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If you want to learn how to include conditionals into your Go program, take this lesson! Else, you can use this lesson to review conditionals.

Learn Go: Conditionals

There are more comparison operators that we haven't covered and they may seem familiar from math class:

Operator | Meaning:
--- | ---
`<` | Less than
`>` | Greater than
`<=` | Less than or equal to
`>=` | Greater than or equal to

Like the previous exercise, we're still using the comparison operators to compare the left hand value against the right hand value. For instance:

```go
100 < 200 // Evaluates to true
```

We can read this example as, "Is `100` less than `200`? Yes, that is true.

Let's look at one more example with `<=`:

```go
100.5 <= 100.5 // Evaluates to true
```

Just like the less than operator (`<`), we want to check if the left hand value is less than OR equal to the right hand value. Since `100.5` is less than or equal to `100.5`, it evaluates as `true`. 




Comparison Operators Continued

We make decisions every day based on certain conditions.

Is the alarm ringing? If so, turn it off.
Is it raining? If so, bring an umbrella.
Is the ice cream truck parked outside? If so, it's time for a treat.

We can also implement this decision-making ability in our programs using _conditionals_. Conditionals check on values and, depending on their status (if the values turn out to be `true` or `false`), executes an appropriate block of code in response. 

In this lesson we'll cover:
* `if`, `else if`, and `else` statements.
* comparison operators.
* logical operators.
* the `switch` statement.
* short declarations in conditionals.
* how to use conditionals in randomized conditions. 

If you're ready, Go on to the next exercise. Else, come back when you're ready! 


The Go gopher is deciding between a bed and a rodent wheel based on how tired it feels. 

What are Conditionals?

We can add different conditions to our `if...else` statements using an `else if` statement. Adding an `else if` statement allows us to check another condition after our `if` statement checks its condition. In fact, we can add as many `else if` statements as we'd like to make more complex conditionals!

The `else if` statement always comes after an `if` statement. If we have an `else` statement, then the `else if` comes before it. The `else if` statement also takes a condition. Let’s take a look at the syntax:

```go
position := 2

if position == 1 {
  fmt.Println("You won the gold!")
} else if position == 2 {
  fmt.Println("You got the silver medal.")
} else if position == 3 {
  fmt.Println("Great job on bronze.")
} else {
  fmt.Println("Sorry, better luck next time?")
}
```

Notice that we're able to use `else if` statements to evaluate separate conditions and allow for different possible outcomes. `if`/`else if`/`else` statements are read from top to bottom, so the first condition that evaluates to `true` is the only block of code that gets executed. 

In the example above, since `position == 1` evaluates to `false`, the first block of code isn't executed. Then, we get to our `else if` statement  and `position == 2` evaluates to `true`, the code inside the first `else if` statement is executed. The rest of the conditions are not evaluated. If none of the conditions evaluated to `true`, then the code in the `else` statement would have executed. 


The else if Statement

We can also include a short variable declaration before we provide a condition in either `if` or `switch` statements. Here's the syntax:

```go
x := 8
y := 9
if product := x * y; product > 60 {
  fmt.Println(product, "  is greater than 60")
}
```

In our `if` statement, we first declare `product`. Notice that `product` is separated from the condition using a semi-colon `;`. We can also have a short variable declaration inside a `switch` statement: 

```go
switch season := "summer" ; season {
case "summer":
  fmt.Println("Go out and enjoy the sun!")
}
```

One thing to keep in mind when using the short variable declaration in `if` or `switch` statements is that the declared variable is _scoped_ to the statement blocks. In programming, scope refers to where variables can be accessed. Having the variable scoped to the `if… else if… else` statement or `switch` statement means that variable is only accessible within the blocks of those statements and not anywhere else. 

```go
x := 8
y := 9
if product := x * y; product > 60 {
  fmt.Println(product, "  is greater than 60")
}
fmt.Println(product)
```

The code above will throw the error:

```
./main.go:11:13: undefined: product
```

Even though we defined `product` in our code snippet, we can only access `product` inside of the `if` block. Therefore, when we try to access it outside of the block, the compiler throws an error. We say that `product` is _out-of-scope_ outside the `if` statement.


Let's use this handy shortcut in our code.


Scoped Short Declaration Statement

If we're hungry we would go to eat something. But if we're not hungry then we don't. The idea is that we have a backup plan or something we can default to in case our condition isn't met.

We can provide a default option to our conditional (`if` statement) by adding an `else` statement: 

```go
isHungry := false
if isHungry {
  fmt.Println("Eat the cookie") 
} else {
  fmt.Println("Step away from the cookie...")
}
```

In the example above, `isHungry` is a variable with a value of `false`. We've set up an `if` statement like we saw in the previous exercise. Immediately after the `if` statement's closing brace is the `else` keyword and its opening brace, all on the same line. The `else` statement also contains a block of code wrapped by a set of curly braces. The code inside the block will execute by default if the `if` condition is `false`. Notice, the `else` statement does *not* accept a condition. 


The else Statement

Our last logical operator is the not (`!`) operator. It _negates_ (reverses) the value of a boolean. For example: 

```go
bored := true
fmt.Println(!bored) // Prints false

tired := false;
fmt.Println(!tired) // Prints true
```
Notice that the `!` operator will either take a `true` value and pass back `false`, or it will take a `false` value and pass back `true`. 

Let's see how to incorporate the `!` operator. 


Logical Operators (Not)

In the previous exercises we checked one condition at a time. But what if we wanted to check multiple conditions at a time? To do so, we can use _logical operators_. There are three logical operators:

Operator | Meaning:
--- | ---
`&&` | And
`\|\|` | Or
`!` | Not

In this exercise, we'll focus on the first two, `&&` and `||`. When we use the And (`&&`) operator, we are checking that both expressions are true:

```go
if storeLights == "on" && doorsOpen {
  fmt.Println("You can enter the store!")
} 
```

When using the `&&` operator, both conditions must evaluate to `true` for the entire condition to evaluate to `true` and execute. Otherwise, if either condition evaluate as `false`, the `&&` condition will evaluate to `false` and the code inside the `if` block will not execute. 

If we only care about either condition being true, we can use the Or (`||`) operator:

``` go
if day == "Saturday" || day == "Sunday" {
  fmt.Println("Enjoy the weekend!")
} else {
  fmt.Println("Do some work.")
}
```

When using the `||` operator, only one of the conditions must evaluate to true for the overall statement to evaluate to true. In the code example above, if either `day == "Saturday"` or `day == "Sunday"` is `true` the statement's code block will execute. If the first operand in the `||` expression evaluates to `true`, the second operand won’t even be checked. Only if `day == "Saturday"` evaluates to `false` will `day == "Sunday"` be evaluated. The code in the `else` statement above will execute only if both comparisons evaluate to `false`.

Let's implement these operators to add an additional layer of logic! 


Logical Operators (And, Or)

What if…? What if we're hungry? If it's raining? If the alarm's ringing?

We would do something in response to these conditions. 

`if` statements work very similarly to our own decision-making process. Let's look at Go's `if` statement:

```go
alarmRinging := true
if alarmRinging {
  fmt.Println("Turn off the alarm!!") 
}
```

In our example, we have a variable `alarmRinging` that has a value of `true`. Then we have an `if` statement that checks if the condition next to the `if` keyword is `true`. Then we have an opening curly brace `{` with code inside followed by a closing curly brace `}`. If the condition is `true`, then the code in between the curly braces `{ }` is executed. In this case, `"Turn off the alarm!!"` is printed to the console.

In our `if` statement we could have provided parentheses, like so:

```go
if (alarmRinging) {
  fmt.Println("Turn off the alarm!!") 
}
```

The parentheses are optional and can make it easier to read, but typically, we'll see the `if` statement written without parentheses.  


The if Statement

Previously, we used _hard coded_ values (values that don't change) and then created conditionals that checked on these values. For example:

```go
alarmRinging := true
if alarmRinging {
    fmt.Println("Turn off the alarm!!") 
}
```

We knew that our condition would be true and the print statement would execute. This level of certainty is usually NOT the reason why we would use a conditional. Instead, we create conditionals to account for different conditions and different possible outcomes. 

So let's introduce some uncertainty to our code by generating a random number. Go has a `math/rand` library that helps us generate a random integer:

```go
import (
  "math/rand"
  "fmt"
)

func main() {
  fmt.Println(rand.Intn(100))
}
```

In our `main` function, we're printing out a random number using `rand` and the `Intn()` method. With the argument of `100`, the maximum value that the method will return is 99. Looking at the entire line `fmt.Println(rand.Intn(100))`, it should print a random integer from `0` to `99`. However, if we run our program multiple times, we'll find that it always prints `81`. 

We'll figure out why this happens in the next exercise, for now let's see `rand.Intn()` in action. 



Randomizing

Previously, we saw how our random numbers weren't entirely random. The reason for this behavior is due to how Go _seeds_ or chooses a number as a starting point for generating random numbers. By default, the seed value is `1`. We can provide a new seed value using the method `rand.Seed()` and passing in an integer. 

However, we encounter the same problem if we pass in a constant, i.e. pass in `5`. Each time we run our program, we'll always print the same set of numbers. Therefore, each time we run our program, we also need a unique number as a seed. One way to get this unique number each time we run our program is to use time. The reason why we can use time to be our unique number is because it's always a different time when our program is run! Take for example:

```go
package main

import (
  "fmt"
  "math/rand"
  "time"
)

func main() {
  rand.Seed(time.Now().UnixNano())
  fmt.Println(rand.Intn(100))
}
```

In the example above, we import the `time` library. We then use the `time` library and call `.Now()` with the `.UnixNano()` method chained to it. This results in the difference in time (in nanoseconds) since Janurary 1st, 1970 UTC. (Check out the [`UnixNano` documentation](https://golang.org/pkg/time/#Time.UnixNano) for more details). The number that we get from `time.Now().UnixNano()` is passed as an argument to `rand.Seed()` resulting in a different seed value each time we run our program. Since each run has a unique seed value, each run will also print out a random number from `0` to `99`.

Let's see this change in our own code.



Seeding

`else if` statements are great for checking multiple conditions. However, we can find ourselves needing to check so many conditions that writing all the necessary `else if` statements can feel tedious. For example:

```go
clothingChoice := "sweater"

if clothingChoice == "shirt" {
  fmt.Println("We have shirts in S and M only.")
} else if clothingChoice == "polos" {
  fmt.Println("We have polos in M, L, and XL.")
} else if clothingChoice == "sweater" {
  fmt.Println("We have sweaters in S, M, L, and XL.")
} else {
  fmt.Println("Sorry, we don't carry that.")
}
```
In the code above, we have a series of conditions checking for a value that matches a  `clothingChoice` variable. Our code works fine, but imagine if we needed to check more values! Having to write additional `else if` statements sounds like a pain!

Instead, we can use a `switch` statement that uses alternative syntax that is easier to read and write. Below is an example of a `switch` statement:

```go
clothingChoice := "sweater"

switch clothingChoice {
case "shirt":
  fmt.Println("We have shirts in S and M only.")
case "polos":
  fmt.Println("We have polos in M, L, and XL.")
case "sweater":
  fmt.Println("We have sweaters in S, M, L, and XL.")
case "jackets":
  fmt.Println("We have jackets in all sizes.")
default:
  fmt.Println("Sorry, we don't carry that")
}
// Prints: We have sweaters in S, M, L, and XL.
```

Let's break down what happens in a `switch` statement: 
* The `switch` keyword initiates the statement and is followed by a value. In the example, the value after `switch` is compared to the value after each `case`, until there is a match.
* Inside the `switch` block, `{ ... }`, there are multiple `case`s. The `case` keyword checks if the expression matches the specified value that comes after it. The value following the first `case` is `"shirt"`. Since the value of `clothingChoice` is not the same as `"shirt"`, that `case`'s code does not run.
* The value of `clothingChoice` is `"sweater"`, so the third `case`'s code runs and `"We have sweaters in S, M, L, and XL."` is printed. No more `case` statements are checked.
* At the end of our `switch` statement is a `default` statement. If none of the `case`s are true, then the code in the `default` statement will run.



The switch Statement

So far we've been checking on boolean values (variable assigned a `true` or `false` value). But, we can check more than a single value using _comparison operators_. Here are two commonly used comparison operators: 

Operator | Meaning:
--- | ---
`==` | Is equal to
`!=` | Is NOT equal to

To use a comparison operator, we have two values (or _operands_) with a comparison operator in between the two values. The combination of values and the operator is _evaluated_ by Go's compiler which looks at the left value, compares it to the right value, and decides on a `true` or `false` value. Let's take a look at `==` first:

```go
"password1" == "password1" // Evaluates to true
"password1" == "badpass"   // Evaluates to false
```

When we use comparison operators, we check the left value against the right value. It can be helpful to think of comparison statements as questions. When the answer is "yes", the statement evaluates to true, and when the answer is "no", the statement evaluates to false. The above code's first example would be asking: is `"password1"` the same as `"password1"`? Yes it is, so `"password1" == "password1"` evaluates to `true`. We can read the second example as, is `"badpass"` the same as `"password1"`? No, `"badpass"` is not the same as `"password1"` so it evaluates to `false`. 

Let's take a look at the `!=` operator in action:

```go
123 != 12 // Evaluates to true
123 != 123 // Evaluates to false
```

Above, the first line checks if `123` and `12` are **not** equivalent and since the integers are different values, it evaluates to `true`. This time we can think of the question as: is `123` NOT the same as `12`? Yes, they're not the same, so it evaluates to `true`. The same idea persists for the second example: is `123` **not** the same as `123`? No, they are the same, so it evaluates to `false`. 

It can be very helpful to take a second to think through how the operands are being compared before committing it to code. Let's practice that now! 


Comparison Operators

If you've made it to this exercise, then you've finished Go's conditionals lesson, great job!

Here are the topics covered In this lesson:
* How to create an `if` statement that checks a condition and executes code if the condition is `true`
* How to add an `else if` statement to check for additional conditions.
* The use of an `else` statement that runs by default if none of the previous conditional statements evaluated to `true`. 
* Comparison operators check between two operands and evaluate to a boolean. 
* Logical operators check between two boolean values and return a single boolean. 
* `switch` statements can be used to check between multiple conditions much like an `if… else if… else` statement. 
* Short variable declarations can be used prior to providing a condition for either `if` or `switch` statements. Declared variables are scoped to the statement blocks. 
* The `math/rand` library's `.Intn()` method is used to generate random numbers.
* Go uses a seed value to as a starting point for generating random numbers.
* Unique seed values can be created using time, namely `rand.seed(time.Now().UnixNano())`


Including conditionals in allows you to add a layer of logic to your code to address many different scenarios. It's an important foundational skill to have under your belt as a developer. 
