error: ‘a’ was not declared in this scope for(unsigned i = 0; i < a.size() - 1; i++){ ^

for(unsigned i = 0; i < a.size() - 1; i++){
    int s = (rand() % (a.size() - 1) );
    temp = a.at(i);
    a.at(i) = a.at(s);
    a.at(s) = temp;

 }

could you help me please I get this error message:\ error: ‘a’ was not declared in this scope for(unsigned i = 0; i < a.size() - 1; i++){