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memory.cpp
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memory.cpp
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#include <iostream>
using std::cout, std::cin, std::endl;
/*
Welcome to the heart and soul of C++, the part that most people struggle with
Pointers:
a pointer stores the address that a data type is located at and is denoted by <type>* <variablename>
an address is just an integer value
New Operators:
&<variable> - reference - get the memory address of <variable>
*<variable> - dereference - access the data stored at the address stored in <variable>
new <type> - allocate memory on the heap for <type> and return the address
delete <variable> - deallocate the memory stored at the address stored in variable
delete[] <variable> - same as above, but for arrays
*/
// see definition below after reading all of main
int nth_fibonacci(int);
int main() {
// initializes a pointer to an int
int *address;
// when initially declared, the pointer doesn't point to anything
// the OS doesn't have to initialize it to nullptr, so it is good practice to do so
address = nullptr;
// just an int
int number = 420;
// you can give a pointer the address of a pre-existing variable
address = &number;
// address now stores the memory address that stores number
// if you dereference a pointer, you can access the data it points to
cout << *address << endl;
// should print 420
//you can also use dereferencing to modify the data
*address += 10;
// remember that address is the memory address that stores number, so modifying the data pointed to by address modifies number
cout << number << endl;
// should print 430
// but you don't have to just work with pre-existing variables
int *address2 = new int(69);
// this allocates a new pointer called address2 with the address of an int on the heap with value 69
cout << address2 << " stores " << *address2 << endl;
// memory allocated with `new` must be deleted by you, otherwise the memory is never freed and you create a memory leak
delete address2;
// address2 still contains an address at this point but it now points to invalid memory
// this is called a dangling pointer
// good practice says to set it to nullptr, but it doesn't matter that much
address2 = nullptr;
// storing a single number is boring
// the real fun here comes when you realize you can now dynamically allocate arrays
// previously, you have been restricted to arrays witha predefined size
// now you can do
int size;
cout << "Give me a positive integer: ";
cin >> size;
int *array = new int[size];
for (int i = 0; i < size; i++) {
array[i] = 0;
}
cout << "Created an array of size " << size << endl;
// array points to an array, use delete[]
delete[] array;
int n;
cout << "Give me a positive integer: ";
cin >> n;
cout << "The " << n << "th fibonacci number is " << nth_fibonacci(n) << endl;
// being able to dynamically allocate memory is useful for operations involving files, math, devices, etc.
// otherwise known as most things
}
// returns the nth fibonacci number
// 0-indexed
// https://oeis.org/A000045
// there are better ways to do this, this is just an example
int nth_fibonacci(int n) {
// base case, no point in doing math
if (n < 2) return 1;
// create array to store calculations
int *array = new int[n + 1];
// manually set the first two fibonacci numbers
array[0] = 1;
array[1] = 1;
// generate the fibonacci sequence through n
for (int i = 2; i < n + 1; i++) {
// use the array to store results as we go so we don't have to recompute them
array[i] = array[i-1] + array[i-2];
}
// store the result so we can delete the array
int result = array[n];
delete[] array;
// return the nth element of the array
return result;
}