lec06-more-memory.md

Lec 06 - More Memory

Lecture Catchup

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https://mediaspace.wisc.edu/media/CS%20368%20S24%20-%20Wed%20Wrapup%3A%20arrays%2C%20const%2C%20references/1_rtz4ox59

• arrays on the heap
• delete[]
• const
• references

References and Auto

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https://mediaspace.wisc.edu/media/CS%20368%20S24%20-%20References%20and%20Auto/1_wkmh738m

Practice

Run the following program:

#include <iostream>
#include <vector>

using namespace std;

struct Coord {
  int x = 0;
  int y = 0;
};

int main() {
  vector<Coord> coords = {{3,3}, {4,5}, {8,10}};
  cout << "addr of Coord at idx 0: " << &coords[0] << "\n";
  cout << "addr of Coord at idx 1: " << &coords[1] << "\n";
  cout << "addr of Coord at idx 2: " << &coords[2] << "\n";

  for (auto c : coords) {
    cout << c.x << " " << c.y << " " << &c << "\n";
  }
}

Note that the c doesn't change, and is different than the addresses of the elements. This is because a Coord is being copied for each iteration of the loop. That could become a problem if the struct gets much bigger (more members).

Modify the code so that c refers each element in turn (instead of copying it).

Pointer Arithmetic

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https://mediaspace.wisc.edu/media/CS%20368%20S24%20-%20Pointer%20Arithmetic/1_o3x9kp3p

Practice

Replace ???? with some pointer arithmetic to get the index of the number within nums (it's 3, as you can see in the getnum function, but pretend you can't see that).

#include <iostream>
#include <vector>

using namespace std;

int nums[] = {1,3,2,7,9};

int *getnum() {
  return &nums[3];
}

int main() {
  int *num = getnum();
  cout << *num << " (value)\n";
  cout << (????) << " (index)\n";
}

If you want even more practice, write a safety check to verify num actually points to an element in nums before printing the index (print a warning instead if it points some number outside of the array).

Double Pointers

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https://mediaspace.wisc.edu/media/CS%20368%20S24%20-%20Double%20Pointers/1_rth7n0ja

Practice

Here is the code from the demo:

#include <iostream>

// double pointers                                                                                                                
using namespace std;

int init_range(int** arrp) {
  int size=8;
  *arrp = new int[size];
  for (int i=0; i<size; i++) {
    (*arrp)[i] = 10-i;
  }
  return size;
}

int main() {
  int* arr;
  int size = init_range(&arr);

  for (int i=0; i<size; i++) {
    cout << arr[i] << "\n";
  }
}

Write a similar float_range function that returns a range of floats. It should have start, end, and step params. For example, if you have start=3, end=3.5, and step=0.1, you would get an array back with these numbers:

3.0 3.1 3.2 3.3 3.4

Strings

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https://mediaspace.wisc.edu/media/t/1_a9yuzqyk

Practice

Complete this program:

#include <iostream>
#include <vector>

using namespace std;

int main(int argc, char *argv[]) {
  std::vector<std::string> args{};

  for (????) {
    args.push_back(????);
  }

  for (auto& s : args) {
    cout << s << "\n";
  }
}

Your goal is to take all the C-style strings from argv (excluding the program name!) and add them as standard stings to the args vector.

Example execution:

./a.out hello 1 2 3
hello
1
2
3

const vs. constexpr

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https://mediaspace.wisc.edu/media/t/1_jlh5t9td