## Flow Control

• `if`
• `cond`
• Boolean logic

### What is flow control?

#### Decisions how to react

“Flow control” is the programming term for deciding how to react to a given circumstance. We make decisions like this all the time. If it’s a nice day out, then we should visit the park; otherwise we should stay inside and play board games. If your car’s tank is empty, then you should visit a gas station; otherwise you should continue to your destination.

#### Testing conditions to react

Software is also full of these decisions. If the user’s input is valid, then we should save her data; otherwise we show an error message. The common pattern here is that you test some condition and react differently based on whether the condition is true or false.

### `if`

In Clojure, the most basic tool we have for the flow control is the `if` operator. Here’s the example how you might code the data validation scenario.

Here’ an example. If adding 40 to `y` is still less than 150, then return `(+ y 40)`; otherwise, returns -150. (As for turtle app’s frame, the top has 150 in y, while the bottom has -150 in y.)

Reference: Conditional `if`

``````(if (< (+ y 40) 150)
(+ y 40)
-150))
``````

#### General form of `if` operator

``````(if conditional-expression
expression-to-evaluate-when-true
expression-to-evaluate-when-false)
``````

#### `if` examples

``````(if (> 3 1)
"3 is greater than 1"
"3 is not greater than 1")
;=> "3 is greater than 1"

(if (> 1 3)
"1 is greater than 3"
"1 is not greater than 3")
;=> "1 is not greater than 3"
``````

#### Truthiness

When testing the truth of an expression, Clojure considers the values `nil` and `false` to be false and everything else to be true. Here are some examples:

Reference: Truthiness

``````(if "anything other than nil or false is considered true"
"A string is considered true"
"A string is not considered true")
;=> "A string is considered true"

(if nil
"nil is considered true"
"nil is not considered true")
;=> "nil is not considered true"

(if (get {:a 1} :b)
"expressions which evaluate to nil are considered true"
"expressions which evaluate to nil are not considered true")
;=> "expressions which evaluate to nil are not considered true"
``````

#### EXERCISE 1: Y value within a frame

• Write a function `y-within-frame` that takes y (vertical position) as an argument.
• You may use if example in the slide.
• The function should return the y value that won’t exceed 150.

``````;; if example
(if (< (+ y 40) 150)
(+ y 40)
-150))
``````
``````;; usage of y-within-frame function
(y-within-frame 80)    ;=> 120
(y-within-frame 180)   ;=> -150
``````

### `cond`

The `if` operator takes only one predicate. When we want to use multiple predicates, `if` is not a good option. We have to write nested, nested, … and nested `if` conditions. To branch to multiple situations, `cond` operator works well.

Here’s the example. If adding 40 to y exceeds 150, evaluate the first form. In this case, it returns -150. If adding 40 to y is less than -150, evaluate the second form. In this case, it returns 150. If both two predicates return false, evaluate the `:else` form. In this case, it returns y plus 40. If we use this function in the turtle app, we can keep our turtle between top and bottom of the frame.

Reference: Conditional `cond`

``````(cond
(> (+ y 40) 150) -150
(< (+ y 40) -150) 150
:else (+ y 40)))
``````

#### General form of `cond` operator

``````(cond
predicate1 expression-to-evaluate-when-predicate1-is-true
predicate2 expression-to-evaluate-when-predicate2-is-true
...
:else expression-to-evaluate-when-all-above-are-false)
``````

#### EXERCISE 2: Y value within a frame - part 2

The function we wrote in the previous exercise, `y-within-frame`, has a flaw. If the given y value is -1000, the function will return -960. Since y value of the frame bottom is -150, -960 is beyond that. Your turtle will go invisible area. Let’s make it real within-frame function using `cond`.

• Write a function `y-within-frame-cond` that takes y (vertical position) as an argument.
• You may use `cond` example in the slide.
• The function should return the y value between -150 and 150.

``````;; usage of y-within-frame-cond function
(y-within-frame-cond 200)    ;=> -150
(y-within-frame-cond -200)   ;=> 150
(y-within-frame-cond 0)      ;=> 40
``````

### Boolean logic with `and`, `or`, and `not`

`if` statements are not limited to testing only one thing. You can test multiple conditions using boolean logic. Boolean logic refers to combining and changing the results of predicates using `and`, `or`, and `not`.

If you’ve never seen this concept in programming before, remember that it follows the common sense way you look at things normally. Is this and that true? Only if both are true. Is this or that true? Yes, if either – or both! – are. Is this not true? Yes, if it’s false.

### Truthy and falsey table

`and`, `or`, and `not` work like other functions (they aren’t exactly functions, but work like them), so they are in prefix notation, like we’ve seen with arithmetic.

x y (`and` x y) (`or` x y) (`not` x) (`not` y)
false false false false true true
true false false true false true
true true true true false false
false true false true true false

#### `and`, `or`, and `not` combination

`and`, `or`, and `not` can be combined. This can be hard to read. Here’s an example:

``````(defn leap-year?
"Every four years, except years divisible by 100, but yes for years divisible by 400."
[year]
(and (zero? (mod year 4))
(or (zero? (mod year 400))
(not (zero? (mod year 100))))))
``````

#### [Bonus] `cond`, `and`, `or`, and `not` combination

We have learned `cond`, `and`, `or`, and `not`. Let’s think what function we can write combining those. Here’s an example:

``````(defn true-or-false?
"Given op, returns true or false"
[op]
(let [x true
y false]
(cond
(= op :and) (and x y)
:else false)))

(defn correct?
"Given op and ans, returns message whether ans was corret or not"
[op ans]
(if (= ans (true-or-false? op))
"You won"
"You lost"))
``````

#### EXERCISE 3: [Bonus] Complete `true-or-false?` function

`true-or-false?` function in previous slide sees only `:and` operation. Add `:or`, `:not` operation in the function.

• Use `core.clj` of `myproject` and InstaREPL
• Add `(= op :or)` and `(= op :not)` in `cond`
• For `:not`, choose either x or y for the argument
``````;; usage of correct? function
(corret? :and false)   ;=> "You won"
(corret? :or false)    ;=> "You lost"
(corret? :not true)    ;=> "You won"  (this may be "You lost")
``````