Unit 6 Loops and Decisions

The if Statement

Use the if statement to specify a block of C++ code to be executed if a condition is true.

Syntax

if (condition) {

// block of code to be executed if the condition is true

}

Note that if is in lowercase letters. Uppercase letters (If or IF) will generate an error.

In the example below, we test two values to find out if 20 is greater than 18. If the condition is true, print some text:

Example

if (20 > 18) {

cout << "20 is greater than 18";

}

We can also test variables:

Example

int x = 20;

int y = 18;

if (x > y) {

cout << "x is greater than y";

}

Example explained

In the example above we use two variables, x and y, to test whether x is greater than y (using the > operator). As x is 20, and y is 18, and we know that 20 is greater than 18, we print to the screen that "x is greater than y".

The else Statement

Use the else statement to specify a block of code to be executed if the condition is false.

Syntax

if (condition) {

// block of code to be executed if the condition is true

} else {

// block of code to be executed if the condition is false

}

Example

int time = 20;

if (time < 18) {

cout << "Good day.";

} else {

cout << "Good evening.";

}

// Outputs "Good evening."

Example explained

In the example above, time (20) is greater than 18, so the condition is false. Because of this, we move on to the else condition and print to the screen "Good evening". If the time was less than 18, the program would print "Good day".

The else if Statement

Use the else if statement to specify a new condition if the first condition is false.

Syntax

if (condition1) {

// block of code to be executed if condition1 is true

} else if (condition2) {

// block of code to be executed if the condition1 is false and condition2 is true

} else {

// block of code to be executed if the condition1 is false and condition2 is false

}

Example

int time = 22;

if (time < 10) {

cout << "Good morning.";

} else if (time < 20) {

cout << "Good day.";

} else {

cout << "Good evening.";

}

// Outputs "Good evening."

Example explained

In the example above, time (22) is greater than 10, so the first condition is false. The next condition, in the else if statement, is also false, so we move on to the else condition since condition1 and condition2 is both false - and print to the screen "Good evening".

However, if the time was 14, our program would print "Good day."

Short Hand If...Else (Ternary Operator)

There is also a short-hand if else, which is known as the ternary operator because it consists of three operands. It can be used to replace multiple lines of code with a single line. It is often used to replace simple if else statements:

Syntax

variable = (condition) ? expressionTrue : expressionFalse;

Instead of writing:

Example

int time = 20;

if (time < 18) {

cout << "Good day.";

} else {

cout << "Good evening.";

}

You can simply write:

Example

int time = 20;

string result = (time < 18) ? "Good day." : "Good evening.";

cout << result;

C++ Switch Statements

Use the switch statement to select one of many code blocks to be executed.

Syntax

switch(expression) {

case x:

// code block

break;

case y:

// code block

break;

default:

// code block

}

This is how it works:

• The switch expression is evaluated once

• The value of the expression is compared with the values of each case

• If there is a match, the associated block of code is executed

• The break and default keywords are optional, and will be described later in this chapter

The example below uses the weekday number to calculate the weekday name:

Example

int day = 4;

switch (day) {

case 1:

cout << "Monday";

break;

case 2:

cout << "Tuesday";

break;

case 3:

cout << "Wednesday";

break;

case 4:

cout << "Thursday";

break;

case 5:

cout << "Friday";

break;

case 6:

cout << "Saturday";

break;

case 7:

cout << "Sunday";

break;

}

// Outputs "Thursday" (day 4)

The break Keyword

When C++ reaches a break keyword, it breaks out of the switch block.

This will stop the execution of more code and case testing inside the block.

When a match is found, and the job is done, it's time for a break. There is no need for more testing.

A break can save a lot of execution time because it "ignores" the execution of all the rest of the code in the switch block.

The default Keyword

The default keyword specifies some code to run if there is no case match:

Example

int day = 4;

switch (day) {

case 6:

cout << "Today is Saturday";

break;

case 7:

cout << "Today is Sunday";

break;

default:

cout << "Looking forward to the Weekend";

}

// Outputs "Looking forward to the Weekend"

Note: The default keyword must be used as the last statement in the switch, and it does not need a break.

C++ Loops

Loops can execute a block of code as long as a specified condition is reached.

Loops are handy because they save time, reduce errors, and they make code more readable.

1. While Loop

The while loop loops through a block of code as long as a specified condition is true:

Syntax

while (condition) {

// code block to be executed

}

In the example below, the code in the loop will run, over and over again, as long as a variable (i) is less than 5:

Example

int i = 0;

while (i < 5) {

cout << i << "\n";

i++;

}

Note: Do not forget to increase the variable used in the condition, otherwise the loop will never end!

2. The Do/While Loop

The do/while loop is a variant of the while loop. This loop will execute the code block once, before checking if the condition is true, then it will repeat the loop as long as the condition is true.

Syntax

do {

// code block to be executed

}

while (condition);

The example below uses a do/while loop. The loop will always be executed at least once, even if the condition is false, because the code block is executed before the condition is tested:

Example

int i = 0;

do {

cout << i << "\n";

i++;

}

while (i < 5);

Do not forget to increase the variable used in the condition, otherwise the loop will never end!

3. For Loop

When you know exactly how many times you want to loop through a block of code, use the for loop instead of a while loop:

Syntax

for (statement 1; statement 2; statement 3) {

// code block to be executed

}

Statement 1 is executed (one time) before the execution of the code block.

Statement 2 defines the condition for executing the code block.

Statement 3 is executed (every time) after the code block has been executed.

The example below will print the numbers 0 to 4:

Example

for (int i = 0; i < 5; i++) {

cout << i << "\n";

}

Example explained

Statement 1 sets a variable before the loop starts (int i = 0).

Statement 2 defines the condition for the loop to run (i must be less than 5). If the condition is true, the loop will start over again, if it is false, the loop will end.

Statement 3 increases a value (i++) each time the code block in the loop has been executed.

Another Example

This example will only print even values between 0 and 10:

Example

for (int i = 0; i <= 10; i = i + 2) {

cout << i << "\n";

}

Break keyword

You have already seen the break statement used in an earlier chapter of this tutorial. It was used to "jump out" of a switch statement.

The break statement can also be used to jump out of a loop.

This example jumps out of the loop when i is equal to 4:

Example

for (int i = 0; i < 10; i++) {

if (i == 4) {

break;

}

cout << i << "\n";

}

Continue keyword

The continue statement breaks one iteration (in the loop), if a specified condition occurs, and continues with the next iteration in the loop.

This example skips the value of 4:

Example

for (int i = 0; i < 10; i++) {

if (i == 4) {

continue;

}

cout << i << "\n";

}

Break and Continue in While Loop

You can also use break and continue in while loops:

Break Example

int i = 0;

while (i < 10) {

cout << i << "\n";

i++;

if (i == 4) {

break;

}

}

Continue Example

int i = 0;

while (i < 10) {

if (i == 4) {

i++;

continue;

}

cout << i << "\n";

i++;

}