View Compilation Error

0_0_34389935_18972.cpp: In function 'int main()':
0_0_34389935_18972.cpp:32:31: error: 'strlen' was not declared in this scope
   for (int i = 0; i < strlen(a); i++)
                               ^
0_0_34389935_18972.cpp:41:30: error: 'strlen' was not declared in this scope
   for (int i = b; i <strlen(a); i++)
                              ^