Find base period of sequence in C-CodePudding

For a sequence of numbers a1, a2,...,an, we say that there is a period if 1≤p<n and if it holds that it is ai=ai p for all values for which this equality makes sense.

For example, the sequence of numbers 1, 3, 1, 4, 2, 1, 3, 1, 4, 2, 1, 3 has period 5, because ai=ai 5 for all values such that both indices i and i 5 are within the allowable range (i.e. for 1 to 7 inclusive). The same sequence also has a period of 10. Next, we say that the sequence of numbers is periodic if it exists at least one number that is the period of that sequence, with the smallest such number being called the base sequence period. If such a number does not exist, the sequence is not periodic. For example, the above the sequence of numbers is periodic with the base period 5, while the sequence of numbers 4, 5, 1, 7, 1, 5 is not periodic.

#include <iostream>
#include <vector>

int period(std::vector<double> vektor) {
  int p;
  for (int i : vektor) {
    for (int j : vektor) {
      if (vektor[i] == vektor[j])
        p = j;
    }
  }
  return p;
}

int main() {
  std::vector<double> vektor{1, 3, 1, 4, 2, 1, 3, 1, 4, 2, 1, 3};
  std::cout << period(vektor);
  return 0;
}

This should be solved using vector.

Could you help me fix this code? This returns 3 as base period of sequence.

CodePudding user response：

For starters it is unclear why you are using a vector with the value type double instead of the type int when all initializers have the type int.

The function period should except a vector by constant reference.

The variable p is not initialized. As a result the function can return the uninitialized value.

The range based for loop does not return indices in a container as you think

for (int i : vektor) {

It returns stored in the vector objects of the type double.

So the condition in the if statement

if (vektor[i] == vektor[j])

makes no sense.

The function can look the following way as it is shown in the demonstration program below.

#include <iostream>
#include <vector>

size_t period( const std::vector<double> &v )
{
    size_t p = 0;

    for (size_t i = 1; !p && i < v.size(); i  )
    {
        size_t j = 0;

        while (j < v.size() - i && v[j] == v[j   i])   j;

        if ( j   i == v.size() ) p = i;
    }

    return p;
}

int main()
{
    std::vector<double> v = { 1, 3, 1, 4, 2, 1, 3, 1, 4, 2, 1, 3 };
    std::cout << period( v ) << '\n';
}

The program output is