx2 += 2 )
{
out_str[str_ctrx2] = in_str[str_ctr];
out_str[str_ctrx2 + 1] = in_str[str_ctr];
}
/* null-terminate the string */
out_str[str_ctrx2] = '\0';
I would also like to mention that you forget to null-terminate your string in your program. I have made a comment about that and have shown you how to do it above.

Answer 4

#include <stdio.h>
int main () {
char str_in[100];
char str_out[200] = "";
int i, j;
printf("Enter a word: ");
scanf("%s", str_in);
j=0;
for (i = 0; i < sizeof(str_in); i++) {
str_out[i] = str_in[j];
str_out[i+1] = str_in[j];
i++;
j++;
}
printf("Duplicated word: %s", str_out);
return 0;
}