How to pass a vector to a function?

Question

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);
}

• Dat Chu: What do you mean by doesn't work? Please post errors.
• Bo Persson: On the updates code: The parameters first and last are values from the vector, not indexes. You also never set a value to search for (search4)!
• Tomask: Using using namespace std; is a bad idea. Why?
• kingsjester: please note that the expected error in the question has been corrected.. making it meaningless..

Answer 1

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]

• Joe: "The values you assign to first and last are wrong (should be 0 and 99 instead of random[0] and random[99]" - but i want "first" to be the first value in random and "last" to be the last one. I do not want them to be the values 0 and 99.
• Jon: Joe: first and last are indexes, not values. You are confused as to what they represent.
• Joe: Then how do i give Last the last value of the vector?
• Joe: oh, i'm sorry, i see my misstake now. Thank you!
• Jon: @lecaruyer: Briefly, passing by value means that the contents of the vector must be copy-constructible, and a copy of everything will actually be created. The effects of this could in practice range from negligible, to very problematic from a performance and/or resource point of view, to code that does not even compile. There's no reason to do it unless you really do want a second copy of the stuff.

Answer 2

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

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

Answer 3

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"

Answer 4

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.

Answer 5

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

Notice the &.

Answer 6

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.

Answer 7

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