A vector, once declared, has all its values initialized to zero. Following is an example code to demonstrate the same.
// C++ program for displaying the default initialization // of the vector vect[] #include<bits/stdc++.h> using namespace std; int main() { // Creating a vector of size 8 vector< int > vect(8); // Printing default values for ( int i=0; i<vect.size(); i++) cout << ' ' << vect[i]; } |
Output :
0 0 0 0 0 0 0 0
What if we wish to initialize the vector to a specific value, say 1 ? For this, we can pass the value along with the size of the vector.
// C++ program for displaying specified initialization // of the vector vect[] #include<bits/stdc++.h> using namespace std; int main () { // Creates a vector of size 8 with all initial // values as 1. vector< int > vect(8, 1); for ( int i=0; i<vect.size(); i++) cout << ' ' << vect[i]; } |
Output :
1 1 1 1 1 1 1 1
What if we wish to initialize the first 4 values to say 100 and rest 6 values as 200 ?
One way to do this is to manually provide a value to each position in the vector. The other methods as provided in STL, the Standard Template Library, are fill and fill_n.
- fill()
The ‘fill’ function assigns the value ‘val’ to all the elements in the range [begin, end), where ‘begin’ is the initial position and ‘end’ is the last position.NOTE : Notice carefully that ‘begin’ is included in the range but ‘end’ is NOT included. Below is an example to demonstrate ‘fill’ :
// C++ program to demonstrate working of fill()
#include <bits/stdc++.h>
using
namespace
std;
int
main ()
{
vector<
int
> vect(8);
// calling fill to initialize values in the
// range to 4
fill(vect.begin() + 2, vect.end() - 1, 4);
for
(
int
i=0; i<vect.size(); i++)
cout << vect[i] <<
" "
;
return
0;
}
Output :
0 0 4 4 4 4 4 0
- fill_n()
In fill_n(), we specify beginning position, number of elements to be filled and values to be filled. The following code demonstrates the use of fill_n.// C++ program to demonstrate working of fil_n()
#include <bits/stdc++.h>
using
namespace
std;
int
main()
{
vector<
int
> vect(8);
// calling fill to initialize first four values
// to 7
fill_n(vect.begin(), 4, 7);
for
(
int
i=0; i<vect.size(); i++)
cout <<
' '
<< vect[i];
cout <<
' '
;
// calling fill to initialize 3 elements from
// "begin()+3" with value 4
fill_n(vect.begin() + 3, 3, 4);
for
(
int
i=0; i<vect.size(); i++)
cout <<
' '
<< vect[i];
cout <<
' '
;
return
0;
}
Output :
7 7 7 7 0 0 0 0 7 7 7 4 4 4 0 0
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