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Learn about control flow: what it is, why programmers use it, and where they use it.

Control Flow

_Repeat a sound four times a measure._

Computers are good at repetitive tasks: they exactly follow instructions and perform them quickly. But if the programmer has to write the same instructions for every repetition, they waste time and increase the chances of a mistake. 

In our example, instead of literally writing "play a sound, play a sound, play a sound, play a sound", programmers can use a _loop_ structure. A loop is a sequence of instructions which is specified once but which may be carried out several times in a row. In this case there is one instruction (play a sound) and it should be carried out 4 times.

Loop structures can also specify that the instructions are repeated until a certain condition is met. For example, "Repeat a sound until you are booed off stage".


Loop

_Collect shipping and email address. Reject a purchase if either is incorrect._

Errors, or exceptions, happen all the time in programming. You can't escape them. But you can handle them gracefully. 

In exception handling, two sets of instructions are defined. The first set is executed, and if an exception occurs, the first set is stopped and the second set of instructions is executed.

In this example,
* The first set is "collect shipping address", "collect email address"
* The second set is "reject the purchase"

If any errors occur in executing the first set—maybe the address is incorrectly formatted, or fake—it is stopped and the second set is executed.

These statements are useful if you know an exception may occur in a set of instructions but you don't know in which one, or you expect the same exception might occur in multiple, sequential instructions.


Exception

Let's recreate the _conditional_ control structure using JavaScript! (The one about social media posts, remember?)

![Diagram of conditional control structure](https://content.codecademy.com/programs/code-foundations-path/bop-ii/conditional-control-flow-4.png "Conditional Control Structure")

The control flow shown above is written as a _conditional_ control structure in **main.js**. It looks something like this:
```js
if (condition) {
  DO SOMETHING
} else {
  DO SOMETHING DIFFERENT
}
```
If the `condition` evaluates to true, the computer will execute the body of that statement. In this case, it's `DO SOMETHING`.

If the `condition` is false, the computer will execute the body of the `else` statement. In this case, it's `DO SOMETHING DIFFERENT`.

Let's adapt this general template to our example:
1. The `condition` is true when the privacy `mode` is set to `'public'`. This is already written for you in **main.js**.
2. You'll replace `DO SOMETHING` with a JavaScript function: `showDetails()`.
3. You'll replace `DO SOMETHING DIFFERENT` with another function: `hideDetails()`.

The instructions below will show you how to do this in code!



Putting It All Together

How do computers make decisions? Here are a few examples:

* Web surfing: Open the browser, load the text, load the images.
* Social media: If the user's profile is private, hide their posts. Otherwise, make them visible.
* Electronic music: Repeat a note four times a measure.
* Shopping: Collect shipping and contact information. Reject a purchase if either is incorrect.

Each of these _programs_ is a set of instructions, executed in a certain order. This order, called _control flow_, is just as important as the instructions themselves.

Programmers use _control structures_ to alter _control flow_. Control structures include conditionals &mdash; "do these instructions if Z is true", loops &mdash; "do these instructions X many times",  and exceptions &mdash; "do instruction set A, but if an error occurs switch to instruction set B". Programmers use these structures in their code to tell a computer which instructions (or in which order) to execute.

In this lesson you will learn the fundamental concepts of control flow and three essential control structures. By the end of the lesson you should be able to: 
* Define control flow
* Explain the difference between conditional, loop, and exception control structures
* Give examples for each 


_If the user's profile is private, hide their posts. Otherwise, make them visible._

Computers can run instructions on their own, leaving us to work on more interesting and creative problems. But computers can't make decisions on their own. Programmers can give them multiple sets of instructions and describe the right conditions in which to use each set. They can describe a structure like the quoted program above: "if X, then do instruction set 1. Otherwise do instruction set 2." This structure is called a _conditional_ control structure because the computer's instructions depend on some _condition(s)_.

In our social media example, all the instructions are shown in the diagram, but only some of them will be executed each time. Here they are in text form:

1. Check profile privacy
2. Hide the post
3. Show the post

When is step 2 executed and when is 3 executed? The answer is: it depends. 
The control flow is 1-2 if the profile is private and 1-3 if the profile is public. The condition is the privacy setting on the profile.

Conditional control structures, or just conditionals, allow programs to do different things in different scenarios. As you can see, they follow a logic similar to how humans think, making it easy to write clear code while still handling complex processes.


Conditional

Well done! Control flow is a fundamental concept in programming, and learning the basics will serve you wherever your path leads, regardless of language or domain. In review:

- _Control flow_ is the order in which instructions are executed.
- _Control structures_ alter control flow in a program. You learned three structures, each suited for a different scenario:
  - _Conditional:_ "if some condition is met, then do X. Otherwise, do Y".
  - _Loop:_ "do something Z number of times" or "do something repeatedly until some condition is met".
  - _Exception:_ "do steps A, B, C. If an error occurs, stop, and do steps J, K, L".