View Compilation Error

0_0_13230787_21754.cpp: In function 'void get_next(char*)':
0_0_13230787_21754.cpp:26:53: error: 'ex' was not declared in this scope
             if( a[ nex[i]+1 ] == a[ nex[i]+i ] && i+ex[i] <= len ) nex[i]++;
                                                     ^