Common Subtleties in Vector STLs
Last Updated :
19 Dec, 2022
Prerequisite – Vector Basics
Following are some important points that can save time on little things in an interview or an important coding contest.
- vector <int> vect(10) vs vector<int> vect[10]
// Creates a vector vect[] of size 10
vector <int> vect(10)
// creates an array of vectors vect[] of size
// 10 where each vector has int members
vector<int> vect[10]
- resize() and push_back():
After the resize() function has been used on a vector, if push_back() is used on the same vector, the elements being pushed back get added at the end of the resized vector, and not into it.
#include<bits/stdc++.h>
using namespace std;
int main()
{
vector< int > vect;
for ( int i = 0; i < 5; i++)
vect.push_back(i);
vect.resize(10);
for ( int i = 0; i < vect.size(); i++)
cout << vect[i] << " " ;
cout << "\n" ;
vect.push_back(50);
for ( int i = 0; i < vect.size(); i++)
cout << vect[i] << " " ;
return 0;
}
|
Output:
0 1 2 3 4 0 0 0 0 0
0 1 2 3 4 0 0 0 0 0 50
- clear() function It makes the vector to have zero elements, i.e- no elements and not making the elements to all 0s.
- Creating a two dimensional vector
// This doesn't work
vector<vector<int>> vect;
// This works fine
vector< vector <int> > vect;
The difference between these two statements is that the first statement has a space between the angular brackets ( > >). Writing without the space doesn’t work because >> is an operator in C++.
- Passing vectors to functions:
When a vector is simply passed to a function, a copy of the vector is created. This might take a lot of time in cases of large vectors.
#include<bits/stdc++.h>
using namespace std;
void func(vector< int > vect)
{
vect.push_back(30);
}
int main()
{
vector< int > vect;
vect.push_back(10);
vect.push_back(20);
func(vect);
for ( int i=0; i<vect.size(); i++)
cout << vect[i] << " " ;
return 0;
}
|
Output :
10 20
In situations where we don’t actually need to have a copy of the vector, the declaration should be made as follows:
// It is recommended to pass vectors by reference
// wherever possible.
int func(vector<int>& vect)
{
}
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