Runtime error in codechef practice questions - c

Question

I'm getting "sigsegv" , a runtime error from the following code when i try to run it on codechef while the code works fine on my computer with various test inputs.I've also kept in mind the constraints given in the problem but I'm still unable to debug it. The question is not from any contest but a practice problem . Please point out any mistake you can find .
Actual codechef question
#include<stdio.h>
int cash[101][101]={0};
int rec[101][2];
int ri=0;
int sumx(int mat[101][101],int i,int j,int lines)
{
int n=0,a=0,b=0;
if(cash[i][j]!=0)
{
return cash[i][j];
}
else if(i==lines-2)
{
n=(mat[i+1][j]>mat[i+1][j+1])?mat[i+1][j]:mat[i+1][j+1];
cash[i][j]=n+mat[i][j];
rec[ri][0]=i;
rec[ri++][1]=j;
return n+mat[i][j];
}
else
{
a=sumx(mat,i+1,j,lines);
b=sumx(mat,i+1,j+1,lines);
n=(a>b)?a:b;
cash[i][j]=n+mat[i][j];
rec[ri][0]=i;
rec[ri++][1]=j;
return n+mat[i][j];
}
}
int main()
{
int i=0,k=0;
int lines=0,n=0;
int r=0;
int tc=0;
int mat[101][101];
scanf("%d",&tc);
while(tc--)
{
scanf("%d",&lines);
i=0;
k=0;
while(i<lines)
{
while(k<=i)
{
scanf("%d",&mat[i][k]);
k++;
}
k=0;
i++;
}
if(lines==1)
{
r=mat[0][0];
}
else
{
r=sumx(mat,0,0,lines);
}
i=0;
while(i<ri)
{
cash[(rec[i][0])][(rec[i][1])]=0;
rec[i][0]=0;
rec[i][1]=0;
i++;
}
ri=0;
printf("%d\n",r);
}
return 0;
}

Answer 1

The error is with lines
while(i<ri)
{
cash[(rec[i][0])][(rec[i][1])]=0;
rec[i][0]=0;
rec[i][1]=0;
i++;
}
the values of rec[i][0] rec[i][1] can be undefined in some cases ie they may return garbage values
You can Use memset instead to change the values to 0
memset(rec,0,sizeof(rec));
memset(cash,0,sizeof(cash));
I ran your solution , there's a bug in your algorithm implementation try finding it yourself
I can provide you a test case(using a testcase generator) for which it fails
21
79
89 28
14 6 63
96 58 67 48
80 8 22 27 8
24 21 23 96 97 72
38 90 95 83 57 60 94
13 96 9 24 65 27 67 40
26 20 58 42 29 8 52 49 37
80 65 65 34 79 10 89 11 20 84
57 59 72 79 51 67 84 70 43 62 96
16 4 18 9 5 40 34 2 15 4 28 50
29 1 60 39 28 92 38 65 95 57 10 71 37
25 78 96 43 17 51 88 19 0 30 20 80 39 35
55 41 63 76 4 20 97 72 43 93 76 11 82 33 25
61 85 41 77 42 90 20 5 69 51 4 54 41 18 83 72
12 56 21 82 7 1 84 26 47 26 22 52 84 39 75 70 89
12 39 83 92 49 20 35 20 31 96 66 75 48 79 13 51 49 50
42 81 0 58 70 40 16 83 27 34 79 64 14 26 19 22 38 55 93
64 81 26 29 47 22 73 61 3 2 61 99 18 43 33 10 13 46 24 53
5 56 0 0 3 0 71 12 82 34 17 11 14 51 1 82 73 53 85 75 89
correct answer is 1431 while your code returns 1299

Answer 2

#Randomizer : The code runs fine with the output as 1431 , not 1299 . Anyways , as you suggested about the garbage being generated from rec[i][0] rec[i][1], It never appeared to me that rec[i][0] rec[i][1] could be generating garbage as I only accessed the used up cells . However , memset eliminated the need of rec , so cash was easily reset to zero . All i did was remove the rec matrix and reset cash using memset and the code ran fine on codechef. I couldn't figure out how it could be generating garbage , still , its accepted on codechef since I removed rec part . Thank you for the help.