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Control structures

Today:

if statements and ternary operator
Loops: for and while

📺 Watch the related YouTube video!

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

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!

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

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

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;
}

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

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

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

🔼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!

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
}
```

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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