by Steve Smith
Programs that always do the same thing aren't nearly as powerful or interesting as those that can make decisions. These decisions can be based on any number of things, from user input to the current state of the program. Based on some kind of condition, the program can choose a different set of instructions to run. The most common C# keyword used to make decisions like this is the if statement. Because the if statement depends on a condition, the statement is sometimes referred to as a conditional statement.
You can use if statements for all kinds of things in your programs. One common use for these statements is validating user input, such as checking to see if any arguments have been passed to a program or method. When you write an if statement, it consists of the if keyword, a logical expression inside of parentheses, and then one or more statements. It's generally considered a best practice to always wrap the statement(s) after an if statement in curly braces ({ }), unless it's a very short statement that can fit on the same line as the if statement. For example, you can exit a method by returning from it if a required argument is not specified:
static void Greet(string name)
{
if (String.IsNullOrEmpty(name)) return;
Console.WriteLine($"Hello, {name}!");
}
using System;
class Program
{
static void Main()
{
Greet(""); // try calling this with "", null, and your name as arguments
}
static void Greet(string name)
{
if (String.IsNullOrEmpty(name)) return;
Console.WriteLine($"Hello, {name}!");
}
}
However, it can cause confusion if you don't use curly braces around the statements that go with your if statement, especially if your indentation is off:
var sum = 10;
if (someCondition)
sum = sum + 5;
sum = sum * 2;
Console.WriteLine(sum);
using System;
class Program
{
static void Main()
{
bool someCondition = false; // try it with both true and false values
var sum = 10;
if (someCondition)
sum = sum + 5;
sum = sum * 2;
Console.WriteLine(sum);
}
}
In this example, when someCondition is true, the value of sum will be 30. Otherwise, it will be 20. However, based on the indentation, the reader might think that the sum = sum * 2 line only occurs when the if condition is true, which isn't the case. If that's the correct behavior, the code should be rewritten as:
var sum = 10;
if (someCondition)
{
sum = sum + 5;
sum = sum * 2;
}
Console.WriteLine(sum);
using System;
class Program
{
static void Main()
{
bool someCondition = false;
var sum = 10;
if (someCondition)
{
sum = sum + 5;
sum = sum * 2;
}
Console.WriteLine(sum);
}
}
Now the above code will print a sum of 30 when the if condition evaluates to true, or the original sum value of 10 otherwise.
Tip {.tip .newLanguage}
It's a good idea to follow a standard code formatting convention when programming. Such conventions often vary, even between similar languages. In C#, for instance, it's recommended that opening curly braces always start on their own line, indented to match their associated keyword. Closing curly braces should also appear on their own line, indented to match their opening brace.
Tip {.tip .vb}
Unlike in VB, C# allows statements on the same line asifstatements. Be aware, too, that unlike VB'sIf-Thenstatements that must end with anEnd Ifstatement, C# uses curly braces to end many kinds of blocks, not justifstatements.
Often, you'll want to have your program do one thing if a condition is true, and another, different thing if the condition is false. Either way, the code should execute only one of these two different paths. This is accomplished using an if-else statement. To create this statement, simply start with any if statement, and follow it with the else keyword and a statement block. Your else block can consist of a single statement without curly braces, but again it's generally much clearer, especially with else statements, to always wrap the statement(s) in curly braces.
For example, let's say you're writing a simple guessing game. You ask the user a question, and they answer it. Either they're correct, or they're not, and in either case you display a message telling them so. You could implement this as follows:
Console.WriteLine("What is the capital of Ohio?");
var answer = Console.ReadLine();
if (answer.ToLower() == "columbus")
{
Console.WriteLine("Correct!");
}
else
{
Console.WriteLine("Sorry, that's not the right answer.");
}
using System;
class Program
{
static void Main()
{
Console.WriteLine("What is the capital of Ohio?");
var answer = "your answer here"; // change this to your answer
if (answer.ToLower() == "columbus")
{
Console.WriteLine("Correct!");
}
else
{
Console.WriteLine("Sorry, that's not the right answer.");
}
}
}
Of course, sometimes the decisions your program needs to make are a little more complicated than just "this or that". You can nest conditional statements within other conditional statements, creating potentially complex trees of conditions. It's generally a good idea to try to limit how deeply you do such nesting, as complex hierarchies of conditional logic are common sources of bugs in software. However, often they're necessary. For example, sorting algorithms often use comparison logic to determine which of two values is greater, or if they're equal, like this:
// two values, x and y, are being compared
if (x < y)
{
return -1;
}
else
{
if (x == y)
{
return 0;
}
else // x must be greater than y
{
return 1;
}
}As you can see, these kinds of nested conditionals can get quite long to write out. One way these can be shortened is by using the else if construct. If your else block is going to contain another if statement, use else if instead, which eliminates the need for one set of curly braces and a level of indentation. Here's the above example, rewritten to use else if:
// two values, x and y, are being compared
if (x < y)
{
return -1;
}
else if (x == y) // == checks for equality between two values
{
return 0;
}
else // x must be greater than y
{
return 1;
}If you find that you have more than 3 independent options in your conditional, you should consider using a different conditional structure: the switch statement.
The switch keyword is used to have your program execute a different section of code for potentially many different cases. The switch statement relies on the value of a single statement. You can use a switch statement to achieve the same behavior as you can with an if-else statement in many cases, but generally switch is preferred when you have more than a few options to deal with. Within a switch block, you define separate case labels, with code to execute. You can have more than one label for a given section of code. Each section must not allow execution to continue to the next section. This is usually achieved by using the break keyword to break out of the switch statement, but you can use other statements (like return) as well.
The following example shows a simple switch statement based on an integer variable:
uint guessedNumber = 1;
switch (guessedNumber)
{
case 0:
Console.WriteLine("Sorry, 0 is not a valid guess.");
break;
case 1:
case 2:
case 3:
Console.WriteLine("You guessed low.");
break;
case 4:
Console.WriteLine("You guessed the right number!");
break;
default:
Console.WriteLine("You guessed high.");
break;
}
using System;
class Program
{
static void Main()
{
uint guessedNumber = 1; // try different numbers to see resulting behavior
switch (guessedNumber)
{
case 0:
Console.WriteLine("Sorry, 0 is not a valid guess.");
break;
case 1:
case 2:
case 3:
Console.WriteLine("You guessed low.");
break;
case 4:
Console.WriteLine("You guessed the right number!");
break;
default:
Console.WriteLine("You guessed high.");
break;
}
}
}
In the example above, you can see how multiple different case labels (in this case, 1, 2, and 3) can be applied to a single switch section ("You guessed low").
Note {.note}
uintis an unsigned integer type, which cannot hold negative values.
Your program can create a random number between 1 and 3 with the following code:
int correctNumber = new Random().Next(3) + 1;Write a program that generates a random number between 1 and 3 and asks the user to guess what the number is. Tell the user if they guess low, high, or get the correct answer. Also, tell the user if their answer is outside of the range of numbers that are valid guesses (less than 1 or more than 3). You can convert the user's typed answer from a string to an int using this code:
int guessedNumber = int.Parse(Console.ReadLine());Note that the above code will crash the program if the user doesn't type an integer value. For this exercise, assume the user will only enter valid guesses.