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I'm learning to program, and C++ is my first language. Don't bother using pointers to show me - I don't understand them yet, and won't bother until I have more free time to dedicate to this.
int mergeSort()
{
const int n = 9;
int originalarray[n] = {1, 3, 5, 7, 9, 2, 4, 6, 8};
const int halfelements = (sizeof(originalarray) / sizeof(int)) / 2;
int farray[halfelements];
int sarray[halfelements];
for (int i = 0; i < halfelements; i++) {
farray[i] = originalarray[i];
}
for (int i = halfelements, x = 0; i < (halfelements * 2); i++, x++) {
sarray[x] = originalarray[i];
}
I was assigned (I'm not taking classes - just learning with a few friends helping me out) a merge sort algorithm, with the algorithm explained but not the implementation. I want to rewrite this so it will work for both odd and even integers. I tried adding this code:
if ((n % 2) != 0) int farray[halfelements + 1];
So that I could use the same integer to iterate over both subsequent arrays. A sizeof(farray) is showing to be 16 bytes, or 4 integers. So it isn't resizing. What I want to know - is it possible to resize arrays after they initialized?
Edit: How would I implement a vector? I don't understand how to use iterators in a loop to iterate over and copy the values.
C++ arrays are fixed in size.
If you need a "resizable array", you'll want to use std::vector instead of an array.
My advice is even stronger: use std::vector<>
(et. al.) unless you have a very good reason to use a C-style array. Since you're learning C++, I doubt you have such a reason: use std::vector<>
.
I would also recommend std::vector
. However if you are stuck with an array you can always malloc
the memory and then realloc
if you need to make the array larger.
Do a search here on SO, there is information about malloc
and realloc
.
If you want to resize an array, you probably want to use a vector, which can be resized automatically.
You can use the [] operator with a vector the same way you would in an array. You could implement this with a vector something like this (if you wanted to use more vector methods):
#include <vector>
const int halfelements = originalarray.size()/2; //use size to get size
vector <int> farray(halfelements);
vector <int> farray(halfelements);
for (int i = 0; i < halfelements; i++) {
farray.push_back(originalarray[i]); //adds element at i to the end of vector
}
for (int i = halfelements, x = 0; i < (halfelements * 2); i++, x++) {
sarray.push_back(originalarray[i]);
}
You can also use .at(index) to add bounds checking to the vector access.
If you want to know why your first idea compiled but didn't seem to work:
When you omit braces in an if-statement:
if ((n % 2) != 0) int farray[halfelements + 1];
it's just the same as if you'd used them:
if ((n % 2) != 0) {
int farray[halfelements + 1];
}
So it is making an 'farray' of the correct size -- and then it immediately goes out of scope and is gone, and you're left with only the original one.