processing dynamic arrays

This is a discussion on processing dynamic arrays within the C++ Programming forums, part of the General Programming Boards category; Hello all, While learning about dynamic arrays, I'm asked by C++ primer to do the following exercise: Write a program ...

  1. #1
    (?<!re)tired Mario F.'s Avatar
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    processing dynamic arrays

    Hello all,

    While learning about dynamic arrays, I'm asked by C++ primer to do the following exercise:

    Write a program to read the standard input and build a vector of ints from values that are read. Allocate an array of the same size as the vector and copy the elements from the vector into the array.
    I came up with what you can see below. I would like to know, is it possible to simplify the for loop where I assign to the dynamic array? Specifically, can I do this without creating a pointer to the dynamic array while making sure I do not overflow?

    Code:
    #include <cstdlib>
    #include <iostream>
    #include <vector>
    #include <iterator>
    
    int main()
    {
        using namespace std;
        
        int input;
        vector<int> iv;
    
        while (cin >> input) {
            iv.push_back(input);
        }
        cout << endl;
    
        for (vector<int>::const_iterator i = iv.begin(); i != iv.end(); ++i) {
            cout << *i << "\t";
        }
        cout << endl;
    
        int *p = new int[iv.size()];
        vector<int>::size_type i = 0;
        for (int *pp = p; pp != p + iv.size(); ++pp, ++i) {
            *pp = iv[i];
            cout << *(p + i) << "\t";
        }
        cout << endl;
    
        delete[] p;
    
    
        system("PAUSE");
    	return 0;
    }
    Last edited by Mario F.; 06-04-2006 at 09:49 AM.
    The programmer’s wife tells him: “Run to the store and pick up a loaf of bread. If they have eggs, get a dozen.”
    The programmer comes home with 12 loaves of bread.


    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

  2. #2
    ZuK
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    If you don't know the size of the array at compile time you will have to dynamically allocate it.
    But you could use copy from <algorithm> to fill the array
    Code:
    #include <iostream>
    #include <vector>
    #include <iterator>
    #include <algorithm>
    
    int main()
    {
        using namespace std;
        
        int input;
        vector<int> iv;
    
        while (cin >> input) {
            iv.push_back(input);
        }
        cout << endl;
    
        for (vector<int>::const_iterator i = iv.begin(); i != iv.end(); ++i)
            cout << *i << "\t" ;
        
        cout << endl;
    
        int *p = new int[iv.size()];
        
        copy(iv.begin(), iv.end(), p );
        
        for ( int i = 0; i < iv.size(); ++i )
            cout << p[i] << "\t" ;
        cout << endl;
    
        delete[] p;
    
        return 0;
    }
    Kurt
    Last edited by ZuK; 06-04-2006 at 10:27 AM.

  3. #3
    Registered User Tonto's Avatar
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    Um, you could use array indices instead of the pointer. You could assign things to the array when you are printing out the vector. I don't know. This is kind of frivolous.

  4. #4
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    Not sure if you are asking if you can do it without creating a "dynamic" array or not, but in case you are asking if you can do it via a static array you can simply just do:

    Code:
    #include <vector>
    #include <algorithm>
    #include <iostream>
    
    using namespace std;
    
    int main(void)
    {
    	vector<int> ivec;
    	int input;
    
    	while (cin >> input) {
    			ivec.push_back(input);
    	}
    
    	int iarray[ivec.size()];
    	copy(ivec.begin(), ivec.end(), iarray);
    
    	// test: print
    	for (int i = 0; i < ivec.size(); i++) {
    			cout << iarray[i] << endl;
    	}
    
    	return 0;
    }
    Not sure what your asking though
    The cost of software maintenance increases with the square of the programmer's creativity.

  5. #5
    ZuK
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    Quote Originally Posted by 0rion
    Not sure if you are asking if you can do it without creating a "dynamic" array or not, but in case you are asking if you can do it via a static array you can simply just do
    That works with gnu compiler only. std C++ doesn't allow that.
    compile with
    Code:
    g++ ex.cc --pedantic
    Kurt

  6. #6
    (?<!re)tired Mario F.'s Avatar
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    I reread my question after the frivolous adjective has been given to it.

    You are right, as was Tonto. It's a confusing question. And more so because I didn't even remember that *(p + i) yields an lvalue. As such, the final loop could have been replaced with...

    Code:
        for (vector<int>::size_type i = 0; i != iv.size(); ++i) {
            *(p + i) = iv[i];
            cout << *(p + i) << "\t";
        }
    Much simpler...
    Dunno, I drew a blank or something. Couldn't see that when I was answering the exercise.
    The programmer’s wife tells him: “Run to the store and pick up a loaf of bread. If they have eggs, get a dozen.”
    The programmer comes home with 12 loaves of bread.


    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

  7. #7
    and the hat of wrongness Salem's Avatar
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    > *(p + i) = iv[i];
    Why not
    Code:
    p[i] = iv[i];
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  8. #8
    (?<!re)tired Mario F.'s Avatar
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    hmm... because i'm a newb, that's why

    You are right. Since I allocated it, I didn't remember that I couldn't simply access it by name without dereferencing it.

    Ugh... arrays and pointers are killing me!
    The programmer’s wife tells him: “Run to the store and pick up a loaf of bread. If they have eggs, get a dozen.”
    The programmer comes home with 12 loaves of bread.


    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

  9. #9
    Registered User Tonto's Avatar
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  10. #10
    and the hat of wrongness Salem's Avatar
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    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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