Getting a memory address in my array? - c++

Question

I'm writing a C++ application that has a user enter a 0 (zero) or a one (1) as input, then stores the numbers as an array and sorts them so that the zeros come first and the ones come last.
However, I think I'm getting a memory address in my array that's messing up the sorting operation.
The function that accepts input looks like this:
cout << "Please enter number " << i+1 << ":\n";
cin >> ar[i];
Then there's a function that's called that sorts the input and displays the sorted list:
sort_elements(ar, number);
... and that function looks like this:
void sort_elements(int ar[], long int num_elements) {
int temp_num;
num_elements -= 1; //since the array starts at 0
cout << "num_elements is " << num_elements << "\n";
for (int i=0; i < (num_elements/2); i++ ) {
if (ar[i] > ar[num_elements-i]) {
temp_num = ar[i];
ar[i] = ar[num_elements-i];
ar[num_elements-i] = temp_num;
}
}
cout << "Here's your neatly sorted list of numbers: \n";
for (int j=0; j <= num_elements; j++) {
cout << ar[j] << ", ";
}
cout << "\n";
}
For a five number input, starting with three "1"s, and ending with two "0"s, this results in an output that looks like this:
1, 0, 1, 1, 1892218304,
I'm assuming the 1892218304 is a memory address, that's messing up the input. Though I don't really know.
Can anyone figure out why my sort operation is getting messed up?
Thanks in advance.

Answer 1

Suggestion
Use vector and sort in standard library
#include <algorithm>
#include <vector>
#include <iostream>
#include <iterator>
int main()
{
std::vector<int> v;
for(int i=0; i < 10; i++)
{
v.push_back(i);
}
std::sort(v.begin(), v.end());
return 0;
}

Answer 2

The number you are seeing is not a memory address, but the value of the 4 bytes either immediately before or immediately after your array, interpreted as an int. Your code has an off-by-one error that causes an access to just outside the array. That much I suspect even though I don't have proof.
However, I can't find anything wrong with the code you posted that would cause it to access outside the array bounds.
Are you sure that num_elements has the correct value when this function is called?
Update to address the pastebin code
Things are going wrong already from the start:
int number;
int ar[number]
This is called a variable-length array and it's not legal in C90 or any flavor of C++. That your program compiles is probably "thanks to" a compiler extension... which helpfully raises a bug: the value of number is not initialized before the array is allocated.
You need to do one of the following:
declare ar as an array of constant size (a hard limit on the number of inputs you can accept)
dynamically allocate ar with new[] after number is entered from the user
(by far preferable) use an std::vector instead of an array

Answer 3

It looks to me like you're just getting an uninitialized value in your output.
Under the circumstances, the simplest "sorting" method is probably a counting sort -- i.e., count the number of 0's and the number of 1's the user enters, and then print out the appropriate number of 0's followed by the appropriate number of 1's.

Answer 4

As Tony said, your sorting algorithm is incorrect.
If we assume the following values:
ar[0] = 0
ar[1] = 1
ar[2] = 0
ar[3] = 1
ar[4] = 0
that gives us num_elements equal to 5.
Running this through your function as written, we get the following sorting behavior:
First pass, i = 0
ar[0] > ar[4] -> not true, so no switch
Secon pass, i = 1
ar[1] > ar[3] -> not true, so no switch
There is no third pass, as your for loop condition is met
i = 2
num_elements/2 = 2
2 is not less than 2
So based on your code, you didn't sort anything. This is the first issue.
Your print problem is interesting, based on what you have shown num_elements has been decremented by 1 at the top of your code snippet in your function - therefore the <= condition is correct and you should not be outside the bounds of your 'ar' array. Are you sure this is the exact code, or perhaps you did not copy it properly here and you are actually having a scoping issue?
(EDIT: Although the other answers suggesting using a built in sorting method via vectors, I'd suggest you still work your current implementation out to figure out WHY this is wrong, and what you need to do to fix it. You will not always have an available type that has built in sorting, so understanding the fundamentals is important for any young programmer.)
EDIT2: Based on the link you provided, you aren't properly defining your integer array. You are defining the array based on an uninitialized integer (int ar[number]) when number has not yet been initialized. You then read a value from the standard input to set number, and assume your array has been dynamically adjusted to be of the size read from cin. It does not work this way. Your entire loop is reading/writing outside the bounds of your array which is a big no-no. You want to read the number first, and then define your array based on the size read. - Looks like Jon beat me again.. BAH! :P

Answer 5

The last element being read and printed to the screen has never been written to. I think it's likely your function which accepts input has a fault, like maybe you have a line like you have here
num_elements -= 1; //since the array starts at 0
at the input function so that the first element you write to is at address 1. Then when you're reading them you read from address zero.

Answer 6

I kind of agree with Jim's point: Use the facilities in the C++ standard library to finish your task as much as possible.
Besides, I suggest you go through the algorithm all by yourself, manually, instead of letting the computer run it for you. Ever heard of the "Rubber Duck Debugging Method"? :-) Have a try.