View Compilation Error

0_0_6904914_9023.cpp: In function `void modify(int)':
0_0_6904914_9023.cpp:40: error: `a' undeclared (first use this function)
0_0_6904914_9023.cpp:40: error: (Each undeclared identifier is reported only once for each function it appears in.)
0_0_6904914_9023.cpp:40: error: `b' undeclared (first use this function)