Lecture 15, Thu 08/23

Standard Library and Vectors

Standard Library

• It’s rare if a programming language provides all of the necessary tools to accomplish a task “out of the box.”
  • Programmers usually use provided building blocks to create something specific to fit their needs.
  • Programmers are also able to build additional building blocks to build upon as well.
• These libraries usually come standard with the language
  • You don’t have to download separate components.
  • The libraries should be cross-platform compatible (you shouldn’t have to code differently based on running it with Windows, Mac, or Linux)
• Implementing and maintaining libraries come with a cost
  • Python has a dedicated organization called the Python Software Foundation.
  • Java was developed and maintained by Sun Microsystems, which has been bought by Oracle.
  • C++ isn’t “owned” by anyone really.
• C++ isn’t a product of a large organization and is kinda organized like opensource
  • http://isocpp.org/, http://www.open-std.org/
  • Individual C++ compilers are then implemented based on the specifications.
    • g++ / clang++ for UNIX
    • Visual Studio for Microsoft
  • There isn’t any guarantee that these behave EXACTLY the same, but do for the most part based on the specifications.
  • In this class, we’ll assume we’re using the C++11 specification unless stated otherwise.

std::vector

• So far, we’ve been writing and maintaining arrays on the stack and the heap.
• std::vector is a container class using arrays and is part of the standard library.
• Conceptually, a vector is a sequence of objects that are stored one after the other just like arrays.
  • But all of the maintenance of this container are managed for us (i.e. dynamicaly memory manage arrays under-the-hood).
• Vectors are implemented with templates, so you can store one data type in the vector container.
  • For now, think of defining the template type in the “< >” brackets.
  • Details of templates are outside the scope of this course.

Example

#include<vector>

using namespace std;

int main() {
	vector<int> v; // a vector containing int elements
	return 0;
}

• The syntax defines the data type the vector variable v will store.
• Under-the-hood, vectors are implemented using arrays and behave similar to arrays.
  • Vectors can be indexed starting from 0 to size – 1
• Yet they’re different than arrays…
  • Vectors are dynamically-resizable
  • Vectors have a size() method associated with it.
    • Arrays do not know their size and the programmer must be aware of it.
  • Vectors do not need an initial capacity when defining it.

Example: Adding to vectors

vector<int> v;
for (int i = 0; i < 5; i++) // adding five elements
	v.push_back(i);
}

// Can access elements in vectors with [] similar to arrays
for (int i = 0; i < v.size(); i++) {
	v[i] *= 2;
}

for (int i = 0; i < v.size(); i++) {
	cout << "v[" << i << "] = " << v[i] << endl;
}

• Like arrays, if you index a vector element that is out of range, you will probably get junk data or make your program crash.

Range based loop

• Syntax shortcut for looping through containers in C++.

int arr[] = {1,2,3};

for (int x : arr) { // C++11 range-based loop extension
	cout << x << endl;
}

// Loops through the entire array, note if we change
// arr[] to arr[10], then the range based loop will
// loop through the ENTIRE array structure (even through
// values we did not define in { }).

// also can be used with vectors

int index = 0;
for (int i : v) {
	cout << "v[" << index << "] = " << i << endl;
	index++;
}

Vector Initialization

• push_back is just one way to create elements in a vector.
  • You can declare a vector with a size initially
  • You can also initialize a vector with a size and default values.

vector<int> v1(100); // initializes vector with 100 elements = 0
vector<int> v2(100, 1); //initializes vector with 100 elements = 1

Pointer to Vectors

• You can create a vector structure on the heap and have a pointer point to this.
  • Vectors are considered a type and we can allocate any type on the heap with “new”

vector<int>* v = new vector<int>(10,1);
cout << v->size() << endl;