How to add a number to a string at the end in a c program?

Question

I need to write a function which will take a string as input and outputs as below

input : aaabbdd
output : a3b2d2

input : aaaaaaaaaaaaaaaabbccc
output : a16b2c3

basically I have to append the count to every character. I should not use itoa() to convert int to string

I wrote the logic. But I got struck at appending the number to the string. For example, if count is 16, how can I add the number 16 to end of a string?

My logic is as given below.

#include <stdio.h>

void str1(char *str)
{
    int i, j, cnt;

    int len = strlen(str);

    char *nstr = (char *) malloc(len * sizeof(char));

    int k = 0;
    cnt = 1;

    for(i = 0, j = 1; i < len - 1;)
    {
        if(str[i] == str[j])
        {
            j++;
            cnt++;
            continue;
        }
        else
        {
            if(cnt == 1)
            {
                nstr[k++] = str[i];
            }
            else
            {
                nstr[k++] = str[i];
                nstr[k++] = cnt; // GOT STUCK HERE
            }
            i = j;
            j = i + 1;
            cnt = 1;
        }
    }

    printf("\n%s\n", nstr);
}

main()
{
    char str[] = "aaaaaaaaaaaaaaaabbcdd";

    str1(str);
}

possible duplicate of [c string and int concatenation](http://stackoverflow.com/questions/5172107/c-string-and-int-concatenation) — Cornstalks, Sep 08 '14 at 18:21
why do you want to store it, you are only asked to output it? — Pieter21, Sep 08 '14 at 18:37
@Pieter21 : How can I output it? Are you suggesting that just print character and number instead of storing it in a buffer and printing entire string at once? — kadina, Sep 08 '14 at 18:40
@Pieter21 : Yeah. That will work. But I am preparing for interview. If they ask me to write the function which will return the string instead of printing, I will have tough times w/o practising :) — kadina, Sep 08 '14 at 18:53
@kadina, they won't! This exercise is about keeping track of state, and not about int to string conversion. Otherwise just say you'd strcat your way out of this with sufficiently allocated buffers. — Pieter21, Sep 08 '14 at 19:08
You might want to be careful with the size of your buffer: If a single character is followed by a "1", your new string can be longer than the old one. Example: abcdefgh -> a1b1c1d1e1f1g1h1 — EOF, Sep 08 '14 at 20:53

score 1 · Accepted Answer · answered Sep 08 '14 at 18:25

You can implement itoa yourself. The logic is as follows:

If the number is zero, append zero (in your case, this should never happen, but in general case this is obviously possible)
Prepare a temporary buffer for your output. The size depends on the number of bits in the integer that you are printing
Store the last digit by computing n % 10
Divide the number by ten using integer division
Continue the last three steps until the remaining part is zero
Append the reversed sequence of digits from the temporary buffer to your output

This is only one way of implementing the logic. Other ways are possible as well - for example, you could build a lookup table of powers of ten, and compute each digit using a combination of integer division and taking the remainder.

score 1 · Answer 2 · answered Sep 08 '14 at 18:59

All the code you need, decorated with comments:

if (len == 0) return;

/* initialize */
char c = str[0];
int  count = 1;

/* include terminating '\0', and that will resolve itself! */
for (i = 1; i <= len; i++)
{
   if (str[i] == str[i-1])
   {
      /* continue sequence */
      count++;
   }
   else
   {
      /* end sequence */
      printf("%c%d", c, count);
      /* start new sequence */
      c = str[i]; count = 1;
   }
}
printf("\n"); /* flush buffer */

How to add a number to a string at the end in a c program?

2 Answers2