Longest Common Subsequence-Segmentation fault - c

Question

I have to write a program to determine the longest common sub sequence.
Input:
The first argument will be a file that contains two strings per line, semicolon delimited. You can assume that there is only one unique subsequence per test case. e.g.
XMJYAUZ;MZJAWXU
Output:
The longest common subsequence. Ensure that there are no trailing empty spaces on each line you print. e.g.
MJAU
I am using Dev C++ .. And it is compiling Fine!...But this question is a programming challenge and when i submit my answer it's showing me a segmentation fault!
I have written the following code and i am getting a Segmentation Fault where am i wrong?
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
char str1[100],str2[100];
int len1;
int len2;
void printLCS(char b[len1][len2],char str1[],int i,int j)
{
if(i==0 || j==0)
return;
if(b[i][j]=='c')
{
printLCS(b,str1,i-1,j-1);
printf("%c",str1[i-1]);
}
else if(b[i][j]=='l')
printLCS(b,str1,i,j-1);
else
printLCS(b,str1,i-1,j);
}
void Seq(char str1[],char str2[])
{
int i,j;
len1=strlen(str1);
len2=strlen(str2);
int LCS[len1+1][len2+1];
char b[len1][len2];
for(i=0;i<=len1;i++)
{
LCS[i][0]=0;
}
for(j=0;j<=len2;j++)
{
LCS[0][j]=0;
}
for(i=1;i<=len1;i++)
{
for(j=1;j<=len2;j++)
{
if(str1[i-1]==str2[j-1])
{
LCS[i][j]=1+LCS[i-1][j-1];
b[i][j]='c';
}
else if(LCS[i-1][j]>=LCS[i][j-1])
{
LCS[i][j]=LCS[i-1][j];
b[i][j]='u';
}
else
{
LCS[i][j]=LCS[i][j-1];
b[i][j]='l';
}
}
}
printLCS(b,str1,len1,len2);
}
int main(int argc,char *argv[])
{
if(argc!=2)
{
printf("Invalid Number of Arguments:\n");
exit(0);
}
FILE *fp;
fp=fopen(argv[1],"r");
if(fp==NULL)
{
printf("File can't be opened:\n");
exit(0);
}
char c;
c=fgetc(fp);
while(c!=EOF)
{
int k=0;
if(c=='\n')
c=fgetc(fp);
while(c!=';')
{
str1[k]=c;
k++;
c=fgetc(fp);
}
str1[k]='\0';
c=fgetc(fp);
k=0;
while(c!=EOF && c!='\n')
{
str2[k]=c;
k++;
c=fgetc(fp);
}
str2[k]='\0';
Seq(str1,str2);
printf("\n");
if(c==EOF)
{
break;
}
else
c=fgetc(fp);
}
return 0;
}

Answer 1

I dont know system of this site but;
i compiled with no error,
and result was true.
You didnt close file. Maybe memory leak etc. didnt allowed by site.
And, dont use global variables, unless you dont know another solution
this usage is very very bad! ISO C90 forbids this, anyway
int len1;
int len2;
void printLCS(char b[len1][len2]...
good luck.

Answer 2

If you've got access to a Mac or Linux system, there's a fantastic tool called valgrind which can help you track down these kinds of errors: basically it runs your program in a virtual machine and monitors what it reads and writes to memory.
Whilst I can't compile your code, I'm pretty suspicious about this for loop:
for(i=1;i<=len1;i++)
{
for(j=1;j<=len2;j++)
{
if(str1[i-1]==str2[j-1])
{
LCS[i][j]=1+LCS[i-1][j-1];
b[i][j]='c';
}
else if(LCS[i-1][j]>=LCS[i][j-1])
{
LCS[i][j]=LCS[i-1][j];
b[i][j]='u';
}
else
{
LCS[i][j]=LCS[i][j-1];
b[i][j]='l';
}
}
}
Arrays in C and C++ start at 0, so the maximum offset you're interested in is probably strlen - 1. Try changing your for loops to
for(i=1;i<len1;i++)
{
for(j=1;j<len2;j++)
{
...
}
}