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5.3. Vectors

Vectors are much more similar to Python lists than arrays are. Vectors use a dynamically allocated array to store their elements, so they can change size, and they have other friendly features as well. Because they use a dynamically allocated array, they use contiguous storage locations which means that their elements can be accessed and traversed, and they can also be accessed randomly using indexes. However, vectors are dynamically sized, so their size can change automatically. A new element can be inserted into or deleted from any part of a vector, and automatic reallocation for other existing items in the vector will be applied. Unlike Python lists, vectors are homogeneous, so every element in the vector must be of the same type.

Vectors are a class that is available through a library called the Standard Template Library (STL), and one uses a < > notation to indicate the data type of the elements. In order to use vectors, One needs to include the vector library.

#include <vector>

Common C++ Vector Operators

Vector Operation	Use	Explanation
[ ]	myvector[i]	access value of element at index i
=	myvector[i]=value	assign value to element at index i
push_back	myvect.push_back(item)	Appends item to the far end of the vector
pop_back	myvect.pop_back()	Deletes last item (from far end) of the vector
insert	myvect.insert(i, item)	Inserts an item at index i
erase	myvect.erase(i)	Erases an element from index i
size	myvect.size()	Returns the actual size used by elements
capacity	myvect.capacity()	Returns the size of allocated storage capacity
reserve	myvect.reserve(amount)	Request a change in capacity to amount

A very common programming task is to grow a vector using the push_back() method to append to the vector as we see in the next example. Because vectors can change size, vectors typically allocate some extra storage to accommodate for possible growth. Thus the vector typically has an actual capacity greater than the storage size strictly needed to contain its elements.

5.3.1. Iterating through Vectors

When iterating vectors, you must first find the length of your container. You can simply call the .length() function. For arrays, the number of elements can be found by getting the size in memory of the array by using the sizeof() function, and then dividing it by the size of the first element of the array using the same sizeof() function. Because all elements in C++ arrays are the same type, they take the same amount of space and that can be used to find the number of elements the Array contains!

#include <iostream> using namespace std; int main() { int nums[] = {1,3,6,2,5}; //Divide the size of the array (in bytes) by the size of a single element (in bytes) // to get the total number of elements in the array. int numsSize = sizeof(nums)/sizeof(nums[0]); // Get size of the nums array for (int index=0; index<numsSize; index++) { cout << nums[index] << endl; } // Simpler Implementation that may only work in // Newer versions of C++ // for (int item:nums) { // cout << item << endl; // } return 0; }

An optional secondary version of the for loop has been commented out of the above code. You can try running this in your version of C++ to see if it works, but in some older versions of C++, such as C++98, it does not.

The above loop assigns the variable index to be each successive value from 0 to numsSize.

Then, the value at that index in the array is printed to the console.

5.3.1.1. Matching

Q-2: Match the Vector operations with their corresponding explination. Feedback shows incorrect matches.
• [ ]
• Accesses value of an element.
• =
• Assigns value to an element.
• push_back
• Appends item to the end of the vector.
• pop_back
• Deletes last item of the vector.
• insert
• Injects an item into the vector.
• erase
• Deletes an element from the choosen index.
• size
• Returns the actual capacity used by elements.
• capacity
• Returns the ammount of allocated storage space.
• reserve
• Request a change in space to amount

// function that uses a vector to square // every number from 0 to 49 // uses the reserve operation to save space in memory #include <iostream> #include <vector> using namespace std; int main(){ vector<int> intvector; intvector.reserve(50); for (int i=0; i<50; i++){ intvector.push_back(i*i); cout << intvector[i] << endl; } return 0; }

"""Uses a list to square every number from 0 to 49 """ def main(): intlist=[] for i in range(50): intlist.append(i*i) print(intlist[i]) main()

In the above example, the use of reserve was optional. However, it is a good idea to use it before growing a vector in this way because it will save time. Because vectors are stored in underlying arrays which require contiguous memory, every time the vector’s size gets too large for the capacity, the entire vector must be moved to a larger location in memory, and all that copying takes time. In a typical implementation, the capacity is doubled each time. as in the example that follows.

// function that uses a vector to square // every number from 0 to 49 // and does not use reserve. // shows amount of space used #include <iostream> #include <vector> using namespace std; int main(){ vector<int> intvector; // without intvector.reserve(50); for (int i=0; i<50; i++){ intvector.push_back(i*i); cout << intvector[i] << endl; cout << "capacity: " << intvector.capacity() << endl; } return 0; }

Remembering that C++ is designed for speed, not protection, we will likely not be surprised by the following:

// Note: counting always starts at 0 // This demonstrates what happens when // accessing datat outside of your vector #include <iostream> #include <vector> using namespace std; int main(){ vector<int> intvector; intvector.reserve(10); for (int i=0; i<10; i++){ intvector.push_back(i); } for (int i=0; i<=10; i++){ cout << "intvector[" << i << "]=" <<intvector[i] << endl; } return 0; }

"""Demonstrates python's protections against iterating outside of a list""" def main(): intlist=[] for i in range(10): intlist.append(i) for i in range(11): print("intlist[" + str(i) + "]=" + str(intlist[i])) main()

Q-8: Which of the following is the biggest difference between a C++ array and a C++ vector?

• Vectors can change size.
• Right! Good job!
• Vectors offer all of the features and protections of Python lists
• Not all of the protections of lists are offered by vectors; one can still iterate off of either end.
• Vectors don't use contiguous memory, so elements can be inserted.
• No. Although elements can be inserted in vectors, they do require contiguous memory.
• more than one of the above
• No. Only one of the above is correct.
• none of the above
• One of the above is indeed correct.

Q-9: What good is the reserve method in a vector?

• Nothing. It is completely optional.
• It is optional but it does serve a purpose. Try again.
• Using it will save time if you know the maximum size needed.
• Right!
• It is required so memory can be allocated.
• It is not required.
• none of the above
• One of the above is indeed correct.

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