Currently I am coding the ALNS algorithm for the vehicle routing problem .

I am stuck in the operator selection phase, I did not understand how I can apply this formula: $$p_i=\frac{f_i}{\sum_{j=1}^Nf_j}$$

Please can someone post some little ALNS java code with the roulette-wheel-selection?


1 Answer 1


The idea behind roulette wheel selection is to use a random number generator to pick an index, so that the probability of index $i$ being selected is your $p_i$. One way to do this is to compute partial sums $s_i = \sum_{j=1}^i p_j$, then generate a random value $u~U(0,1)$ and pick the largest index $i$ for which $u\le s_i$.

Below is a Java function that does it slightly differently. Rather than computing the partial sums, it subtracts each $p_i$ from $u$ until $u$ goes negative, at which point you have the "winning" index. The input vector to the function is your $f$. Since Java uses 0-based indexing, it return an index in $\lbrace 0,\dots,N-1\rbrace$ rather than $\lbrace 1,\dots,N\rbrace$.

   * Selects an input entry using roulette wheel selection,
   * where the probability of an entry being chosen is proportional to its
   * input value.
   * @param input the vector of input values
   * @param rng the random number generator to use for selection
   * @return the 0-based index of the selected entry
  private int roulette(final double[] input, final Random rng) {
    // Sum the input vector.
    double sum = Arrays.stream(input).sum();
    // Calculate probabilities by dividing inputs by the sum.
    double[] prob = Arrays.stream(input).map((x) -> x / sum).toArray();
    // Generate an observation from the U[0,1] distribution.
    double u = rng.nextDouble();
    // Find the first index for which the cumulative sum of the probabilities
    // is greater than the observation. Rather than summing the probabilities,
    // we subtract each probability from the observation until the difference
    // becomes negative.
    for (int i = 0; i < input.length; i++) {
      u -= prob[i];
      if (u < 0) {
        return i;
    // We should never reach this point, but to appease the compiler we need
    // a return value here. It would have to be the highest index.
    return input.length - 1;

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