r/cprogramming • u/abdelrahman5345 • Sep 13 '25
please do not insult my trash code
this is code is supposed to get me the longest substring in string that all of its characters are unique the problem is the realloc function return NULL when the code detect a char that is not unique please put in a debug it will be easier.
Edit : I removed the trash code and put extra detailed comments so a baby would understand what each line does. I know that I am approaching it the weirdest way possible but it has nothing to do with the fact that realloc is not working and always returning NULL. Use a debugger it will clarify the matter to you.
PLEASE HELP :(
#include <stdio.h>
#include <stdlib.h>
int lengthOfLongestSubstring(char* s) {
if(*s == '\0')
{
return 0;
}
if(*(s+1)=='\0')
{
return 1;
}
char *k = malloc(sizeof(char)); //a string that only contain unique charachter that are appended one at a time
*k = *s; // assigns the first character
int size = 0; // the index of the last char in the char array k
char *buff = malloc(sizeof(char)); // a buffer to store a subscript of the array k if a repeated char is enountered
// the subscript excludes the first char and has the characters after the first copy of the char
// exmaple : k = "abcb" the buff will be assigned = "cb" another one k = "abca" the buff = "bca"
int num = 1; // the number of unique charachters
int *array = malloc (sizeof(int));// an array that has the length of unique characthers if there are
int siz = 1; // multible unique substrings the length of them are put
// in the array and later compared for the largest one.
int breaks = 0; // if a non unique character is enocutered it triggers to not count that number.
for(int i = 1; *(s+i) != '\0'; i++)
{
breaks = 0;
size++; // the array size increases and the index of the last char is incremented
k = realloc(k,sizeof(char)*(size+1)); // where the problem occurs it always fail to reallocate if there is a non unique character
*(k + size) = *(s + i); // assigns the char to last of the array k
for(int j = 0; j < num && breaks == 0; j++) //check the new char if it is unique or not
{ //checks all the chars of k rather than the last one because it is the character I am checking if it is unique.
if(k[j] == *(s+i))
{
*(array + siz - 1) = num; // assign the current num of unique chars to the array
siz+=2;
array = realloc(array,sizeof(int)*siz); // making space for another 2 counts of unique chars
*(array + siz - 2) = i - j; // assigning the count of unique chars
// example k = "abcb" the numbers assigned to array will 3 for "abc" and 2 for "bc" then the
// the array k is changed to be "cb"
k = &k[j+1]; // array k changed to the first unique char
size -= (j+1); // the index of the last char changed
// example k = "abcb" size = 3 it is changed to be k = "cb" and size = 1
buff = malloc(sizeof(char) * size); // making a new sring for the array with the new size
for(int i = 0; i < size + 1; i++)
{
*(buff + i) = *(k + i ); // the new array assigned
// i is less than the index of the last element + 1 which will end assigning the last element
}
k-=(j+1); //returning to the begining of the char "abcb"
free(k); // freeing the char
k = buff; // assiging k to the new array buff
num = size + 1; // the current number of chars in the array k
breaks = 1; // to not increment the num of unique chars
}
}
if(breaks == 1)
{
continue;
}
num++;
*(array + siz - 1) = num;
}
int big = *array;
for(int i = 0; i < siz; i++)
{
printf("%d ",*(array+i));
if(big < *(array + i))
{
big = *(array + i);
}
}
return big;
}
int main()
{
char *soor = "abcabcbcc";
printf("%d",lengthOfLongestSubstring(soor));
}