What do I need to know before studying this lesson?You should have a knowledge of vectors and vector iterators
What does rotate do?Rotate will shift the values of a vector to the left or right. Example output:
QUOTE("rotate algorithm")
before rotation: 1 2 3 4 5 6 7 8 9
after rotation: 6 7 8 9 1 2 3 4 5
Before we get started, here is the general syntax for the rotate function:
rotate (<vector position to start>, <how many values to shift across>, <where to end>);DON'T BE INTIMIDATED BY THIS, IT REALLY IS SIMPLE!So how do we start?What do we need to include? Well, we need a vector, an algorithm and some I/O (to see what's going on) So:
cpp
#include <iostream>
#include <algorithm>
#include <vector>
Now, what are we going to use from the
std namespace? We need:
cpp
using std::cout; // output the values to see what's happening
using std::endl; // make our output easier to read
using std::cin; // pause for input
using std::vector; // and of course, we need a vector
And then it's on to our
int main() function
First off, we're going to need a vector and an iterator:
cpp
vector <int> myVec; // create a vector called myVec
vector <int> :: iterator it; // create a vector iterator called it
And then we simply need to fill our vector with some values, and output what we have so far:
cpp
// fill myVec with some values
for (int i = 1; i < 10; i++)
myVec.push_back (i);
// output the values before rotation
cout << "Before rotation: ";
for (it = myVec.begin(); it < myVec.end(); ++it)
cout << *it << " ";
Simple enough, right? Now it's time to use the rotate function, to shift our values.
cpp
rotate (myVec.begin(), myVec.begin()+5, myVec.end());
So, this will begin the rotation at the
beginning of the vector, shift it by
5 values, and stop when it reaches the
end of the vector.
Now, let's output the contents of our vector, using our iterator:
cpp
cout << endl; // add a line between our previous output
cout << "After rotation: ";
for (it = myVec.begin(); it < myVec.end(); ++it)
cout << *it;
And that's all there is to it!
Here is the final code, commented: (some people can learn better from just the code)
cpp
#include <iostream> // standard I/O
#include <algorithm> // we will use the rotate algorithm
#include <vector> // we need a vector to rotate!
using std::cout; // output the values to see what's happening
using std::endl; // make our output easier to read
using std::cin; // pause for input
using std::vector; // and of course, we need a vector
int main () // beginning of main
{
vector <int> myVec; // create a vector of ints called myVec
vector <int>::iterator it; // create a vector iterator called it
// now let's fill our vector with some values
for (int i = 1; i < 10; i++) // 9 loops
myVec.push_back (i); // add the value of i to our vector
cout << "Before rotation: "; // before rotation (simple enough!)
for (it = myVec.begin(); it < myVec.end(); ++it) // use it to iterate through myVec
cout << *it << " "; // output it
// now it's time for rotate
// we're going to start at the beginning - myVec.begin()
// we're going to shift the values across by 5 - myVec.begin()+5
// and we're going to stop when we reach the end of the vector - myVec.end()
rotate (myVec.begin(), myVec.begin()+5,myVec.end());
cout << endl; // add an empty line for our next output
cout << "After rotation: "; // after rotation
for (it = myVec.begin(); it < myVec.end(); ++it) // use it to iterate through myVec
cout << *it << " "; // output it
cin.get (); // pause for input
return EXIT_SUCCESS; // everything went OK
}
Using the STL algorithm::rotate()
19 May, 2008 - 04:08 AM
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