foo.cpp:15:13: error: empty character constant
 if(a[i][j]=='') {
             ^
foo.cpp: In function 'int main()':
foo.cpp:9:9: warning: unused variable 'k' [-Wunused-variable]
 int i,j,k,l,n,m;
         ^
foo.cpp:9:11: warning: unused variable 'l' [-Wunused-variable]
 int i,j,k,l,n,m;
           ^