View Compilation Error

0_0_15115321_27977.cpp: In function 'int main()':
0_0_15115321_27977.cpp:13:28: error: 'strlen' was not declared in this scope
   int carry=0,i=strlen(l[x])-1,j=strlen(r[x])-1;
                            ^
0_0_15115321_27977.cpp:14:16: error: 'j' was not declared in this scope
   for (; i>=0||j>=0; i--,j--,num[x]++) 
                ^