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How to pass a vector to a function?

Posted by: admin November 29, 2017 Leave a comment

Questions:

I’m trying to send a vector as an argument to a function and i can’t figure out how to make it work. Tried a bunch of different ways but they all give different error messages.
I only include part of the code, since it’s only this part that doesn’t work.
(the vector “random” is filled with random, but sorted, values between 0 and 200)

Updated the code:

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

using namespace std;

int binarySearch(int first, int last, int search4, vector<int>& random);

int main()
{
    vector<int> random(100);

    int search4, found;
    int first = 0;
    int last = 99;

    found = binarySearch(first, last, search4, random);

    system("pause");    
    return(0);      
}

int binarySearch(int first, int last, int search4, vector<int>& random)
{
    do
    {
        int mid = (first + last) / 2;  
        if (search4 > random[mid]) 
            first = mid + 1;  
        else if (search4 < random[mid]) 
            last = mid - 1; 
        else
            return mid;     
    } while (first <= last); 

    return -(first + 1);
}
Answers:

It depends on if you want to pass the vector as a reference or as a pointer (I am disregarding the option of passing it by value as clearly undesirable).

As a reference:

int binarySearch(int first, int last, int search4, vector<int>& random);

vector<int> random(100);
// ...
found = binarySearch(first, last, search4, random);

As a pointer:

int binarySearch(int first, int last, int search4, vector<int>* random);

vector<int> random(100);
// ...
found = binarySearch(first, last, search4, &random);

Inside binarySearch, you will need to use . or -> to access the members of random correspondingly.

Issues with your current code

  1. binarySearch expects a vector<int>*, but you pass in a vector<int> (missing a & before random)
  2. You do not dereference the pointer inside binarySearch before using it (for example, random[mid] should be (*random)[mid]
  3. You are missing using namespace std; after the <include>s
  4. The values you assign to first and last are wrong (should be 0 and 99 instead of random[0] and random[99]
Questions:
Answers:

You’ll have to pass the pointer to the vector, not the vector itself. Note the additional ‘&’ here:

found = binarySearch(first, last, search4, &random);

Questions:
Answers:

You’re passing in a pointer *random but you’re using it like a reference &random

The pointer (what you have) says “This is the address in memory that contains the address of random”

The reference says “This is the address of random”

Questions:
Answers:

Anytime you’re tempted to pass a collection (or pointer or reference to one) to a function, ask yourself whether you couldn’t pass a couple of iterators instead. Chances are that by doing so, you’ll make your function more versatile (e.g., make it trivial to work with data in another type of container when/if needed).

In this case, of course, there’s not much point since the standard library already has perfectly good binary searching, but when/if you write something that’s not already there, being able to use it on different types of containers is often quite handy.

Questions:
Answers:
found = binarySearch(first, last, search4, &random);

Notice the &.

Questions:
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You’re using the argument as a reference but actually it’s a pointer. Change vector<int>* to vector<int>&. And you should really set search4 to something before using it.

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If you use random instead of * random your code not give any error