Week 3

What are we doing today?

  • An introduction to the Standard Template Library
    • Built-in Data Structures
    • Built-in Algorithms
  • Color Detection with the RJRobot

The Standard Template Library (STL)

  • a collection of general purpose containers and algorithms that are included in C++
  • Containers store things; algorithms perform operations on those things
  • Everything we work with is declared under the std namespace.

Containers

  • Data structures in the STL that store a collection of other objects are called containters
  • Examples of Containers include std::set, std::priority_queue, or std::vector. Today we'll look at the latter
  • Containers are parameterized by a template arguments, which dictate what type of data the container stores.

Vectors

  • an object that can contain a variable number of other objects (like ArrayList in Java)
  • you can keep adding elements to the vector
  • order of insertion is maintained
#include <vector>

std::vector<int> numbers = {1, 2, 3};
  • In this example, the vector is passed the type int as its template argument, meaning it can only store type int

Vector methods

at() or [] returns the value at a specific index
front() returns the first item
back() returns the last item
size() returns the number of elements
clear() removes all items
insert() inserts an item at a specific index
push_back() adds a given element to the end
pop_back() removes the last element

Playing with Vectors

#include <vector>
#include <string>
#include <iostream>
using namespace std;
int main() {
      vector<string> trainers = {"Evan 1", "Evan 2", "Jason", "Andrew"};
      cout << "There are: " << trainers.size() << " trainers" << endl;

      trainers.push_back("Dallas");
      trainers.push_back("Woodward");

      cout << "Actually, there are: " << trainers.size() << " trainers" << endl;
}

More vector examples

#include <vector>
#include <string>
#include <iostream>
using namespace std;

int main() {
      vector<string> trainers = {"Evan 1", "Evan 2", "Jason", "Andrew"};
      trainers.push_back("Dallas");
      trainers.push_back("Woodward");

      //Can't forget about the OG trainer. He goes first
      trainers.insert(trainers.begin(), "Matt");
      trainers.insert(trainers.end(), "Sahit");

      //This part is a metaphor for something
      cout << "There are "  << trainers.size() << " trainers" << endl;
      cout << "The 1st trainer is " << trainers.at(0) << endl;
      cout << "The last trainer is " << trainers.at(trainers.size() -1) << endl;
}

Iterators

  • An object that lets you access and modify objects in a container
  • Depending on their type, they will let you read or write data in the container, and move forwards, backwards, or both
  • The containers library includes the most common ones you'll need, but you can also write your own

Iterator methods

begin() starts at the first item and moves forwards when incremented
end() starts at the last item and moves forwards when incremented
rbegin() starts at the last item and moves backwards when incremented
rend() starts at the first item and moves backwards when incremented
  • NOTE: these methods are called on the container
    • i.e. container.begin()

Iterator Methods

XOfZ5kf.png

Iterator operations

* gets the value at the current index
++ increments the iterator forwards
-- decrements the iterator backwards

Iterator practice

vector<int> vec = {66,89,0,60,17,90,8};
vector<int>::iterator it = vec.begin();

MTaVFFM.png 

vector<int> vec = {66,89,0,60,17,90,8};
vector<int>::iterator it = vec.begin();
it++;

gOXGy4i.png 

vector<int> vec = {66,89,0,60,17,90,8};
vector<int>::iterator it = vec.begin();
it++;
it--;

394eVwQ.png 

vector<int> vec = {66,89,0,60,17,90,8};
vector<int>::reverse_iterator it = vec.rbegin();
it += 3;

wrsXiAZ.png 

vector<int> vec = {66,89,0,60,17,90,8};
vector<int>::reverse_iterator it = vec.rbegin();
it += 3;

BMO9nL9.png

Algorithms

  • algorithms is a header that includes useful operators that can be used on vectors, arrays and other containers
  • uses iterators to interact with these containers
  • three cateorgies of algorithm methods
    • does not modify the container
    • modifies the container
    • modifies a destination container

Modifies the container

sort() sorts a container in increasing order
nth_element() finds the nth smallest element
fill() fills a container with copies of a given element
transform() manipulates each element using a function
reverse() Reverses the order of the elements

nth_element example

  • std::nth_element(first, nth, last);
  • first is an iterator to the beginning
  • last is an iterator to the end
  • nth is an iterator to the element you would want if the container were sorted
  • Live example

Algorithm examples

#include <vector>
#include <iterator>
#include <algorithm>

int main()
{
      std::vector<int> v{2, 1, 5, 4, 3};

      //this changes v to {1, 2, 3, 4, 5}
      std::sort(v.begin(), v.end()); 

      //this changes v to {5, 4, 3, 2, 1}
      std::reverse(v.begin(), v.end());
}

Does not modify the container

count() counts the number of items in a container that match a given item
find() returns an iterator to the first element that matches a given item
accumulate() sums all elements in a container

Challenge

  1. Drive over the several gray strips, stopping at the black strip
  2. Determine which strip is the line sensor value
  3. Drive back to the median strip, to show that you have calculated it
  4. Notes
    • Modify the code in median_line
    • You don't know ahead of time how many colored sections there are
    • This is kind of hard