View Compilation Error

0_0_19811939_19865.cpp: In function 'int main()':
0_0_19811939_19865.cpp:10:11: error: 'i' was not declared in this scope
   if(!((s[i]>='A'&&s[i]<='Z')||(s[i]>='a'&&s[i]<='z')||s[i]=='_')){
           ^