View Compilation Error

0_0_16546410_19733.cpp: In function 'int main()':
0_0_16546410_19733.cpp:16:20: error: 'strlen' was not declared in this scope
        len=strlen(a);  
                    ^
0_0_16546410_19733.cpp:50:33: error: 'strcpy' was not declared in this scope
                    strcpy(b[j],c);  
                                 ^
0_0_16546410_19733.cpp:62:35: error: 'strcmp' was not declared in this scope
                if(strcmp(b[i],b[k])==0 && b[k][0]!='#')  
                                   ^