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#include "stdafx.h"
#include "iostream"
#include <stdlib.h>
#include <string.h>
using namespace std; 
#define SIZE 100

void reset(char temp[]) //clears the array
for (int i=0;i <= strlen(temp);i++)
temp[i] = 0;

void lcs(char str1[], char str2[], char temp[], int x, int y, int i)
if (str1[x]==0)//end of string exit
if (str1[x]==str2[y])//finds similar cells
temp[i]=str1[x];//copy to temp
if ((str1[x+1]!=0) && (str2[y+1]!=0))//checks that this is not the end of the strings and send 
return lcs (str1, str2, temp,x+1 ,y+1 , i+1);//the next cell to compare
else//the x&y are not the same
if (str2[y] == 0)//if the program finished going on the second string it reset the counter using 
return lcs (str1, str2, temp,x+1 ,strlen(temp), i+1);//temp and return the next cell from str1
if (str2[y+1] != 0)//compare x from str1 to y+1 from str2
return lcs (str1, str2, temp, x, y + 1, i);
if (str1[x] == 0)//compare x+1 from str1 to y from str2
return lcs (str1, str2, temp, strlen(temp), y, i);
void main()
char str1[SIZE]={0},str2[SIZE]={0},temp[SIZE]={0}, big[SIZE] = {0};//, biggest2[SIZE] = {0};
cout<<"Please enter the first string:";
cout<<"Please enter the second string:";
lcs(str1, str2, temp, 0, 0, 0);
for(int i=0;i<SIZE;i++)
lcs(str2, str1, temp, 0, 0, 0);
cout<<"The longest common string is:";
if ((strlen(big)) > (strlen(temp)))

Above code returns a, but when the inputs are:

it should return gsc.

Please help me.
I can't find it's problem
Updated 13-May-15 22:26pm
CHill60 14-May-15 4:22am    
"gsc" is not a substring of either of those inputs

gsc is longest common sub sequence (not sub-sting).
Decription and Algorithm.
Algorithm implementation in C++ and several other languages.

But if you are looking for longest common sub-string then a is one of the correct answer.
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CPallini 15-May-15 2:47am    
Ashish Tyagi 40 15-May-15 3:28am    
Thanks :-)
As CHill60 says, it shouldn't return "gsc" at all - gsc isn't a substring of either input.
The LCS of those two inputs is a single character: either "g", "s", or "c" (and possibly "A" if you accept case independence)

A Common Substring is a short string of characters which appears in all the inputs, not a sequence of characters which appear in the same order without considering any extra characters between them.

I'd suggest you need to change your algorithm considerable if you want "gsc" returned - and learn to use the debugger to work out what is goign on while your code is running; you are going to need it! :laugh:
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you named it as "longest common substring"
but your program is just returning the common alphabets of both the strings. Did not debug your code but a quick look suggests that this code correctly returns the common alphabets.
I think your intention is to use dynamic programming. Look at the implementation on this page[^].

If your intent is to find the common substring then another way(data structure) to check for sub string (of any length) is to implement a suffix tree.
Here[^] is a very helpful article regarding this approach.

Also as the Griff has suggested, you need to learn how to use your debugger. Its your best friend after google :)

hope this helps !!
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