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L8.Functions and Algorithms

约 277 个字 97 行代码 预计阅读时间 2 分钟

Algorithms

Algorithms we will explore:

  1. std::sort
  2. std::nth_element
  3. std::stable_partition
  4. std::copy_if
  5. std::remove_if

Find the median course rating

If we want to calculate the median course rating, we have two ways to do it:

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struct Course {
    string code,
    double rating;
};

auto compRating = [](const auto& s1, const auto &s2) {
    return s1.rating < s2.rating;
};

size_t size = classes.size();

// O(N log N) sort
std::sort(classes.begin(), classes.end(), compRating);

Course median = classes[size/2];
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struct Course {
    string code,
    double rating;
};

auto compRating = [](const auto& s1, const auto &s2) {
    return s1.rating < s2.rating;
};

size_t size = classes.size();

// O(N) sort, sorts so nth_element is in correct position
// all elements smaller to left, larger to right
Course median = *std::nth_element(classes.begin(), classes.end(), size/2, compRating);

Stable Partition

Imagine we have a vector of strings, we can use std::stable_partition to partition the vector into two groups(for example cs courses and non-cs courses) without breaking their relative order.

CS61A CS106L Math Physics English CS61B

After calling std::stable_partition, we will get two groups:

CS61A CS106L CS61B Math Physics English

And std::stable_partition will return an iterator pointing to the first element of the second group.

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string dep = "CS";
auto isDep = [dep](const auto& course) {
    return course.name.size() >= dep.size() && course.substr(0, dep.size()) == dep;
};

auto iter = std::stable_partition(courses.begin(), courses.end(), isDep);

courses.erase(iter, courses.end());

Copy

What if we want to copy all the cs courses to another vector?

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string dep = "CS";
auto isDep = [dep](const auto& course) {
    return course.name.size() >= dep.size() && course.substr(0, dep.size()) == dep;
};

std::cpy_if(courses.begin(), courses.end(), csCourses, isDep);

Sometimes the code above doesn't work beacuse the space of the vector csCourses is not enough, we may write into unintialized memory.

So we need a special iiterator to solve this problem, std::back_inserter.

It will expand the vector when needed.

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string dep = "CS";
auto isDep = [dep](const auto& course) {
    return course.name.size() >= dep.size() && course.substr(0, dep.size()) == dep;
};

std::cpy_if(courses.begin(), courses.end(), back_inserter(csCourses), isDep);

Stream Iterators

Look at the code below:

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string dep = "CS";
auto isDep = [dep](const auto& course) {
    return course.name.size() >= dep.size() && course.substr(0, dep.size()) == dep;
};

std::copy_if(csCourses.begin(), csCourses.end(),
std::ostream_iterator<Course>(cout, "\n"), isDep);

We can use std::ostream_iterator to write the elements to the output stream.

Remove

One thing to note that std::remove_if doesn't actually remove the elements, we can check it by the code below:

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#include <iostream>
#include <vector>

using namespace std;

int main()
{
  auto isLessThan5 = [](int x) { return x < 5; };

  vector<int> v = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
  cout << "Before remove the size:" << v.size() << endl;
  for (auto i : v)
  {
    cout << i << " ";
  }
  cout << endl;
  remove_if(v.begin(), v.end(), isLessThan5);
  cout << "After remove the size:" << v.size() << endl;
  for (auto i : v)
  {
    cout << i << " ";
  }
}

We get the output:

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Before remove the size:10
1 2 3 4 5 6 7 8 9 10
After remove the size:10
5 6 7 8 9 10 7 8 9 10

We can clearly see that the size of the vector doesn't change and the elements are just moved forward.

So we need to use std::erase to remove the elements.

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#include <iostream>
#include <vector>

using namespace std;

int main()
{
  auto isLessThan5 = [](int x) { return x < 5; };

  vector<int> v = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

  cout << "Before remove the size:" << v.size() << endl;
  for (auto i : v)
  {
    cout << i << " ";
  }
  cout << endl;

  v.erase(remove_if(v.begin(), v.end(), isLessThan5), v.end());

  cout << "After remove the size:" << v.size() << endl;
  for (auto i : v)
  {
    cout << i << " ";
  }
  cout << endl;
}

We get the output:

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Before remove the size:10
1 2 3 4 5 6 7 8 9 10
After remove the size:6
5 6 7 8 9 10

We have correctly removed the elements.

Why does std::remove_if not remove the elements?

Because the Algorithm remove_if is not a member of std::vector (or any other collection). So it doesn't have the ability to remove elements from the vector.

Must be used in conjunction with std::erase to remove the elements.

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