View Compilation Error

0_0_17657361_29373.cpp: In function 'int main()':
0_0_17657361_29373.cpp:19:31: error: 'strlen' was not declared in this scope
   for (int i = 0; i < strlen(a); i++)
                               ^
0_0_17657361_29373.cpp:42:31: error: 'strlen' was not declared in this scope
   for (int i = 0; i < strlen(a); i++)
                               ^