How to run part of the code with a specified probability?

Question

I have a variable cnt, whose value is checked via an if/else statement as follows:
If cnt<=2, then call func
Else if cnt > 2, then call func with the probability of P=3/(2*cnt).
How can I implement this probability-based execution of code in C++?

void func() {
    sendMsg();
}

You generate a random double `D` between 0 and 1, and execute the code if `P` header and many examples in the internet. — deviantfan, Jan 22 '16 at 05:28
You can use rand() function to generate random number between 1 and 0 and check if it satisfies your condition to run the function. — sunkuet02, Jan 22 '16 at 05:29
PS: Do NOT use `rand()`. It's not really random or near it. It's just easy to use, but the values you get are bad. — deviantfan, Jan 22 '16 at 05:29
@LưuVĩnhPhúc I did know how to generate random numbers. What I did not know was how to use random numbers to follow a probability. — Mehdi Haghgoo, Jan 22 '16 at 06:26
related http://stackoverflow.com/questions/9432226/how-do-i-select-a-range-of-values-in-a-switch-statement — Ciro Santilli OurBigBook.com, Feb 19 '17 at 18:16

score 6 · Accepted Answer · answered Jan 22 '16 at 05:36

6

Use something like std::uniform_real:

#include <random>

std::random_device rd;
std::mt19937 gen(rd());
std::uniform_real_distribution<> dis(0, 1);

void func() {
    // Probability 0.3
    if(dis(gen) < 0.3)
        sendMsg();
}

answered Jan 22 '16 at 05:36

Ami Tavory

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Are these classes for C++? My compiler does not seem to understand std::random_device and std::uniform_real_distribution. – Mehdi Haghgoo Jan 22 '16 at 10:04
You should find out how to get your compiler to use C++11 (which is the current mainstream version). Which compiler is it? – Ami Tavory Jan 22 '16 at 10:13
actually, I am typing just make to build the project after modification, but `gcc --version` reports `gcc (GCC) 5.1.1 20150618 (Red Hat 5.1.1-4)` – Mehdi Haghgoo Jan 22 '16 at 10:42
@JasonStack OK, so try adding `--std=c++11` to your gcc command line invocation. – Ami Tavory Jan 22 '16 at 11:20
Thank you Ami. I am modifying file aodv.cc of ns-2 c++ code and have to use `make` to build it. I added `CXXFLAGS=-g -std=c++11 -Wall -pedantic` as given in [this post](http://stackoverflow.com/questions/16886591/how-do-i-enable-c11-in-gcc) to [Makefile.in](http://nsnam.cvs.sourceforge.net/viewvc/nsnam/ns-2/Makefile.in?revision=1.321&view=markup) in the project directory, but it doesn't work. – Mehdi Haghgoo Jan 22 '16 at 12:03
@JasonStack Pretty sure it should be `--std=c++11` (note two `-` chars). Just tried building it by me, and it built fine. If it doesn't build for you, I suggest you start a separate question about getting contemporary C++ (C++11) working with your build setting. – Ami Tavory Jan 22 '16 at 12:08

score 2 · Answer 2 · edited May 23 '17 at 12:00

std::map::lower_bound + partial_sum + C++11 lambdas

Here is a generalization where you can easily give a probability for each lambda.

#include <functional>
#include <iostream>
#include <map>
#include <numeric>
#include <random>
#include <vector>

int main() {
    typedef std::pair<double, std::function<void()>> P;
    auto v = std::vector<P> {
        {0.2, [](){std::cout << "a" << std::endl;}},
        {0.5, [](){std::cout << "b" << std::endl;}},
        {0.3, [](){std::cout << "c" << std::endl;}}
    };
    std::partial_sum(v.begin(), v.end(), v.begin(),
        [](const P& x, const P& y){return P(x.first + y.first, y.second);}
    );
    auto m = std::map<P::first_type, P::second_type>(v.begin(), v.end());
    std::random_device r;
    std::mt19937 prng(r());
    std::uniform_real_distribution<> dist(0.0, 1.0);
    for (auto i = 0u; i < 10u; ++i) {
        auto r = dist(prng);
        std::cout << r << std::endl;
        m.lower_bound(r)->second();
        std::cout << std::endl;
    }
}

Sample output:

Related: How do I select a range of values in a switch statement?

How to run part of the code with a specified probability?

2 Answers2

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