Changing Arrays To Vectors!

**JM1082** · 08-14-2011

Hi all,

I'm teaching myself C++ from a textbook ( C++ HOW TO PROGRAM by Deitel&Deitel ) but after successfully creating a program using C style arrays I'm having trouble converting it into vetors.

Any help much appreciated!!!

Please see both programs code below:

Code:

// recursiveFindMinimum.cpp ( WITH ARRAYS )




#include <iostream>
using namespace std;



const int arraySize = 5;



void initializeArray( int[] ); // to ask the user for numbers
int findMinimum( int[], int, int ); // to recursively find the smallest number



int main()
{

    cout << "Welcome!  This program recursively locates the smallest number in an array!" << endl;

    int array1[ arraySize ];
    int startSubscript = 0;

    initializeArray( array1 );

    int smallest = array1[ 0 ];

    smallest = findMinimum( array1, startSubscript, smallest );

    cout << "\nYour smallest number was: " << smallest << "!" << endl;

} // end main



// to recursively find the smallest number
int findMinimum( int array[], int counter, int smallest )
{

    while ( counter < arraySize )
    {

        if ( smallest > array[ counter ] )
        {

            smallest = array[ counter ];

        } // end if

        counter++;

        findMinimum( array, counter, smallest );

    } // end while

    return smallest;

} // end function findMinimum



// to ask the user for numbers
void initializeArray( int array[] )
{

    cout << endl;

    for ( int i = 0; i < arraySize; i++ )
    {

        cout << "Enter a number into the array: ";
        cin >> array[ i ];

    } // end for

} // end function initializeArray

Code:

// recursiveFindMinimumWithVectors.cpp ( WITH VECTORS )




#include <iostream>
#include <vector>
using namespace std;



const int arraySize = 5;



void initializeArray( vector < int > ); // to ask the user for numbers
int findMinimum( vector < int >, int, int ); // to recursively find the smallest number



int main()
{

    cout << "Welcome!  This program recursively locates the smallest number in an array!" << endl;

    vector < int > array1( arraySize );
    int startSubscript = 0;

    initializeArray( array1 );

    int smallest = array1[ 0 ];

    smallest = findMinimum( array1, startSubscript, smallest );

    cout << "\nYour smallest number was: " << smallest << "!" << endl;

} // end main



// to recursively find the smallest number
int findMinimum( vector < int > array, int counter, int smallest )
{

    while ( counter < arraySize )
    {

        if ( smallest > array[ counter ] )
        {

            smallest = array[ counter ];

        } // end if

        counter++;

        findMinimum( array, counter, smallest );

    } // end while

    return smallest;

} // end function findMinimum



// to ask the user for numbers
void initializeArray( vector < int > array )
{

    cout << endl;

    for ( int i = 0; i < arraySize; i++ )
    {

        cout << "Enter a number into the array: ";
        cin >> array[ i ];

    } // end for

} // end function initializeArray

**King Mir** · 08-14-2011

What exactly is the problem?

It looks like you're program runs in O(n^2) time when it should be O(n), because your looping and recurring through the array. Just use a loop; no need for recursion here.

**grumpy** · 08-14-2011

All you've done is change the types of the declarations from arrays (or pointers) into vector<int> without any further thought. There is much more needed than that: starting with some thought on your part. You might also actually want to read the documentation for the vector, such as this link.

If you want to pass a vector to a function, and change the vector passed, pass it by reference. Passing it by value (which you are) passes a copy of the vector, so the changes are not visible to the caller.

Finding the minimum value of an array using recursion is not a particularly good method. It is a really bad method when you are passing vectors by value around.

**AndrewHunter** · 08-17-2011

@OP: If you are learning C++, you should learn about the standard library.

Code:

#include <iostream>
#include <vector>
#include <algorithm>

int main(void){

	//this would be user input
	int userints[] = {5,3,40,25,6,1,34};
	
	std::vector<int>myVector(userints,userints+7);
	std::vector<int>::iterator ii;

	std::cout<<"Before sort:\n";
	for(ii=myVector.begin();ii!=myVector.end();ii++)
		std::cout<<(*ii)<<" ";

	std::sort(myVector.begin(),myVector.end());
	std::cout<<"\nAfter sort:\n";
	for(ii=myVector.begin();ii!=myVector.end();ii++)
		std::cout<<(*ii)<<" ";

	return(0);
}

**laserlight** · 08-17-2011

Originally Posted by AndrewHunter

Code:

for(ii=myVector.begin();ii!=myVector.end();ii++)

It probably won't matter because vector iterators may well be pointers, but generally, prefer pre-increment to post-increment since post-increment may be less efficient than pre-increment. Actually, since <algorithm> has already been included, I might write instead:

Code:

#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>

int main() {
    //this would be user input
    int userints[] = {5, 3, 40, 25, 6, 1, 34};

    std::vector<int> myVector(userints,
                              userints + sizeof(userints) / sizeof(userints[0]));

    std::cout << "Before sort:\n";
    std::copy(myVector.begin(), myVector.end(),
              std::ostream_iterator<int>(std::cout, " "));
    std::sort(myVector.begin(),myVector.end());
    std::cout<<"\nAfter sort:\n";
    std::copy(myVector.begin(), myVector.end(),
              std::ostream_iterator<int>(std::cout, " "));
    std::cout << std::endl;
}

**AndrewHunter** · 08-17-2011

@Laser: Thanks for the note on prefix operators. As for your solution, that honestly didn't even cross my mind; thanks for the improvements.

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