Birthday problem: Difference between revisions

=={{header|C++}}==

{{trans|Java}}

<lang cpp>#include <iostream>

#include <random>

#include <vector>

double equalBirthdays(int nSharers, int groupSize, int nRepetitions) {

std::default_random_engine generator;

std::uniform_int_distribution<int> distribution(0, 364);

std::vector<int> group(365);

int eq = 0;

for (int i = 0; i < nRepetitions; i++) {

std::fill(group.begin(), group.end(), 0);

for (int j = 0; j < groupSize; j++) {

int day = distribution(generator);

group[day]++;

}

if (std::any_of(group.cbegin(), group.cend(), [nSharers](int c) { return c >= nSharers; })) {

eq++;

}

}

return (100.0 * eq) / nRepetitions;

}

int main() {

int groupEst = 2;

for (int sharers = 2; sharers < 6; sharers++) {

// Coarse

int groupSize = groupEst + 1;

while (equalBirthdays(sharers, groupSize, 100) < 50.0) {

groupSize++;

}

// Finer

int inf = (int)(groupSize - (groupSize - groupEst) / 4.0f);

for (int gs = inf; gs < groupSize + 999; gs++) {

double eq = equalBirthdays(sharers, groupSize, 250);

if (eq > 50.0) {

groupSize = gs;

break;

}

}

// Finest

for (int gs = groupSize - 1; gs < groupSize + 999; gs++) {

double eq = equalBirthdays(sharers, gs, 50000);

if (eq > 50.0) {

groupEst = gs;

printf("%d independant people in a group of %d share a common birthday. (%5.1f)\n", sharers, gs, eq);

break;

}

}

}

return 0;

}</lang>

{{out}}

<pre>2 independant people in a group of 23 share a common birthday. ( 50.6)

3 independant people in a group of 87 share a common birthday. ( 50.2)

4 independant people in a group of 186 share a common birthday. ( 50.0)

5 independant people in a group of 313 share a common birthday. ( 50.5)</pre>