Conditional Constructs

Conditional constructs are features of a programming language, which perform different computations or actions depending on whether a programmer-specified conditional expression evaluates to true or false.

The if Statement

The statements inside your source files are generally executed from top to bottom (in the order that they appear). Control flow statements, however, break up the flow of execution by employing decision making, enabling your program to conditionally execute particular blocks of code. This section describes the if and if-else statements that allow code to be executed based on a given condition.

The if statement is the most basic of all the control flow statements. It allows an application to execute a certain section of code only if a particular condition evaluates to true.

Examine the following example where the user is requested to enter a temperature. Next the given value is evaluated and if it is above (or equal to) a certain threshold value (85 in this case) a warning message is outputted to the terminal.

int temperature = 0;

std::cout << "Please enter a temperature: ";
std::cin >> temperature;

if (temperature >= 85) {
  std::cout << "Warning, temperature is too high!" << std::endl;
}

Output

Please enter a temperature: 123
Warning, temperature is too high!

If this test evaluates to false (meaning that the temperature is below 85), control jumps to the end of the if statement.

The if-else Statement

The if-else statement provides a secondary path of execution when an "if" clause evaluates to false. Taking the previous example you could output an "all is good" message when the temperature is below the threshold value.

int temperature = 0;

std::cout << "Please enter a temperature: ";
std::cin >> temperature;

if (temperature >= 85) {
  std::cout << "Warning, temperature is too high!" << std::endl;
} else {
  std::cout << "All is good" << std::endl;
}

Output

Please enter a temperature: 12
All is good

The if-else statement can be extended with even more if-else statements. Each if-else will need a new condition that needs to be checked. The first one that evaluates to true is executed, after which control jumps to the end of the if-else statements.

Let us extend the temperature example with a number of ranges.

int temperature = 0;

std::cout << "Please enter a temperature: ";
std::cin >> temperature;

if (temperature < 85) {
  std::cout << "All is good" << std::endl;
} else if (temperature < 100) {
  std::cout << "Warning, temperature is too high!" << std::endl;
} else if (temperature <= 200) {
  std::cout << "Time to run!" << std::endl;
} else {
  std::cout << "We are doomed!!!!" << std::endl;
}

Output

Please enter a temperature: 105
Time to run!

You may have noticed that the value of temperature can satisfy more than one expression in the combined statements. However the conditions are checked sequentially and once a condition is satisfied, the appropriate statements are executed and the remaining conditions are not evaluated anymore. Control then jumps to the end of the if-else statements.

The switch Statement

A switch statement allows a variable to be tested for equality against a list of known values. Each value is called a case, and the variable being switched on is checked for each case.

Take a look at some code that will allow the user to enter the number of the day of the week. The program will than determine the name of the day and output it to the user.

int dayOfTheWeek = 0;

std::cout << "What day of the week is it today [1-7]? ";
std::cin >> dayOfTheWeek;

if (dayOfTheWeek == 1) {
  std::cout << "Than it's Monday today" << std::endl;
} else if (dayOfTheWeek == 2) {
  std::cout << "Than it's Tuesday today" << std::endl;
} else if (dayOfTheWeek == 3) {
  std::cout << "Than it's Wednesday today" << std::endl;
} else if (dayOfTheWeek == 4) {
  std::cout << "Than it's Thursday today" << std::endl;
} else if (dayOfTheWeek == 5) {
  std::cout << "Than it's Friday today" << std::endl;
} else if (dayOfTheWeek == 6) {
  std::cout << "Than it's Saturday today" << std::endl;
} else if (dayOfTheWeek == 7) {
  std::cout << "Than it's Sunday today" << std::endl;
} else {
  std::cout << "That is not a valid week day" << std::endl;
}

Output

What day of the week is it today [1-7]? 2
Than it's Tuesday today

When checking a single variable for equality using multiple if-else statements, it can be replaced with another structure called a switch structure. The template of the switch structure is shown below. Each case needs a literal integral value to compare the variable against. If it matches (equals), than the code between the colon : and the break; statement is executed. The break is required for the switch to be stopped when a match is found. If no break is placed, the execution falls through to the next case.

switch (<variable>) {
  case <integral_literal_1>:
    // Code to be executed
    break;
  case <integral_literal_2>:
    // Code to be executed
    break;
  case <integral_literal_3>:
    // Code to be executed
    break;
  // ...
  default:
    // Code to be executed in case no match found
}

Replacing the if-else structure of the day-of-the-week example with a switch statement results in the following code.

int dayOfTheWeek = 0;

std::cout << "What day of the week is it today [1-7]? ";
std::cin >> dayOfTheWeek;

switch(dayOfTheWeek) {
  case 1:
    std::cout << "Than it's Monday today" << std::endl;
    break;
  case 2:
    std::cout << "Than it's Tuesday today" << std::endl;
    break;
  case 3:
    std::cout << "Than it's Wednesday today" << std::endl;
    break;
  case 4:
    std::cout << "Than it's Thursday today" << std::endl;
    break;
  case 5:
    std::cout << "Than it's Friday today" << std::endl;
    break;
  case 6:
    std::cout << "Than it's Saturday today" << std::endl;
    break;
  case 7:
    std::cout << "Than it's Sunday today" << std::endl;
    break;
  default:
    std::cout << "That is not a valid number" << std::endl;
}

Output

What day of the week is it today [1-7]? 2
Than it's Tuesday today

No general rule exists for when to use which construct. Some programmers don't like the switch statement. In most cases it is a case of preference.

Some important points about the switch statement:

• The expression provided in the switch should result in a constant value otherwise it would not be valid.

  • Valid expressions for switch:

    • switch(1+2+23)
    • switch(1*2+3%4)

  • Invalid switch expressions for switch:

    • switch(ab+cd)
    • switch(a+b+c)

• Duplicate case values are not allowed.

• The default statement is optional. Even if the switch case statement did not have a default statement, it would run without any problem.

• The break statement is optional. If omitted, execution will continue on into the next case. The flow of control will fall through to subsequent cases until a break is reached.

• Nesting of switch statements is allowed, which means you can have switch statements inside another switch. However nested switch statements should be avoided as it makes code more complex and less readable.

Scope in Switch

One final warning with switch statements requires a bit more explanation. Care must be taken when one defines variables inside the case's, for example for holding a temporary math result.

Consider the following example that requests two numbers from the user and than let's the user choose what action to perform on them:

int a = 0;
int b = 0;
std::cout << "Please enter two numbers: ";
std::cin >> a >> b;

int action = 0;
std::cout << "What would you like to do with the numbers?" << std::endl;
std::cout << "[1] Add" << std::endl;
std::cout << "[2] Multiply" << std::endl;
std::cout << "Choice: ";
cin >> action;

switch(action) {
  case 1:
    int result = a + b;
    cout << "a + b = " << result << endl;
    break;
  case 2:
    int result = a * b;
    cout << "a * b = " << result << endl;
    break;
}

Output

test.cpp: In function ‘int main()’:
test.cpp:25:8: error: jump to case label
   25 |   case 2:
      |        ^
test.cpp:22:9: note:   crosses initialization of ‘int result’
   22 |     int result = a + b;
      |         ^~~~~~
test.cpp:26:9: error: redeclaration of ‘int result’
   26 |     int result = a * b;
      |         ^~~~~~
test.cpp:22:9: note: ‘int result’ previously declared here
   22 |     int result = a + b;

Case statements are actually only labels. This means the compiler will interpret this as a jump directly to the label. In C++, the problem here is one of scope. The curly brackets define the scope as everything inside the switch statement. This means that the first variable result is defined until the closing curly brace of the switch statement. This conflicts with the second definition which would also live till the closing curly brace.

This can be solved by adding curly bracing around the statements in each case, giving each case it's own scope:

int a = 0;
int b = 0;
std::cout << "Please enter two numbers: ";
std::cin >> a >> b;

int action = 0;
std::cout << "What would you like to do with the numbers?" << std::endl;
std::cout << "[1] Add" << std::endl;
std::cout << "[2] Multiply" << std::endl;
std::cout << "Choice: ";
cin >> action;

switch(action) {
  case 1: {
      int result = a + b;
      cout << "a + b = " << result << endl;
    }
    break;
  case 2: {
      int result = a * b;
      cout << "a * b = " << result << endl;
    }
    break;
}

Output

Please enter two numbers: 2 5
What would you like to do with the numbers?
[1] Add
[2] Multiply
Choice: 1
a + b = 7

Exercises

Try to solve the exercises yourself. Don't go copy pasting other people's solutions.

Mark the exercises using a ✅ once they are finished.