random writing Markov Model efficiency - algorithm

Question

Here is my implementation
However, it is a bit slow when analyzing the textfile,
Anyone have a better idea or better data structure to implement Random writing?
Im not using the STL library so dun worry about the syntax.
instead of using push_back, vector here is using .add
randomInteger will generate randome integer between ranges
I would like to produce 2000 character if possible;
I think the slowest part is reading the file char by char?
void generateText(int order, string initSeed, string filename){
Map<string , Vector<char> > model;
char ch;
string key;
ifstream input(filename.c_str());
for(int i = 0; i < order; i++){
input.get(ch);
key+=ch;
}
while(input.get(ch)){
model[key].add(ch);
key = key.substr(1,key.length()-1) + ch;
}
string result;
string seed = initSeed;
for(int i = 0;i<2000;i++){
if (model[seed].size() >0) {
ch = model[seed][randomInteger(0, model[seed].size()-1)];
cout << ch;
seed = seed.substr(1,seed.length()-1) + ch;
}
else
return;
}
}

Answer 1

You need to determine that it is taking too long. (How is this code not running in less than a second on an average laptop?)
If it is, you need to profile.
For example, a likely candidate is the cost of generating random numbers...
You'll only disprove me by profiling ;)

Answer 2

I think it is a bit slow because it creates lots of temporary strings during the analysis phase.
for(int i = 0; i < order; i++){
input.get(ch);
key+=ch; // key = key + ch, at least one new string created
}
while(input.get(ch)){
model[key].add(ch); // key copied to hash table
key = key.substr(1,key.length()-1) + ch; // a couple of temp strings created
}
You could do instead like this:
char key[order + 1]; // pseudo code, won't work because order is not constant
key[order] = 0; /* NUL terminate */
for (int i = 0; i < order; i++) {
input.get(key[i]);
}
while (!(input.eof())) {
for (int j = 0; j < order - 1; k++) {
key[j] = key[j + 1];
}
input.get(key[order]);
model[key].add(ch);
}
Here the only string that is actually created is the string that ends up as a key in the hash table. The key is rotated in a simple character array, avoiding string temporaries.