View Compilation Error

0_0_35234130_21915.cpp: In function 'int main()':
0_0_35234130_21915.cpp:24:78: error: 'ch' was not declared in this scope
         while(op != 'I'&&op != 'D'&&op != 'L'&&op != 'R'&&op != 'Q') op = ch();
                                                                              ^