Learn conditional logic with branch and loop statements

This tutorial teaches you how to write code that examines variables and changes execution path based on those variables. You'll use your browser to write C# interactively and see the results of compiling and running your code. This tutorial contains a series of lessons that explore branching and looping constructs in C#. These lessons teach you the fundamentals of the C# language.

Make decisions using the if statement

• 48 minutes remaining

Run the following code in the interactive window. Select the Enter focus mode button. Then, type the following code block in the interactive window and select Run:

int a = 5;
int b = 6;
if (a + b > 10)
    Console.WriteLine("The answer is greater than 10.");

If you are running this on your environment, you should follow the instructions for the local version instead.

Modify the declaration of b so that the sum is less than 10:

int b = 3;

Select the Run button again. Because the answer is less than 10, nothing is printed. The condition you're testing is false. You don't have any code to execute because you've only written one of the possible branches for an if statement: the true branch.

This first sample shows the power of if and boolean types. A boolean is a variable that can have one of two values: true or false. C# defines a special type, bool for boolean variables. The if statement checks the value of a bool. When the value is true, the statement following the if executes. Otherwise, it's skipped.

This process of checking conditions and executing statements based on those conditions is powerful. Let's explore more.

Make if and else work together

• 44 minutes remaining

To execute different code in both the true and false branches, you create an else branch that executes when the condition is false. Try this:

int a = 5;
int b = 3;
if (a + b > 10)
    Console.WriteLine("The answer is greater than 10");
else
    Console.WriteLine("The answer is not greater than 10");

The statement following the else keyword executes only when the condition being tested is false. Combining if and else with boolean conditions provides all the power you need.

Because indentation isn't significant, you need to use { and } to indicate when you want more than one statement to be part of the block that executes conditionally. C# programmers typically use those braces on all if and else clauses. The following example is the same as what you created. Try it.

int a = 5;
int b = 3;
if (a + b > 10)
{
    Console.WriteLine("The answer is greater than 10");
}
else
{
    Console.WriteLine("The answer is not greater than 10");
}

You can test more complicated conditions:

int a = 5;
int b = 3;
int c = 4;
if ((a + b + c > 10) && (a == b))
{
    Console.WriteLine("The answer is greater than 10");
    Console.WriteLine("And the first number is equal to the second");
}
else
{
    Console.WriteLine("The answer is not greater than 10");
    Console.WriteLine("Or the first number is not equal to the second");
}

The == symbol tests for equality. Using == distinguishes the test for equality from assignment, which you saw in a = 5.

The && represents "and". It means both conditions must be true to execute the statement in the true branch. These examples also show that you can have multiple statements in each conditional branch, provided you enclose them in { and }.

You can also use || to represent "or":

int a = 5;
int b = 3;
int c = 4;
if ((a + b + c > 10) || (a == b))
{
    Console.WriteLine("The answer is greater than 10");
    Console.WriteLine("Or the first number is equal to the second");
}
else
{
    Console.WriteLine("The answer is not greater than 10");
    Console.WriteLine("And the first number is not equal to the second");
}

Modify the values of a, b, and c and switch between && and || to explore. You'll gain more understanding of how the && and || operators work.

Use loops to repeat operations

• 34 minutes remaining

Another important concept to create larger programs is loops. You'll use loops to repeat statements that you want executed more than once. Try this code in the interactive window:

int counter = 0;
while (counter < 10)
{
  Console.WriteLine($"Hello World! The counter is {counter}");
  counter++;
}

The while statement checks a condition and executes the statement following the while. It will repeat checking the condition and executing those statements until the condition is false.

There's one other new operator in this example. The ++ after the counter variable is the increment operator. It adds 1 to the value of counter, and stores that value in the counter variable.

The while loop tests the condition before executing the code following the while. The do ... while loop executes the code first, and then checks the condition. It looks like this:

int counter = 0;
do
{
  Console.WriteLine($"Hello World! The counter is {counter}");
  counter++;
} while (counter < 10);

This do loop and the earlier while loop work the same.

Let's move on to one last loop statement.

Work with the for loop

• 28 minutes remaining

Another common loop statement that you'll see in C# code is the for loop. Try this code in the interactive window:

for (int counter = 0; counter < 10; counter++)
{
  Console.WriteLine($"Hello World! The counter is {counter}");
}

This does the same work as the while loop and the do loop you've already used. The for statement has three parts that control how it works.

The first part is the for initializer: int counter = 0; declares that counter is the loop variable, and sets its initial value to 0.

The middle part is the for condition: counter < 10 declares that this for loop continues to execute as long as the value of counter is less than 10.

The final part is the for iterator: counter++ specifies how to modify the loop variable after executing the block following the for statement. Here, it specifies that counter should be incremented by 1 each time the block executes.

Experiment with these yourself. Try each of the following:

• Change the initializer to start at a different value.
• Change the condition to stop at a different value.

When you're done, let's move on to write some code yourself to use what you've learned.

There's one other looping statement that isn't covered in this tutorial: the foreach statement. The foreach statement repeats its statement for every item in a sequence of items. It's most often used with collections, so it is covered in the next tutorial.

Created nested loops

• 23 minutes remaining

A while, do, or for loop can be nested inside another loop to create a matrix using the combination of each item in the outer loop with each item in the inner loop. Let's do that to build a set of alphanumeric pairs to represent rows and columns.

One for loop can generate the rows:

for (int row = 1; row < 11; row++)
{
  Console.WriteLine($"The row is {row}");
}

Another loop can generate the columns:

for (char column = 'a'; column < 'k'; column++)
{
  Console.WriteLine($"The column is {column}");
}

You can nest one loop inside the other to form pairs:

for (int row = 1; row < 11; row++)
{
  for (char column = 'a'; column < 'k'; column++)
  {
    Console.WriteLine($"The cell is ({row}, {column})");
  }
}

You can see that the outer loop increments once for each full run of the inner loop. Reverse the row and column nesting, and see the changes for yourself.

Combine branches and loops

• 13 minutes remaining

Now that you've seen the if statement and the looping constructs in the C# language, see if you can write C# code to find the sum of all integers 1 through 20 that are divisible by 3. Here are a few hints:

• The % operator gives you the remainder of a division operation.
• The if statement gives you the condition to see if a number should be part of the sum.
• The for loop can help you repeat a series of steps for all the numbers 1 through 20.

Try it yourself. Then check how you did. As a hint, you should get 63 for an answer.

Complete challenge

• 1 minutes remaining

Did you come up with something like this?

int sum = 0;
for (int number = 1; number < 21; number++)
{
  if (number % 3 == 0)
  {
    sum = sum + number;
  }
}
Console.WriteLine($"The sum is {sum}");

Congratulations!

• 100% complete!

You've completed the "branches and loops" interactive tutorial. You can select the list collection link below to start the next interactive tutorial, or you can visit the .NET site to download the .NET SDK, create a project on your machine, and keep coding. The "Next steps" section brings you back to these tutorials.

You can learn more about these concepts in these articles:

• Selection statements
• Iteration statements