Passing Vector as a Reference

This is a discussion on Passing Vector as a Reference within the C++ Programming forums, part of the General Programming Boards category; I am having a problem passing a vector as a reference parameter and can not figure out why the program ...

  1. #1
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    Passing Vector as a Reference

    I am having a problem passing a vector as a reference parameter and can not figure out why the program is not compiling or working. It is meant to read in text from a file, manipulate the vector into a new one, then print out the new one to a file. Everything is working except for the method that manipulates the vector. The syntax seems correct from reading several websites which is what confuses me. I have a header file but there is nothing wrong with it. Any hints are greatly appreciated!

    Code:
    #include <iostream>
    #include <fstream>
    #include <vector>
    #include "hw1.h"
    
    using namespace std;
    
    int main() {
    	int x;
    	vector<int> first;
    	vector<int> second;
    	ifstream inFile;
    	inFile.open("input.txt");
    		while (inFile >> x) {
    			first.push_back(x);
    		}
    		inFile.close();
    		method(first, second);
    		
    	ofstream outFile;
    	outFile.open("output.txt");
    	for (int i = 0; i < second.size(); i++) {
    		outFile << second[i] << endl;
    	}
    		outFile.close();
    		return 0;
    		
    }
    
    void method(vector<int>& x, vector<int>& y) {
    	int step = 0;
    	int j = 1;
    	vector<int> temp(x.size() / 2);
    	for (int i = 0; i < x.size() / 2; i++) {
    		y[i] = x[step] + x[j];
    		j+= 2;
    		step+= 2;
    	}
    }

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    What doesn't work about it (assuming hw1.h contains a legitimate prototype of the function)?

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    hw1.h:8: error: variable or field ‚addEven2OddLines‚ declared void
    hw1.h:8: error: ‚vector‚ was not declared in this scope
    hw1.h:8: error: expected primary-expression before ‚int‚
    hw1.h:8: error: ‚vector‚ was not declared in this scope
    hw1.h:8: error: expected primary-expression before ‚int‚
    hw1prob2.cpp: In function ‚int main()‚:
    hw1prob2.cpp:15: error: request for member ‚push_back‚ in ‚first‚, which is of non-class type ‚std::vector<int, std::allocator<int> >()‚
    hw1prob2.cpp:18: error: ‚addEven2OddLines‚ was not declared in this scope
    hw1prob2.cpp:22: error: request for member ‚size‚ in ‚second‚, which is of non-class type ‚std::vector<int, std::allocator<int> >()‚
    hw1prob2.cpp:23: warning: pointer to a function used in arithmetic


    the header file is:

    Code:
    #ifndef HW1_H
    #define HW1_H
    
    template <class T>
    void make2dArray(T ** &x, int numberOfRows, int rowsize[]);
    template <class T>
    void freeGen2dArray(T ** &x, int numberOfRows);
    void method(vector<int>& x, vector<int>& y);
    
    #endif
    the method signatures match so they should not cause a compiling error.

  4. #4
    and the Hat of Guessing tabstop's Avatar
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    No such thing as vector. There is such a thing as std::vector, though.

  5. #5
    Algorithm Dissector iMalc's Avatar
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    Your vector 'y', otherwise known in main as 'second' is of zero length, so writing to y[i] is a buffer overrun.

    Are you sure it's 'temp' you want to resize to x.size()/2 and not 'y'? Afterall, you aren't using 'temp' anywhere. Or perhaps you meant to write to 'temp' and swap 'temp' and 'y' afterwards.
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    The reason for the compilaton errors is that #include "hw1.h" precedes the "using namespace std;". So, when compiling the contents of hw1.h, the compiler cannot find the vector class.

    It is generally VERY bad practice have a header file rely on a "using namespace" directive. So change the references to vector<int> in hw1.h to std::vector<int>.

    It would also be advisable that hw1.h include <vector> before the function declarations. That makes it self-contained.
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    Thumbs up

    Thank my for the advice about including <vector> in the header class regarding the method signature. I changed that one simple line and the program compiled perfectly and executed flawlessly. C++ is so different in many aspects from Java but so similar in others.

  8. #8
    Algorithm Dissector iMalc's Avatar
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    Quote Originally Posted by gbman88 View Post
    C++ is so different in many aspects from Java but so similar in others.
    IMHO if you're learning C++, the less you know about Java, the better!
    My homepage
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  9. #9
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    Quote Originally Posted by iMalc View Post
    IMHO if you're learning C++, the less you know about Java, the better!
    The reverse is also true, unless you're a C++ programmer who is a glutton for punishment
    Right 98% of the time, and don't care about the other 3%.

    If I seem grumpy or unhelpful in reply to you, or tell you you need to demonstrate more effort before you can expect help, it is likely you deserve it. Suck it up, Sunshine, and read this, this, and this before posting again.

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