control_structures.md

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Control structures

Today:

• if statements and ternary operator
• Loops: for and while

📺 Watch the related YouTube video!

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Special symbols used in slides

• 🎨 - Style recommendation
• 🎓 - Software design recommendation
• 😱 - Not a good practice! Avoid in real life!
• ✅ - Good practice!
• ❌ - Whatever is marked with this is wrong
• 🚨 - Alert! Important information!
• 💡 - Hint or a useful exercise
• 🔼1️⃣7️⃣ - Holds for this version of C++(here, 17) and above
• 🔽1️⃣1️⃣ - Holds for versions until this one C++(here, 11)

Style (🎨) and software design (🎓) recommendations mostly come from Google Style Sheet and the CppCoreGuidelines

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If statement

if (STATEMENT) {
  // This is executed if STATEMENT == true
} else if (OTHER_STATEMENT) {
  // This is executed if:
  // (STATEMENT == false) && (OTHER_STATEMENT == true)
} else {
  // This is executed if neither is true
}

• Used to conditionally execute code
• All the else cases can be omitted if needed
• STATEMENT can be any boolean expression
• 💡 Curly brackets can be omitted for single-line statements

  if (STATEMENT)
    DoThis();

• 🎨 Always use curly brackets as it's harder to make a mistake
• ✅ Use clang-format to indent your code for you!

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Ternary operator

• There is a short-hand version of writing an if statement
• Let's look at an example

  bool condition = GetRandomTrueOrFalse();
  condition ? CallIfTrue() : CallIfFalse();

• This is equivalent to the following if statement:

  bool condition = GetRandomTrueOrFalse();
  if (condition) {
    CallIfTrue();
  } else {
    CallIfFalse();
  }

• ✅ You can use it for simple cases
• ❌ Don't use for complex scenarios

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Switch statement

const int statement = GetNumber();
switch(statement) {
  case kConst1:
    // This runs if statement == kConst1
    break;
  case kConst2:
    // This runs if statement == kConst2
    break;
  default:  // optional
    // This runs if no other options worked
    break;
}

• Used to conditionally execute code
• Can have many case statements
• break exits the switch block
• statement can be any integer-like type
• If you covered all cases, you don't need default

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Omitting the break

• If the break is omitted from a case we call it a fallthrough
• It means that the next case will be executed too
• ✅ Always use [[fallthrough]] C++ attribute to annotate situation where you want a fallthrough 🔼1️⃣7️⃣

// Somewhere in main
const int statement = 1;
switch (statement) {
case kConst1:
  // This runs if (statement == kConst1)
  [[fallthrough]];  // Explicitly fall through
case kConst2:
  // This runs if:
  // (statement == kConst2) or (statement == kConst1)
  break;
default:  // optional
  break;
}

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While loop

while (STATEMENT) {
  // Loop while STATEMENT == true.
}

• Example while loop:

bool condition = true;
while (condition) {
  condition = GetRandomBool();
}

• Usually used when the exact number of iterations is unknown before-wise
• Easy to form an endless loop by mistake

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Do while loop

• Sometimes you want the first iteration to always run
• Use do ... while(); construct for this!

  #include <iostream>
  
  int main() {
    int number = 42;
    do {
      number--;
      std::cout << number << std::endl;
    } while (number != 0);
    return 0;
  }

• Notice that the check only happens after the first iteration!
• 💡 This is not used very often

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For loop

for (INITIAL_CONDITION; END_CONDITION; INCREMENT) {
  // This happens until END_CONDITION == false
}

• Example for loop:

for (int i = 0; i < kIterationCount; ++i) {
  // This happens kIterationCount times.
}

• In C++ for loops are very fast. Use them!
• Use for when the number of iterations is known and while otherwise

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🔼1️⃣1️⃣ Range for loop

• ✅ Iterate over a standard containers with simpler syntax: std::array, std::vector, or even std::map, etc.
• Allows us to avoid mistakes with indices and shows intent

  const std::vector<int> numbers{42, 23};
  for (const auto &number : numbers) {
    std::cout << number << std::endl;
  }

  const std::map<int, double> container{{42, 23.23}, {23, 42.42}};
  for (const auto &[key, value] : container) {
    std::cout << key << " " << value << std::endl;
  }

• 🚨 Make sure to use & to get a reference when iterating over containers. Otherwise, the data is copied, which is slow!
• 💡 Modify data in the container by not using const Try it out on your own!

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Endless loops

• We can create loops that never end with both while and for
• ✅ while expresses the intent better

  while(true) {
    // Do this forever
  }

• Syntax for for is less obvious:

  for(;;) {
    // Do this forever
  }

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Exit loops and iterations

• We have control over loop iterations
• Use continue to skip to next iteration
• Use break to exit the loop

#include <iostream>
int main() {
  while (true) {
    int i = GetRandomInt();
    if (i % 2 == 0) {
      std::cout << i << std::endl;
      continue;
    }
    break;
  }
}

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