Dart if statements

Dart if statements

Learn Dart Programming: If Statements, Ternary Operators, and Conditional Expressions

Introduction:
Explore the fundamentals of Dart programming language with a focus on if statements, else statements, ternary operators, and conditional expressions. This comprehensive lesson provides hands-on examples and explanations to help you master these essential concepts in Dart.

In Dart, if statements are used for conditional execution of code blocks. The basic syntax of an if statement in Dart is as follows:

dart

if (condition) {
// code to be executed if the condition is true
} else {
// code to be executed if the condition is false
}

Here’s a simple example:

dart

void main() {
int x = 10;

if (x > 5) {
print("x is greater than 5");
} else {
print("x is not greater than 5");
}
}

In this example, if the condition x > 5 is true, the code inside the first block will be executed; otherwise, the code inside the else block will be executed.

 if statements without an else block:

dart

void main() {
int y = 7;

if (y % 2 == 0) {
print("y is an even number");
}
}

In this case, if the condition y % 2 == 0 is true, the code inside the block will be executed; otherwise, nothing will happen.

Dart with else if to handle multiple conditions:

dart

void main() {
int z = 0;

if (z > 0) {
print("z is positive");
} else if (z < 0) {
print("z is negative");
} else {
print("z is zero");
}
}

Here, the first condition checks if z is positive, the second condition checks if it’s negative, and the else block handles the case when z is zero.

Dart with ternary operator for concise if-else expressions:

dart

void main() {
int a = 15;
String result = (a > 10) ? "a is greater than 10" : "a is not greater than 10";
print(result);
}

This is a shorter way of writing a simple if-else statement.

If the condition (a > 10) is true, the first expression is evaluated; otherwise, the second expression is evaluated.

Types of if statements in dart 

In Dart, if statements come in a few different forms, and here are some examples to illustrate them:

1. Basic if statement:

void main() {
int x = 10;

if (x > 5) {
print("x is greater than 5");
} else {
print("x is not greater than 5");
}
}
2. if statement without an else block:
dart

void main() {
int y = 7;

if (y % 2 == 0) {
print("y is an even number");
}
}

3. if-else if-else statement:

dart

void main() {
int z = 0;

if (z > 0) {
print("z is positive");
} else if (z < 0) {
print("z is negative");
} else {
print("z is zero");
}
}

4. Ternary operator:

dart

void main() {
int a = 15;
String result = (a > 10) ? "a is greater than 10" : "a is not greater than 10";
print(result);
}

5. Conditional expressions:

Dart supports a concise form of conditional expressions using ??:

dart

void main() {
int value = 42;
String text = value > 0 ? "Positive" : value < 0 ? "Negative" : "Zero";
print(text);
}

6. Cascade notation with if statement:

void main() {
List<int> numbers = [1, 2, 3, 4, 5];

numbers
..add(6)
..add(7)
..add(8)
..forEach((number) {
if (number.isEven) {
print('$number is even');
} else {
print('$number is odd');
}
});
}

In this example, cascade notation (..) is used to perform a sequence of operations on the numbers list.

These examples cover various scenarios where if statements are used in Dart, including basic if-else, if without else, else-if chains, ternary operators, and conditional expressions. Choose the appropriate form based on the complexity and requirements of your code.

Examples
Basic if statement:complete example with explanation

Here’s a complete example of a basic if statement in Dart with an explanation:

void main() {
// Define a variable
int x = 10;

// Use an if statement to check a condition
if (x > 5) {
// Code inside this block will execute if the condition is true
print("x is greater than 5");
} else {
// Code inside this block will execute if the condition is false
print("x is not greater than 5");
}
}

Explanation:

Variable Declaration:

We start by declaring a variable x and assigning it the value 10.

int x = 10;

If Statement:

The if statement is used to check whether the value of x is greater than 5.

if (x > 5) {

True Block:

If the condition x > 5 is true, the code inside the following block will be executed.

print(“x is greater than 5”);

In this case, it will print “x is greater than 5” to the console.

Else Block:

If the condition is false, the code inside the else block will be executed.

} else {
print(“x is not greater than 5”);
}

Here, it will print “x is not greater than 5” to the console.

Result:

When you run this Dart program, it will output:

x is greater than 5

This is because the value of x (10) is indeed greater than 5, so the code inside the if block gets executed.

If the value of x were, for example, 3, then the output would be:

x is not greater than 5

as the condition would be false, and the code inside the else block would be executed.

if statement without an else block:complete example 

Here’s a complete example of an if statement without an else block in Dart, along with an explanation:

void main() {
// Define a variable
int y = 7;

// Use an if statement without an else block
if (y % 2 == 0) {
// Code inside this block will execute if the condition is true
print("y is an even number");
}

// The program continues here after the if statement
print("Program continues...");
}

Explanation:

Variable Declaration:

We declare a variable y and assign it the value 7.

int y = 7;

If Statement Without Else:

The if statement checks whether the value of y is even (y % 2 == 0).

if (y % 2 == 0) {

True Block:

If the condition is true (meaning y is an even number), the code inside the following block will execute.

print(“y is an even number”);

In this case, since y is not an even number (it’s 7), this block will be skipped.

Program Continues:

Regardless of whether the condition is true or false, the program continues with the next statement after the if block.

print(“Program continues…”);

Result:

When you run this Dart program, the output will be:

Program continues…
Since the condition in the if statement is false, the code inside the block is not executed, and the program continues to the next statement after the if block.

This example illustrates how an if statement without an else block allows the program to continue executing the following code regardless of whether the condition is true or false.

if-else if-else statement:complete example with explanation

Here’s a complete example of an if-else if-else statement in Dart, along with an explanation:

void main() {
// Define a variable
int z = 0;

// Use an if-else if-else statement
if (z > 0) {
// Code inside this block will execute if the first condition is true
print("z is positive");
} else if (z < 0) {
// Code inside this block will execute if the first condition is false
// and this condition is true
print("z is negative");
} else {
// Code inside this block will execute if both previous conditions are false
print("z is zero");
}
}

Explanation:

Variable Declaration:

We declare a variable z and assign it the value 0.

int z = 0;

If-Else If-Else Statement:

The if-else if-else statement checks multiple conditions in order.

if (z > 0) {

First Condition (if):

If the first condition is true (meaning z is greater than 0), the code inside the first block will execute.

print(“z is positive”);

Since z is 0 in this example, this block will be skipped.

} else if (z < 0) {

Second Condition (else if):

If the first condition is false and the second condition is true (meaning z is less than 0), the code inside the second block will execute.

print(“z is negative”);
Since z is 0 in this example, this block will also be skipped.

} else {

Default (else):

If both previous conditions are false, the code inside the else block will execute.

print(“z is zero”);

In this case, since z is 0, this block will be executed, and it will print “z is zero.”

Result:

When you run this Dart program, the output will be:

z is zero

This example demonstrates how an if-else if-else statement allows you to check multiple conditions in a structured manner and execute the corresponding block of code based on the first true condition.

Ternary operator:complete example with explanation

Here’s a complete example of using the ternary operator in Dart, along with an explanation:

void main() {
// Define a variable
int a = 15;

// Use a ternary operator to assign a value based on a condition
String result = (a > 10) ? "a is greater than 10" : "a is not greater than 10";

// Print the result
print(result);
}

Explanation:

Variable Declaration:

We declare a variable a and assign it the value 15.

int a = 15;

Ternary Operator:

The ternary operator (condition) ? expression_if_true : expression_if_false allows us to write a concise conditional expression.

String result = (a > 10) ? “a is greater than 10” : “a is not greater than 10”;
In this case, if the condition (a > 10) is true, it evaluates to the value on the left of : (i.e., “a is greater than 10”). If the condition is false, it evaluates to the value on the right of : (i.e., “a is not greater than 10”).

Result:

The result of the ternary operator is assigned to the variable result.

Print the Result:

Finally, we print the value of the result variable to the console.

print(result);

Output:

When you run this Dart program, the output will be:

a is greater than 10
This is because the condition (a > 10) is true (15 is indeed greater than 10), so the expression on the left of : is chosen.

The ternary operator is a concise way to express conditional statements and is often used when you need to assign a value based on a simple condition.

Conditional expressions:complete example with explanation

Here’s a complete example of using conditional expressions in Dart, along with an explanation:

void main() {
// Define a variable
int value = 42;

// Use a conditional expression to determine a message based on the value
String text = value > 0 ? "Positive" : value < 0 ? "Negative" : "Zero";

// Print the result
print(text);
}

Explanation:

Variable Declaration:

We declare a variable value and assign it the value 42.

int value = 42;

Conditional Expression:

Dart supports a concise form of conditional expressions using the ? and : operators.

The syntax is condition ? expression_if_true : expression_if_false.

String text = value > 0 ? “Positive” : value < 0 ? “Negative” : “Zero”;

In this example, if value > 0 is true, it evaluates to “Positive.” If it’s false, it checks value < 0. If that is true, it evaluates to “Negative.” If both conditions are false, it evaluates to “Zero.”

Result:

The result of the conditional expression is assigned to the variable text.

Print the Result: Finally, we print the value of the text variable to the console.

print(text);

Output:

When you run this Dart program, the output will be:

Positive
This is because the condition value > 0 is true (42 is indeed greater than 0), so the first expression “Positive” is chosen.

Conditional expressions in Dart are a compact way to express conditional logic, especially when you have a series of conditions to check.

An application simple Dart console application with explanation

Let’s create a simple Dart console application that incorporates the concepts of if statements, if-else statements, ternary operators, and conditional expressions. We’ll create a program that takes a ‘s input, checks if it’s a positive or negative number, and prints a corresponding message.

import 'dart:io';

void main() {
// Prompt the to enter a number
stdout.write("Enter a number: ");

// Read input
String Input = stdin.readLineSync() ?? "";

// Convert the input to an integer
int number = int.tryParse(Input) ?? 0;

// Use if-else statement to check if the number is positive, negative, or zero
if (number > 0) {
print("The entered number is positive.");
} else if (number < 0) {
print("The entered number is negative.");
} else {
print("The entered number is zero.");
}

// Use a ternary operator to print a message based on the number being even or odd
String parityMessage = (number % 2 == 0) ? "The entered number is even." : "The entered number is odd.";
print(parityMessage);
}

Explanation:

We import the dart:io library to use stdout for output and stdin for input.

The main function is the entry point of the program.

We prompt the to enter a number using stdout.write.

We read the ‘s input using stdin.readLineSync().

We use int.tryParse() to convert the ‘s input to an integer. If conversion fails, it defaults to 0.

An if-else statement is used to check whether the entered number is positive, negative, or zero.

We use a ternary operator to determine if the entered number is even or odd.

The results are printed to the console.

Now, you can run this Dart program, enter a number when prompted, and see messages about whether the number is positive, negative, zero, even, or odd. This example demonstrates the application of if statements, if-else statements, ternary operators, and conditional expressions in a simple console application.

Quiz about this lesson:15 questions with explanation

Here’s a quiz with 15 questions related to Dart programming, specifically focusing on if statements, if-else statements, ternary operators, and conditional expressions. Each question is followed by an explanation of the correct answer.

Quiz:
1-What does the following Dart code print to the console?

dart

int x = 7;
if (x > 5) {
print(“x is greater than 5”);
} else {
print(“x is not greater than 5”);
}
a) “x is greater than 5”
b) “x is not greater than 5”
c) Both a and b
d) None of the above

2-In Dart, what is the purpose of the else keyword in an if-else statement?

a) To indicate the beginning of the else-if block.
b) To execute a block of code if the condition in the if statement is false.
c) To create a loop.
d) To end the if statement.

3-Which of the following is a valid usage of the ternary operator in Dart?

a) result = (x > 10) ? “true” : “false”;
b) result = if (x > 10) then “true” else “false”;
c) result = (x > 10) ? “true” : else “false”;
d) result = (x > 10) then “true” else “false”;

4-What is the purpose of the ?? operator in Dart?

a) Null-aware operator for conditional expressions.
b) Ternary operator.
c) Logical AND operator.
d) Logical OR operator.

5-In Dart, what does the % operator do?

a) Calculates the power of a number.
b) Finds the remainder when one number is divided by another.
c) Checks for equality.
d) Performs a bitwise AND operation.

6-How is a conditional expression written in Dart?

a) condition ? expression_if_true : expression_if_false;
b) if (condition) { /* code */ } else { /* code */ }
c) condition ? { /* code */ } : { /* code */ };
d) if (condition) ? expression_if_true : expression_if_false;

7-Which Dart operator is used for cascade notation?

a) ..
b) =>
c) ~
d) ++

8-What is the output of the following Dart code?

dart

int a = 15;
String result = (a > 10) ? “High” : “Low”;
print(result);
a) “High”
b) “Low”
c) “True”
d) “False”

9-In Dart, how is a single-line comment denoted?

a) // comment
b) /* comment */
c) — comment
d) # comment

10-Which Dart keyword is used to declare a variable?

a) var
b) int
c) variable
d) declare

11-What is the purpose of the stdout.write function in Dart?

a) Reads input from the console.
b) Writes output to the console without a newline character.
c) Declares a new variable.
d) Performs string concatenation.

12-How can you check if a variable is null in Dart?

a) if (variable == null) { /* code */ }
b) if (variable isNull) { /* code */ }
c) if (variable === null) { /* code */ }
d) if (variable.notNull) { /* code */ }

13-What is the purpose of the ?? operator in Dart conditional expressions?

a) Logical AND operator.
b) Null-aware operator for conditional expressions.
c) Ternary operator.
d) Logical OR operator.

14-Which Dart library is used for input and output operations?

a) dart:math
b) dart:convert
c) dart:async
d) dart:io

15-In Dart, what is the purpose of stdin.readLineSync()?

a) Reads a line of text from the console.
b) Reads an integer from the console.
c) Reads a boolean value from the console.
d) Reads a double-precision floating-point number from the console.

Explanation:

1-Correct Answer: a) “x is greater than 5”

Explanation: The condition x > 5 is true, so the code inside the if block will be executed.

2-Correct Answer: b) To execute a block of code if the condition in the if statement is false.

Explanation: The else block is executed when the condition in the if statement is false.

3-Correct Answer: a) result = (x > 10) ? “true” : “false”;

Explanation: The ternary operator syntax is condition ? expression_if_true : expression_if_false.

4-Correct Answer: a) Null-aware operator for conditional expressions.

Explanation: The ?? operator is used for null-aware conditional expressions.

5-Correct Answer: b) Finds the remainder when one number is divided by another.

Explanation: The % operator in Dart is used to find the remainder.

6-Correct Answer: a) condition ? expression_if_true : expression_if_false;

Explanation: This is the correct syntax for a conditional expression in Dart.

7-Correct Answer: a) ..

Explanation: The .. operator is used for cascade notation in Dart.

8-Correct Answer: a) “High”

Explanation: The condition (a > 10) is true, so “High” is chosen.

9-Correct Answer: a) // comment

Explanation: Single-line comments in Dart are denoted by //.

10-Correct Answer: a) var

Explanation: The var keyword is used to declare a variable without specifying its type.

11-Correct Answer: b) Writes output to the console without a newline character.

Explanation: stdout.write is used to write output to the console without adding a newline.

12-Correct Answer: a) if (variable == null) { /* code */ }

Explanation: To check if a variable is null in Dart, use the equality check

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