Lecture 05: Booleans and Decisions

Where we left off

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

Today’s topics

• Boolean expressions
• if-else statements
• Some C++ specific boolean behaviour
• Intro to loops

Textbook Sections 2.4, 3.1-3.2

The bool data type

• A primitive just like int and double
• bool can be declared and initialized like any other primitive

  bool thunder_only_happens_when_its_raining = true;
  

• bool can also be returned from a function

  bool is_valid_account_number(int account_number);
  

• But often expressions are used directly without assigning to a variable

  if (temperature < 0) {
      cout << "It's freezing!\n";
  }
  

Reading and printing booleans

• bool values can’t really be read in or printed out, but they can be implicitly converted to int values
• Caution: there’s no loss of precision, so no compiler warning!
  • false is converted to 0, true is converted to 1
  • 0 is converted to false, and any other number is true
• Safer to read a char and convert to bool explicitly

  char c;
  cin >> c;
  bool is_valid = c == 'y' || c == 'Y';
  

Boolean operators

• Example: assign a boolean timed_out that is a function of two ints: total_time and num_records.

• timed_out should be true if the time per record exceeds 1 second, and false otherwise.

cout and precedence

• In Python, print is a function, so the whole expression is evaluated first:

  print(x > 0 and x < 10) # prints True or False
  

• In C++, << is an operator:

  cout << x > 0 && x < 10; // what happens?
  

• Easiest solution: use parentheses, or assign to a variable:

  cout << (x > 0 && x < 10);
  

if syntax

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

• Remember the {} defines blacks in C++
• boolean_expression can be a compound condition, a function returning a bool, a single boolean variable… anything that evaluates to a bool
• Caution: sometimes things evaluate to a bool when you didn’t expect it!

Caution!

• = is assignment, == is comparison
• Unlike Python, this is not a syntax error
• Better to avoid comparison with bool

Single line if statements

• If the code to execute is a single line, you can omit the curly braces

  if (x > 0)
      cout << "x is positive\n";
  

• Ditto for else:

  if (x > 0)
      cout << "x is positive\n";
  else
      cout << "x is negative\n";
  

• This can be risky though - remember C++ doesn’t care about indentation!

Nested if statements

Just like Python, you can nest if statements inside each other:

if (is_valid_account_number(account_number) {
    if (max_disk_usage > allotment) {
        // surcharge calculation
    }
}

• Indentation is not required, but it’s a good idea
• Emacs will indent for you, but if it’s not, that could mean you have an error

Multiple branching with else if

if x > 0:
    print("x is positive")
elif x < 0:
    print("x is negative")
else:
    print("x is zero")

if (x > 0) {
    cout << "x is positive\n";
} else if (x < 0) {
    cout << "x is negative\n";
} else {
    cout << "x is zero\n";
}

• No special elif keyword, just else followed by if
• As many else if branches as you like, including zero

Tricky mistakes

• ; after if statement

  if (x > 0);
      cout << "x is positive\n";
  

• The “dangling else” problem

  if (x > 0)
      if (y > 0)
          cout << "x and y are positive\n";
  else
      cout << "x is negative\n";
  

if statement check-in

In the following code snippet, x has a value of 15. What is the output?

if (x % 3 == 0)
    cout << "Fizz";
if (x % 5 == 0)
    cout << "Buzz";

A trickier one

What is the output of the following code snippet? x is again 15.

if (x == 0)
    cout << "x is 0\n";
    cout << "Try again\n";

Branching in functions

C++ does not restrict you to a single return statement in a function:

double relu(double x) {
    if (x > 0)
        return x;
    else
        return 0;
}

• Multiple returns can make code harder to read and debug, though unlike Python, the compiler will protect you from a forgotten return
• My recommendation: stick to a single return, unless it’s a “guard clause”
  • return at the start or the end of the function, not in the middle

Tangent: Guard clauses

A “guard clause” is an if statement that returns early if inputs are invalid

bool is_valid_account_number(int account_number) {
    if (account_number < 0) {
        return false;
    }
    
    // rest of function
}

• This can prevent nesting and make code easier to read
• Guard clauses should be short and at the very start of the function

And now, loops!

while condition:
    # code to execute

while (condition) {
    // code to execute
}

• You basically know while loop syntax already! Just remember:
• Each loop has at least one loop control variable (LCV)
• The LCV must be initialized prior to the loop
• The LCV must be updated inside the loop
• Eventually the condition must become false to exit the loop

Complete while loop example

int x = 1;              // Initialization
while (x <= 100) {      // Condition
    if (x % 3 == 0)
        cout << "Fizz";
    if (x % 5 == 0)
        cout << "Buzz";
    cout << "\n";
    x++;                // Update
}

• Forgetting to update the LCV leads to an infinite loop
• Initializing with the wrong value can lead to the loop never executing

for loops - a bit more different

for i in range(10):
    # code to execute

for (int i = 0; i < 10; i++) {
    // code to execute
}

• Notice the semicolons! Inside the parentheses, there are three statements:
  1. Initialization
  2. Condition
  3. Update
• The LCV is declared inside the loop, and only exists inside the loop
• BUT this isn’t actually mandatory - it’s a good idea though

FizzBuzz as a for loop

Since FizzBuzz is counting from 1 to 100, it’s a good candidate for a for loop:

for (int x = 1; x <= 100; x++) {
    if (x % 3 == 0)
        cout << "Fizz";
    if (x % 5 == 0)
        cout << "Buzz";
    cout << "\n";
}

• for loops help protect you from forgetting to initialize or update the LCV
• More readable for counted scenarios, as all 3 steps are in one place
• BUT you can’t shouldn’t use a for loop for event controlled repetition

Coming up next

• Lab: Decisions
• Lecture: Loops in more detail, plus new loop constructs and C++ gotchas
• Get cracking on assignment 1! 🎉

Textbook sections 3.3-3.4