Control Flow

So far our programs have been straight-line: do this, then this, then this. Real programs need to make decisions ("if the user is an admin, show the admin panel") and repeat work ("for every item in the cart, add up the price"). These ideas together are called control flow.

C# gives you four core control-flow tools:

We'll go through each one with small, runnable examples.

if / else if / else

The most fundamental decision-making tool.

A few things to notice:

• The condition must be a bool. In C#, you cannot write if (score) and have the language interpret it as "non-zero is true" the way C does.
• Branches are evaluated top-down, and only the first matching branch runs. Once one matches, the rest are skipped.
• You always want { } around the body, even for one-line bodies. It prevents subtle bugs when you add a second line later.

Boolean expressions

The condition inside an if is just a boolean expression. The basic operators:

&& and || are short-circuit — they stop as soon as the answer is known. That can matter when one side might be expensive or unsafe to evaluate.

switch: branch on a value

When you have many possible values for one variable, a chain of if/else if gets noisy. switch is cleaner.

That's a modern switch expression (introduced in C# 8). The older switch statement form also exists, but the expression form is cleaner for the simple "pick one of these values" case.

The _ (underscore) is the catch-all "anything else" pattern.

Loops: doing the same thing many times

A loop is a block of code that runs repeatedly. C# has four loop forms; you'll mostly use two (foreach and for).

while — "as long as this is true"

The condition is checked before each iteration. If it's false to begin with, the body never runs.

do-while — "run, then check"

The body runs at least once, and the condition is checked after each iteration.

You'll use this less often than while. Mostly when you want a "prompt-and-validate" loop and the first prompt always happens.

for — "counter loop"

Used when you know up front how many iterations you want, or you need a counter variable.

A for loop has three parts in its header:

1. Initializer: runs once before the loop starts (int i = 0).
2. Condition: checked before each iteration (i < 5).
3. Increment: runs after each iteration (i++).

foreach — "do this to every item"

Almost always your best choice when iterating a collection.

foreach works on arrays, lists, dictionaries, sets, strings, and anything else that implements an interface called IEnumerable<T>. You'll meet that interface later. For now, just know: if it looks like a collection, foreach works on it.

break and continue

Two useful escape hatches:

• break exits the current loop immediately.
• continue skips to the next iteration of the current loop.

That prints 1, 3, 5 and then stops when i reaches 7.

A worked example: FizzBuzz

The classic small problem: print numbers 1 to 15, but for multiples of 3 print "Fizz", for multiples of 5 print "Buzz", and for multiples of both print "FizzBuzz".

This brings together loops, if/else if/else, and the modulo (%) operator. The order of the branches matters: check "divisible by 15" first, because every multiple of 15 is also a multiple of 3 and 5.

Picking the right loop

A quick rule of thumb:

If you're not sure, start with foreach. It's the safest and clearest in most situations.

Practice

vowels = 5

Test your understanding