View Compilation Error

0_0_13350987_24696.cpp: In function 'int main()':
0_0_13350987_24696.cpp:13:25: error: 'strlen' was not declared in this scope
   for(int i=0;i<strlen(a);i++){
                         ^
0_0_13350987_24696.cpp:19:31: error: 'strlen' was not declared in this scope
   for(int i=k+1,j=1;i<strlen(a);i++){
                               ^