Vectors
NOTE: This article is part of a C++ guide, therefore only C++ code will be displayed.
Vectors
Vectors in C++ are containers that store elements in a certain order. They can have duplicate elements and can change in size.
Initialization
Vectors can be initialized in many ways:
#include <bits/stdc++.h>
using namespace std;
int main(){
// this creates an empty list of numbers
vector<int> numbers;
// this creates a list of 5 numbers with unkown values
vector<long long> other_numbers(5);
// this list has 7 elements which are all set to 100
vector<int> my_grades(7,100);
}
Accessing and changing values
Accessing a vector has the following syntax:
#include <bits/stdc++.h>
using namespace std;
int main(){
vector<int> numbers(5,0); // numbers = {0,0,0,0,0}
// 0-based indexing
numbers[2] = 5; // numbers = {0,0,5,0,0}
cout << numbers[2] << endl; // prints 2
numbers.push_back(7); // adds an element to the end of the list; numbers = {0,0,5,0,0,7}
// numbers.size() is the number of elements in the list
for(int i = 0;i < numbers.size();i++){
// here all the numbers would get printed like this: 0 0 5 0 0 7
cout << numbers[i] << " ";
}
cout << "\n";
}
getting a list of numbers from input
A vast number of problems have an input format similar to the following:
Input consists of two lines. The first has an an integer () denoting the number of elements, while the second line of input contains number of elemements.
Example Input:
5 2 5 10 2 8
There are two ways to get these numbers in vectors:
The first way is by making a list and then adding elements to the end of the list like so:
#include <bits/stdc++.h>
using namespace std;
int main(){
int n;
cin >> n;
vector<int> numbers;
int temporary;
for(int i = 0;i < n;i++){
cin >> temporary;
numbers.push_back(temporary);
}
// solution to the problem
// ...
}
An easier method is to create the list to be elements in length and then use cin to put the elements there directly
#include <bits/stdc++.h>
using namespace std;
int main(){
int n;
cin >> n;
vector<int> numbers(n);
for(int i = 0;i < n;i++){
cin >> numbers[i];
}
// solution to the problem
// ...
}