Multimap

[workman845]

I'm working on a project right now where my program reads a text file, and encodes the text file with the character locations from another text file.

Right now I have a multimap, that inputs the characters from the text file, and there locations. What I want to do now is randomly pick one of the numbers from the list of numbers that are related to that character.

For example:
A:0
A:26
A:48
A:128

I want to somehow randomly pick one of those numbers.
Any help would be greatly appreciated.

[chris]

I suggest that you load all those values into an array - of integers, for example - once that is done generate a random number, modulus it with the size of your array & then that is the number to choose, for example:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main()
{
  int myArray[] = {0, 26, 48, 128};
  srand(time(NULL));
  printf("%d\n", myArray[rand() % 4]);
  return 0;
}

Rand reference: http://www.cplusplus.com/reference/clibrary/cstdlib/rand/
Srand reference: http://www.cplusplus.com/reference/clibrary/cstdlib/srand/

[jsmith]

You can:

// Assume:
std::multimap<char, int> m;

std::multimap<char, int>::const_iterator i( m.begin() );
std::advance( m, rand() % m.size() );
std::cout << "The number is " << i->second << std::endl;

(Note that std::advance() is O(n) complexity in this case, though not worse
than converting the data structure to an array first).

[workman845]

I'm trying to do something like the Array but I'm getting an error how would I go about getting this to work?

int myArray[] = bookMap.equal_range('a');
			srand(time(NULL));
			printf("%d\n", myArray[rand() % (int)bookMap.count('a')]);

I imagine you can see what I'm trying to do, get the numbers that correspond to a into the array, would creating a for loop, and iterating over the equal range then putting the *it into the array work?

[jsmith]

workman845: I gave you a complete solution that doesn't require copying the
data into an array. I suggest you use it and forget the array approach.

[workman845]

I've never heard/used "advance" before, I'm getting errors, and I don't know why.

[workman845]

This is the code I tried to use

std::multimap<char, int>::const_iterator it( bookMap.begin() );
			std::advance( bookMap, rand() % bookMap.size() );
			std::cout << "The number is " << it->second << std::endl;

[workman845]

Okay, I figured it out, but it's never random, it just seems to use the last element of that type.

multimap<char,int>::iterator a; a = bookMap.equal_range('a').first;
			advance( a, rand() % (int)bookMap.count('a') );
			cout << "The number is " << a->second << endl;

[jsmith]

Did you call srand() in your program to seed the rng?

[workman845]

Oops! haha works now thank you very much.