Print out the longest substring in c - c

Question

Suppose that we have a string "11222222345646". So how to print out subsequence 222222 in C.
I have a function here, but I think something incorrect. Can someone correct it for me?
int *longestsubstring(int a[], int n, int *length)
{
int location = 0;
length = 0;
int i, j;
for (i = 0, j = 0; i <= n-1, j < i; i++, j++)
{
if (a[i] != a[j])
{
if (i - j >= *length)
{
*length = i - j;
location = j;
}
j = i;
}
}
return &a[location];
}

Answer 1

Sorry,I don't really understand your question.
I just have a little code,and it can print the longest sub string,hope it can help.
/*breif : print the longest sub string*/
void printLongestSubString(const char * str,int length)
{
if(length <= 0)
return;
int i ;
int num1 = 0,num2 = 0;
int location = 0;
for(i = 0; i< length - 1; ++i)
{
if(str[i] == str[i+1])
++num2;//count the sub string ,may be not the longest,but we should try.
else
{
if(num2 >num1)//I use num1 store the sum longest of current sub string.
{ num1 = num2;location = i - num2;}
else
;//do nothing for short sub string.
num2 = 0;
}
}
for(i = location;str[i]== str[num1];++i)
printf("%c",str[i]);
printf("\n");
}
int main()
{
char * str = "1122222234566";
printLongestSubString(str,13);
return 0;
}

Answer 2

From your code it appears you want to return the longest sub-sequence (sub-string). Since I'm relearning C I thought I would give it a shot.
I've used strndup to extract the substring. I'm not sure how portable it is but I found an implementation if needed, just click on the link. It will allocate memory to store the new cstring so you have to remember to free the memory once finished with the substring. Following your argument list, the length of the sub-string is returned as the third argument of the extraction routine.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
char *extract_longest_subsequence(const char *str, size_t str_len, size_t *longest_len);
int main()
{
char str[] = "11222234555555564666666";
size_t substr_len = 0;
char *substr = extract_longest_subsequence(str, sizeof(str), &substr_len);
if (!substr)
{
printf("Error: NULL sub-string returned\n");
return 1;
}
printf("original string: %s, length: %zu\n", str, sizeof(str)-1);
printf("Longest sub-string: %s, length: %zu\n", substr, substr_len);
/* Have to remember to free the memory allocated by strndup */
free(substr);
return 0;
}
char *extract_longest_subsequence(const char *str, size_t str_len, size_t *longest_len)
{
if (str == NULL || str_len < 1 || longest_len == NULL)
return NULL;
size_t longest_start = 0;
*longest_len = 0;
size_t curr_len = 1;
size_t i = 0;
for (i = 1; i < str_len; ++i)
{
if (str[i-1] == str[i])
{
++curr_len;
}
else
{
if (curr_len > *longest_len)
{
longest_start = i - curr_len;
*longest_len = curr_len;
}
curr_len = 1;
}
}
/* strndup allocates memory for storing the substring */
return strndup(str + longest_start, *longest_len);
}

Answer 3

It looks like in your loop that j is supposed to be storing where the current "substring" starts, and i is the index of the character that you are currently looking at. In that case, you want to change
for (i = 0, j = 0; i <= n-1, j < i; i++, j++)
to
for (i = 0, j = 0; i <= n-1; i++)
That way, you are using i to store which character you're looking at, and the j = i line will "reset" which string of characters you are checking the length of.
Also, a few other things:
1) length = 0 should be *length = 0. You probably don't actually want to set the pointer to point to address 0x0.
2) That last line would return where your "largest substring" starts, but it doesn't truncate where the characters start to change (i.e. the resulting string isn't necessarily *length long). It can be intentional depending on use case, but figured I'd mention it in case it saves some grief.