Lecture 04: Pass by reference

Where we left off

#include <iomanip>
cout.precision(2);
cout << fixed;
cout << "Total: $" << setw(8) 
     << bill << endl;
cout << "With GST: $" << setw(8) 
     << bill*1.05 << endl;

Textbook Sections 4.1-4.5

Today’s topics

• More on variable scope
• Pass by reference
• Bottom-up vs top-down testing

Textbook Sections 4.5, 5.1-5.5

Variable scope

• Python only has two scopes: global and local

• C++ has block-level scope

  • Variables declared inside {} are only accessible inside that block
  • This applies to functions, but also if statements, for loops, etc.

  int global_var = 23; // bad idea, but legal
  int main() {
        int local_var = 42; // only accessible within main
        cout << global_var << endl; // 23
        cout << local_var << endl; // 42
  }
  

Function parameters are local variables

• Parameters are local variables that are initialized with the values of the arguments

• Do not redefine function parameters!

  Common error in COMP 1701:

  def some_function(arg_1: int, arg_2: int) -> int:
      arg_1 = 42
      arg_2 = input("Enter a number: ")
      return arg_1 + arg_2
  

Scope guidelines

• Declare variables in the smallest scope possible
• Variable names can be repeated in different scopes, but make sure the usage is consistent
  • e.g. if temp is used for temperature in one scope, don’t use it for temporary values in another
  • Similarly if x is an int in one scope, don’t use it as a double in another
• Avoid global variables altogether (except constants shared across scopes)
  • Improper use of global variables will affect your grade on assignments

Functions check-in 1/2

Based on the following function prototype (declaration), which of the following is a valid function call?

double compute_interest(double balance, double rate, int years);

Functions check-in 2/2

Predict the output of the following code:

void fun(int x);

int main() {
    int y = 0;
    fun(y);
    cout << y << endl;
}

void fun(int x) {
    x = x + 10;
}

Returning multiple things

• Functions allow you to return either nothing (void) or one thing (any other type)
• Python (sort of) allows you to return multiple values from a function by implicitly packing them into a tuple:

  def get_initial_and_age() -> tuple[str, int]:
      initial = input("Enter your initial: ")
      age = int(input("Enter your age: "))
      return initial, age
  

• How can we do this in C++ (98)?
  • Objects and data structures, pointers, and references

Pass by value

So far, all of our functions have used pass by value

• The value of the argument is assigned to the parameter
• Example: given the following function:

  void increase_salary(double salary, double percent_increase) {
      salary = salary * (1 + percent_increase);
  }
  

  trace the execution of the following code, showing memory locations:

  double wage = 10000.0;
  increase_salary(wage, 0.05);
  cout << wage << endl;
  

Pass by reference

• Instead of passing a value, we can pass a reference to a memory location
• This allows us to modify the original value, in a different scope!

  Use with caution! Side effects can lead to chaos

• Syntactically, one tiny change: & after the type in the parameter list

  void increase_salary(double& salary, double percent_increase) {
      salary = salary * (1 + percent_increase);
  }
  

• The & is called the reference operator

Rules and conventions for pass by reference

• Only things with an address can be passed to a reference parameter
  • Variables only, no literals or expressions
• Reference parameters can be both read and written
  • The called function can modify or even destroy the original value!

Example 1

Write a prototype for a function that will “move” a point in a 2D plane according to an angle and a distance. Assume the point is represented by two double parameters x and y.

Example 2

Implement a function with the prototype void swap(int& a, int& b) that exchanges the values of two ints

Try this on paper for a few minutes, then we’ll go through a solution

Testing: drivers and stubs

• Functions are great because they let us break our code into smaller pieces
• Don’t wait until you’ve written everything to test!
• Two approaches:
  • Top-down: Start with the main logic and then fill in the pieces
  • Bottom-up: Start with the pieces and then put them together

Testing review 1/2

A function with an int parameter num implements the following logic. How many test values are needed to exhaustively test it?

result = num
if num < 0
    result = -num

return result

Testing review 2/2

It is acceptable to hard-code magic numbers for test purposes.

Test Drivers

• Used in bottom-up testing
• After writing a complete function, a test driver is a “dummy” main function that calls the function with a variety of test values
• At its simplest, the test driver should print the results of the function calls along with a label for context

  cout << "my_func(2) = " << my_func(2) << endl;
  cout << "my_func(-2) = " << my_func(-2) << endl;
  cout << "my_func(0) = " << my_func(0) << endl;
  

Function Stubs

• While a driver acts as the calling function, a stub acts as the called function
• Used in top-down design
• Write your main logic first, then write stubs for the functions you need
• Stubs match the data type, name, and number of parameters for a function you want to write, but don’t do anything useful

  int my_func(int whole_num, double dec) {
      return 0;
  }
  

Side Tangent: input redirection

• We talked about testing functions individually with hard-coded values, but eventually you need to test with input as well
• You can repeatedly type your input…

    $ ./a1
    Enter the range of R0 values (0 - 20): 0.5 12
    Enter the range of p values (0 - 1): 0.1 0.95

• But it’s easier to redirect input from a file:

    $ ./a1 < input.txt

Boolean preview

bool is a data type that can only have two values: true or false

def is_even(num: int) -> bool:
    return num % 2 == 0

bool is_even(int num) {
    return num % 2 == 0;
}

Compound Boolean expressions

• Same behaviour and precedence as Python, just different symbols
• Example: bool in_range = x > 0 && x < 10

if statement syntax

if (boolean_expression) {
    // code to execute if true
} else {
    // code to execute if false
}

• note the () around the boolean expression - this is mandatory in C++
• Like Python, the else is optional
• More nuance on if and booleans next lecture

Coming up next

• Lab: Pass by reference
• Lectures: Decisions and Loops
• Assignment 1 now available! 🎉

Textbook Sections 2.4, 3.1-3.2

Extra: another pass-by-reference example

Write a function called sort2 that takes two int parameters and sorts them in ascending order - that is, after a call to sort2(m, n), the smaller value is in m and the larger value is in n.

Hint: you can use the swap function from earlier