Splitting an array at a given value - c++

Question

Hello I am trying to split an array any time there is a negative value (excluding the negative value) and am a bit stuck at the moment. I tried an approach as seen in my code but I am not getting the desired output.
#include <iostream>
#include <string>
#include <vector>
#include <fstream>
using namespace std;
int main()
{
string line;
string filename;
int n,length;
std::vector<int>arr1;
fstream file("t1.txt");
if(file.is_open())
{
while(file >> n)
arr1.push_back(n);
for(int i =0; i < (int)arr1.size(); i++)
cout << arr1.at(i);
}
cout << endl;
int* arr2 = &arr1[0];
int arr3[arr1.size()/2];
int arr4[arr1.size()/2];
for(int i = 0; i < arr1.size(); i++)
{
cout << arr2[i];
}
for (int i =0; i < arr1.size(); i++)
{
if(i == -1)
break;
else
arr3[i] = arr2[i];
}
return 0;
}

Answer 1

The main problem is here:
int arr3[arr1.size()/2];
int arr4[arr1.size()/2];
This doesn't compile, and can be replaced with
std::vector<int> arr3; arr3.reserve(arr1.size() / 2);
std::vector<int> arr4; arr4.reserve(arr1.size() / 2);
I've added the "reserve" function so that the program doesn't have to allocate memory over and over in the loop.
Next, you are checking i in your loop, and your i loops from 0 to arr1.size() (which is unsigned so can't be negative) therefore i will never be negative.
What you really wanna check is what is in the arr1 vector at "i" position, and you can do so with the [] operator like
for (int i =0; i < arr1.size(); i++)
{
if (arr1[i] >= 0) //if the value is positive, we push it inside our arr3 vector
arr3.push_back(arr1[i]);
else
{
i++; //skip negative value
//
while (i < arr1.size())
{
if (arr1[i] > 0)
arr4.push_back(arr1[i]);
i++;
}
//
//or
//insert all the elemenents we haven't processed yet in the arr4 vector
//this code assumes those elements are positive values
//arr4.insert(arr4.begin(), arr1.begin() + i, arr1.end());
//break;
}
}
Of course this could be done in a different way, like instead of creating 2 vectors, you could just use the one you have generated already.
Hope this helps.

Answer 2

There are several problems in your code
you should not access the vector's data this way unless you really need to
you prepare arrays with predefined size without knowing where to expect the negative values
you do not assign anything to your array 4
you check the index for being negative, not the value
according to your text there could be several negative values leading to multiple result-arrays. You seem to be prepared for only two.
Here is some code that actually splits when encountering negative values:
std::vector<vector<int> > splitted;
for (int i = 0; i < arr1.size(); ++i)
{
if (i ==0 or arr1[i] < 0)
splitted.push_back(std::vector<int>());
if (arr1[i] >= 0)
splitted.back().push_back(arr1[i]);
}
Testing it:
for (int i = 0; i < splitted.size(); ++i)
{
for (int k = 0; k < splitted[i].size(); ++k)
{
std::cout << splitted[i][k];
}
if (splitted[i].empty())
std::cout << "(emtpy)";
std::cout << '\n';
}
Using the following test input
1 2 3 -1 1 -1 -1
You get the following output:
123
1
(emtpy)
(emtpy)