Discussion

Written by John Pham
Modified by Josh Beto

Lesson Plan

We will be reviewing CS10 material this week.

  • Variables
  • Types
    • int
    • double
    • string
  • Vectors
  • Pass by value vs. pass by reference
  • Functions

Functions

  • Improve readability
  • Modularize your code/avoid writing redudant code
#include <iostream>
#include <vector>
using namespace std;

void printVector(const vector<int>& v) {
    for (auto index : v) {
        cout << index << " ";
    }
    cout << endl;
}

int main() {
    vector<int> items = {1, 2, 3, 4, 5};

    // we clearly know what's going on here and can be reused as many times as needed
    printVector(items);

    // we have to read these 4 lines to figure out what's going on, must copy-paste to keep printing vector
    for (auto index : items) {
    cout << index << " ";
    }
    cout << endl;

    return 0;
}

Testing

As programmers, we need to strive to writing correct code. This means our code should work properly for any set of inputs.

  • Asserts
    • #include <cassert>
      • Need to include cassert to use the assert function
    • assert(func(a, b) == c)
      • This will test whether the output of func(a, b) equals c.
#include <iostream>
#include <math>
using namespace std;

int pow(int a, int b) {
  return pow(a, b);
}
int main() {
  assert(pow(2, 2) == 4);
  assert(pow(2, 3) == 8);
  assert(pow(3, 3) == 10); // this will throw an error

  return 0;
}

Pass by Value vs. Pass by Reference

These are 2 options for when you need to pass variables into functions

  • Pass by value
    • The function creates a local variable with the same value
    • Uses time and memory to create a new variable
    • Changes to the local variable do not affect the original variable
  • Pass by reference
    • The function creates a reference that points to the original variable
    • Does not use additional memory to create a copy
    • Changes to the referenced variable will apply to the original variable
      • You can prevent this by prefixing with const.
#include <iostream>
using namespace std;

void passByValue(int x) {
    x = x + 1; // Copy of x is incremented, but then thrown away
}

void passByReference(int &x) {
    x = x + 1; // Reference to x is actually incremented
}

int main() {
    int variable = 0;

    passByValue(variable);
    cout << variable << endl; // prints '0'

    passByReference(variable);
    cout << variable << endl; // prints '1'

    return 0;
}

Function Overloading

Function overloading allows us to create functions that share the same name but accept different argument types and/or a different number of arguments

#include <iostream>
#include <vector>
#include <string>
using namespace std;

void printVector(const vector<int>& v) {
    for (auto index : v) {
        cout << index << " ";
    }
    cout << endl;
}

// Different parameter type
void printVector(const vector<string>& v) {
    for (auto index : v) {
        cout << index << " ";
    }
    cout << endl;
}

// Different number of parameters
void printVector(const vector<string>& v, string title) {
    cout << title << endl;
    for (auto index : v) {
        cout << index << " ";
    }
    cout << endl;
}

int main() {
    vector<int> itemsA = {1, 2, 3, 4, 5};
    vector<string> itemsB = {"Foo", "Boo", "Bar"}

    printVector(itemsA);
    printVector(itemsB);
    printVector(itemsB, "Example");

    return 0;
}

Common Pitfalls

Integer divison

double a = 5 / 2; // WRONG: Performs integer division, then converts to double!
cout << a << endl; // prints 2

#include <iostream>
using namespace std;

double a = 5 / 2.0; //CORRECT: Converts to double before division!
cout << a << endl; // prints 2.5

Comparison between signed and unsigned

vector<int> v;
for(int i = 0; i < v.size(); i++) { // WRONG: v.size() returns an unsigned!
    cout << v.at(i) << endl;
}

vector<int> v;
for(unsigned i = 0; i < v.size(); i++) { // CORRECT: unsigned compares to another unsigned
    cout << v.at(i) << endl;
}

Out of bounds errors

vector<int> v;
v.at(v.size()); // WRONG: Accesses array element AFTER last one

vector<int> v;
v.back(); // CORRECT: Accesses last array element

vector<int> v;
for (unsigned i = v.size() - 1; i >= 0; i--) {
    cout << v.at(i) << endl; // WRONG: At the last iteration, i goes to -1, check the type!
}

Exercises

  1. Write a function that swaps the value in 2 variables
  2. Write a function that reverses a string Hint: try swapping
  3. Write a function that prints out a vector<int> backwards