Flow control in C#
In this part of the C# tutorial, we will talk about the flow control. We will define several keywords that enable us to control the flow of a C# program.
In C# language there are several keywords that are used to alter the flow of the program. When the program is run, the statements are executed from the top of the source file to the bottom. One by one. This flow can be altered by specific keywords. Statements can be executed multiple times. Some statements are called conditional statements. They are executed only if a specific condition is met.
The if statement
The if statement has the following general form:
if (expression)
{
statement;
}
The if keyword is used to check if an expression
is true. If it is true, a statement is then executed. The statement can
be a single statement or a compound statement. A compound statement
consists of multiple statements enclosed by the block. A block is code
enclosed by curly brackets.
using System;
public class CSharpApp
{
static void Main()
{
int num = 31;
if (num > 0)
{
Console.WriteLine("num variable is positive");
}
}
}
We have a num variable. It is assigned 31. The if keyword
checks for a boolean expression. The expression is put between square
brackets. 31 > 0 is true, so the statement inside the block is executed.
$ ./ifstatement.exe num variable is positive
The condition is met and the message is written to the console.
using System;
public class CSharpApp
{
static void Main()
{
int num = 31;
if (num > 0)
{
Console.WriteLine("num variable is positive");
Console.WriteLine("num variable equals {0}", num);
}
}
}
More statements can be executed inside the block, enclosed by curly brackets.
We can use the else keyword to create a simple branch.
If the expression inside the square brackets following the if keyword
evaluates to false, the statement following the else
keyword is automatically executed.
using System;
public class CSharpApp
{
static void Main()
{
string sex = "female";
if (sex == "male") {
Console.WriteLine("It is a boy");
} else {
Console.WriteLine("It is a girl");
}
}
}
We have a sex variable. It has "female" string. The boolean expression evaluates to false and we get "It is a girl" in the console.
$ ./branch.exe It is a girl
We can create multiple branches using the else if keyword.
The else if keyword tests for another condition, if and only if
the previous condition was not met. Note, that we can use multiple
else if keywords in our tests.
using System;
public class CSharpApp
{
static void Main()
{
int a = 0;
if (a < 0) {
Console.WriteLine("a is negative");
} else if (a == 0) {
Console.WriteLine("a equals to zero");
} else {
Console.WriteLine("a is a positive number");
}
}
}
We have a numerical variable and we test it, if it is a negative number or
positive or if it equals to zero. The first expression evaluates to false.
The second condition is met. The program prints 'a equals to zero' to the
console. The rest of the branch is skipped. If the previous conditions were
not met, than the statement following the else keyword would
be executed.
The switch statement
The switch statement is a selection control flow statement.
It allows the value of a variable or expression to control the flow of program
execution via a multi way branch. It creates multiple branches in a simpler way
than using the combination of if, else if
statements.
We have a variable or an expression. The switch keyword is used
to test a value from the variable or the expression against a list of values.
The list of values is presented with the case keyword.
If the values match, the statement following the case is executed.
There is an optional default statement. It is executed, if no other
match is found.
using System;
public class CSharpApp
{
static void Main()
{
string domain = Console.ReadLine();
switch (domain) {
case "us":
Console.WriteLine("United States");
break;
case "de":
Console.WriteLine("Germany");
break;
case "sk":
Console.WriteLine("Slovakia");
break;
case "hu":
Console.WriteLine("Hungary");
break;
default:
Console.WriteLine("Unknown");
break;
}
}
}
In our program, we have a domain variable. We read a value for the variable
from the command line. We use the case statement to
test for the value of the variable. There are several options. If the value
equals for example to "us" the "United States" string is printed to the console.
$ ./selectcase.exe hu Hungary
We have entered "hu" string to the console and the program responded with "Hungary".
The while statement
The while statement is a control flow statement
that allows code to be executed repeatedly based on a given boolean condition.
This is the general form of the while loop:
while (expression)
{
statement;
}
The while keyword executes the statements inside the
block enclosed by the curly brackets. The statements are executed each time
the expression is evaluated to true.
using System;
public class CSharpApp
{
static void Main()
{
int i = 0;
int sum = 0;
while (i < 10)
{
i++;
sum += i;
}
Console.WriteLine(sum);
}
}
In the code example, we calculate the sum of values from a range of numbers.
The while loop has three parts. Initialization, testing and
updating. Each execution of the statement is called a cycle.
int i = 0;
We initiate the i variable. It is used as a counter.
while (i < 10)
{
...
}
The expression inside the square brackets following the while
keyword is the second phase, the testing. The statements in the body are
executed, until the expression is evaluated to false.
i++;
The last, third phase of the while loop. The updating. We increment
the counter. Note that improper handling of the while loops may lead
to endless cycles.
It is possible to run the statement at least once. Even if the condition is not met.
For this, we can use the do while keywords.
using System;
public class CSharpApp
{
static void Main()
{
int count = 0;
do {
Console.WriteLine(count);
} while (count != 0);
}
}
First the iteration is executed and then the truth expression is evaluated.
The for statement
When the number of cycles is know before the loop is initiated,
we can use the for statement. In this construct
we declare a counter variable, which is automatically increased
or decreased in value during each repetition of the loop.
using System;
public class CSharpApp
{
static void Main()
{
for (int i=0; i<9; i++)
{
Console.WriteLine(i);
}
}
}
In this example, we print numbers 0..9 to the console.
for (int i=0; i<9; i++)
{
Console.WriteLine(i);
}
There are three phases. First, we initiate the counter i to zero. This phase is done only once. Next comes the condition. If the condition is met, the statement inside the for block is executed. Then comes the third phase; the couter is increased. Now we repeat the 2, 3 phases until the condition is not met and the for loop is left. In our case, when the counter i is equal to 9, the for loop stops executing.
The foreach statement
The foreach construct simplifies traversing over
collections of data. It has no explicit counter. The foreach
statement goes through the array or collection one by one and
the current value is copied to a variable defined in the construct.
using System;
public class CSharpApp
{
static void Main()
{
string[] planets = { "Mercury", "Venus",
"Earth", "Mars", "Jupiter", "Saturn",
"Uranus", "Neptune" };
foreach (string planet in planets)
{
Console.WriteLine(planet);
}
}
}
In this example, we use the foreach statement to go
through an array of planets.
foreach (string planet in planets)
{
Console.WriteLine(planet);
}
The usage of the foreach statement is straightforward.
The planets is the array, that we iterate through. The planet is the
temporary variable, that has the current value from the array.
The foreach statement goes through all the planets
and prints them to the console.
$ ./planets.exe Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune
Running the above C# program gives this output.
The break, continue statements
The break statement can be used to terminate
a block defined by while, for
or switch statements.
using System;
public class CSharpApp
{
static void Main()
{
Random random = new Random();
while (true)
{
int num = random.Next(1, 30);
Console.Write(num + " ");
if (num == 22)
break;
}
Console.Write('\n');
}
}
We define an endless while loop. We use the break
statement to get out of this loop. We choose a random value from 1 to 30.
We print the value. If the value equals to 22, we finish the endless while loop.
$ ./break.exe 29 21 26 6 29 3 10 3 18 6 3 22
We might get something like this.
The continue statement is used to skip a
part of the loop and continue with the next iteration of the loop.
It can be used in combination with for and
while statements.
In the following example, we will print a list of numbers, that cannot be divided by 2 without a remainder.
using System;
public class CSharpApp
{
static void Main()
{
int num = 0;
while (num < 1000)
{
num++;
if ((num % 2) == 0)
continue;
Console.Write(num + " ");
}
Console.Write('\n');
}
}
We iterate through numbers 1..999 with the while loop.
if ((num % 2) == 0)
continue;
If the expression num % 2 returns 0, the number in question can be divided by 2.
The continue statement is executed and the rest of the cycle is skipped.
In our case, the last statement of the loop is skipped and the number is not printed to the
console. The next iteration is started.
In this part of the C# tutorial, we were talking about control flow structures.