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.
Tip
To paste a code snippet inside the focus mode you should use your keyboard shortcut (Ctrl + v, or cmd + v).
Make decisions using the if statement
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.
Tip
As you explore C# (or any programming language), you'll make mistakes when you write code. The compiler will find those errors and report them to you. When the output contains error messages, look closely at the example code, and the code in the interactive window to see what to fix. That exercise will help you learn the structure of C# code.
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
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.
Important
The indentation under the if and else statements is for human readers.
The C# language doesn't treat indentation or white space as significant.
The statement following the if or else keyword will be executed based
on the condition. All the samples in this tutorial follow a common
practice to indent lines based on the control flow of statements.
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");
}
Tip
Through the rest of this tutorial, the code samples all include the braces, following accepted practices.
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
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($"Hello World! The counter is {counter}");
counter++;
}
The while statement checks a condition and executes the statement
following the while. It will repeat checking the condition and
executing those statements until the condition is false.
There's one other new operator in this example. The ++ after
the counter variable is the increment operator. It adds 1
to the value of counter, and stores that value in the counter variable.
Important
Make sure that the while loop condition does switch to
false as you execute the code. Otherwise, you create an
infinite loop where your program never ends. Let's
not demonstrate that, because the engine that runs your
code will time out and you'll see no output from your program.
The while loop tests the condition before executing the code
following the while. The do ... while loop executes the
code first, and then checks the condition. It looks like this:
int counter = 0;
do
{
Console.WriteLine($"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
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($"Hello World! The counter is {counter}");
}
This does the same work as the while loop and the do loop you've
already used. The for statement has three parts that control
how it works.
The first part is the for initializer: int counter = 0; declares
that counter is the loop variable, and sets its initial value to 0.
The middle part is the for condition: counter < 10 declares that this
for loop continues to execute as long as the value of counter is less than 10.
The final part is the for iterator: counter++ specifies how to modify the loop
variable after executing the block following the for statement. Here, it specifies
that counter should be incremented by 1 each time the block executes.
Experiment with these yourself. Try each of the following:
- Change the initializer to start at a different value.
- Change the condition to stop at a different value.
When you're done, let's move on to write some code yourself to use what you've learned.
There's one other looping statement that isn't covered in this tutorial: the foreach statement. The foreach statement repeats its statement for every item in a sequence of items. It's most often used with collections, so it is covered in the next tutorial.
Created nested loops
A while, do, or for loop can be nested inside another loop to create a matrix
using the combination of each item in the outer loop with each item in the inner
loop. Let's do that to build a set of alphanumeric pairs to represent rows and columns.
One for loop can generate the rows:
for (int row = 1; row < 11; row++)
{
Console.WriteLine($"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
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
ifstatement gives you the condition to see if a number should be part of the sum. - The
forloop 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
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!
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:
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