C++ :: Incrementing Values In Vector Based On Variable Input

I'm trying to increment the values in a vector, not the vector size, based on variable input. Basically I have a vector of size 10, and all of its values are initialized at zero. The program counts the frequency of numbers 0-9 in a four digit user input. This is what I have (I want it to work so badly but the compiler says that I'm using a pointer to a function used in arithmetic):

for (int i=0; i < num_slots; ++i) {
++guess_frequency[guess[i]];
}

I just want to know if you can increment values within a vector:

E.g.
change
0 0 0 0 0 0 0 0 0 0
to
1 0 0 0 2 0 0 1 0 0