if and else

Picture a fork in the road with a signpost: "If it's hot, go left, to the river. Otherwise go right, to the park." You read the condition and pick one road, and this time you don't take the other one.

So far, the programs in this course have run all their statements in a row, from top to bottom. But a program needs forks too: one answer for hot weather, another for cold.

Such a choice is written with the if statement. After it, the parentheses hold a condition, and the braces hold code that runs only when the condition is true:

int t = 20;

if (t > 30)
{
    Console.WriteLine("hot");
}
else if (t > 15)
{
    Console.WriteLine("warm"); // warm
}
else
{
    Console.WriteLine("cold");
}

Let's walk through the program step by step:

1. int t = 20; puts the temperature outside into the variable t.
2. The condition t > 30 is false: 20 is not greater than 30. The code in the braces under it is skipped.
3. The condition t > 15 is true, so the code under else if runs, and the program prints warm.
4. The program never gets to else: one branch has already run.

With t = 35, the program prints hot. With t = -5, both conditions are false, and the last part runs: the program prints cold.

A condition is a bool

A condition is an expression of type bool: its value is true or false. Usually it is a comparison like t > 30, or several comparisons joined with && and ||:

int age = 20;
bool hasTicket = true;

if (age >= 18 && hasTicket)
{
    Console.WriteLine("Welcome"); // Welcome
}

Here age >= 18 is true and hasTicket is true, so the whole condition is true, and the program prints Welcome. Without a ticket, the program would print nothing: this if has no else, and when the condition is false the block is skipped entirely.

A number or a string cannot be a condition. if (t) does not compile: C# does not turn an int into a bool on its own. You need a comparison, such as if (t != 0).

For the same reason, the typo if (t = 30) does not compile either. As you remember from the lesson on comparisons, a single = is assignment, and the value of such code is the number 30, not a bool.

Three kinds of branches

The code in braces after if, else if or else is called a branch. The first example has three of them, and each has its own role.

The if branch

The if branch opens the fork. An if is written in three parts:

• the keyword if;
• a condition in parentheses, for example (t > 30);
• a block in braces that runs when the condition is true.

If the condition is false, the program skips the block and moves on. An if can also stand without else, as in the ticket example.

The else if branch

An else if branch adds one more condition to the fork. There can be any number of them between if and else.

The conditions are checked from top to bottom, and only the first branch with a true condition runs. With t = 35, both t > 30 and t > 15 are true, but the program prints only hot.

The order of branches matters. If you swap the branches with t > 30 and t > 15, the program prints warm for t = 35: the first true condition is now t > 15. The stricter condition goes higher.

This is where the road analogy breaks down. A traveler can read every signpost and pick the best one. The program follows the first signpost with a true condition and does not read the rest.

The else branch

The else branch has no condition. It runs when every condition above it is false, and it always comes last.

If a branch holds one statement, you may leave out the braces. In this course's code they are always there, each on its own line: that way the branch's borders are visible at once.

The conditional operator ?:

Sometimes you need to choose a value rather than code: one of two strings or one of two numbers. That is what the conditional operator ?: is for. It is also called the ternary operator, because it has three parts: condition ? a : b. If the condition is true, the whole expression equals a; otherwise it equals b:

int age = 16;
string status = age >= 18 ? "adult" : "minor";

Console.WriteLine(status); // minor

Let's go step by step:

1. The condition age >= 18 is false: 16 is less than 18.
2. So the whole expression equals the second value, "minor", and that goes into status.
3. Console.WriteLine(status) prints minor.

Both values must fit the variable on the left. In status, of type string, you can choose one of two strings, but the compiler rejects a number in place of "minor".

if is a statement: it decides which code runs. ?: is an expression: it has a value, which you can store in a variable or pass straight to Console.WriteLine. For three or more options, an if and else if chain is easier to read.

Common mistake

A semicolon follows the condition:

int t = 10;

if (t > 30); // ✗
{
    Console.WriteLine("hot");
}

warning CS0642: Possible mistaken empty statement

The program compiles with this warning and prints hot, even though 10 is not greater than 30. The message means that an empty statement may have been put there by mistake.

The semicolon ended the if with an empty statement: with a true condition, the if runs it and does nothing. The block in braces no longer belongs to the if and always runs. Remove the ; after (t > 30), and the block becomes a branch again.

Key points

• if runs its branch when the condition is true, and else runs when every condition above it is false.
• An else if chain checks the conditions from top to bottom and runs only the first matching branch.
• A condition is a bool value: C# does not accept a number or a string as a condition.
• The conditional operator condition ? a : b picks one of two values.