Simple Values

  • Strings
  • Booleans and nil
  • Keywords
  • Numbers
    • Arithmetic
  • Assignment: def

Simple values

In order to do anything in a programming language, you need to have values to do stuff with. In Clojure, simple values are numbers, strings, booleans, nil and keywords.

Numbers

Clojure has several different types of numbers.

First up are integers. Integers include zero, the positive whole numbers, and the negative whole numbers, and you write them just like we write them normally.

0
12
-42

Then we have decimal numbers, which are also called floats. They include any numbers that have a decimal point in them.

0.0000072725
10.5
-99.9

Finally, we have fractions, which are also called ratios. Computers cannot perfectly represent all floats, but ratios are always exact. We write them with a slash, like so:

Note that, just like with pen-and-paper math, the denominator of your ratio cannot be equal to 0.

1/2
-7/3

Arithmetic

You can add, subtract, multiply, and divide numbers. In Clojure, arithmetic looks a little different than it does when you write it out with pen and paper. Look at these examples:

(+ 1 1)  ;=> 1 + 1 = 2
(- 12 4) ;=> 12 - 4 = 8
(* 13 2) ;=> 13 * 2 = 26
(/ 27 9) ;=> 27 / 9 = 3

Infix vs. prefix notation

In Clojure, +, -, * and / appear before two numbers. This is called prefix notation. What you’re used to seeing is called infix notation, as the arithmetic operator is in-between the two operands.

Languages such as JavaScript use infix notation, while Clojure only uses prefix notation. Prefix notation is useful for many reasons. Look at this example of an infix expression and the prefix equivalent:

Infix:  1 + 2 * 3 / 4 + 5 - 6 * 7 / 8 + 9

Prefix: (+ (- (+ (+ 1 (/ (* 2 3) 4)) 5) (/ (* 6 7) 8)) 9)

The benefits of prefix notation

Imagine both are unclear, but notice that in the prefix version, you do not have to ever think about the precedence of operators. Because each expression has the operator before all the operands and the entire expression is wrapped in parentheses, all precendence is explicit.

Infix:  1 + 2 / 3
Prefix: (+ 1 (/ 2 3))

Another reason prefix notation can be nice is that it can make long expressions less repetitive. With prefix notation, if we plan to use the same operator on many operands, we do not have to repeat the operator between them.

Infix:  1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9
Prefix: (+ 1 2 3 4 5 6 7 8 9)

Arithmetic with all number types

So far, we looked at arithmetic operations by integers only. However, we can use floats or ratios for those operations as well. See these examples:

(+ 4/3 7/8)   ;=> 53/24
(- 9 4.2 1/2) ;=> 4.3
(/ 27/2 1.5)  ;=> 9.0

Strings

What is a string? A string is just a piece of text. To make a string, you enclose it in quotation marks. Look at the last example. A backslash is how we put a quotation mark inside a string. Do not try using single quotes to make a string.

Reference: String

"Hello, World!"
"This is a longer string that I wrote for purposes of an example."
"Aubrey said, \"I think we should go to the Orange Julius.\""

Booleans and nil

A boolean is a true or false value, and you type them just like that, true and false. Often in programming, we need to ask a true or false question, like “Is this class in the current semester?” or “Is this person’s birthday today?” When we ask those questions, we get a boolean back.

There is another value nil, which behaves like a boolean in terms of truthiness. But, nil means no value at all and not a boolean

Reference: Truthiness

true
false
nil

Keywords

Keywords are the strangest of the basic value types. Some computer languages have similar one. However, keywords don’t have a real world analog like numbers, strings, or booleans. You can think of them as a special type of string, one that’s used for labels. They are often used as keys of key-value pair for maps (data structure; will learn later).

:forename
:surname
:date-of-birth

Assignment: def

If we had to type the same values over and over, it would be very hard to write a program. What we need are names for values, so we can refer to them in a way we can remember. This is called assignment.

Assigning names to values: def

We can assign a name to value using def. When a name is assigned a value, that name is called a symbol.

Reference: Assignment def

(def mangoes 3)
(def oranges 5)
(+ mangoes oranges)
;=> 8

You can assign more than simple values to symbols. Try the following. Look at the last line, and see how we can use symbols by themselves to refer to a value.

(def fruit (+ mangoes oranges))
(def average-fruit-amount (/ fruit 2))
average-fruit-amount
;=> 4

EXERCISE 1: Basic arithmetic

  • How many minutes have elapsed since you arrived at the workshop today?
  • Convert this value from minutes to seconds.

EXERCISE 2 [BONUS]: Minutes and seconds

  • Convert 1000 seconds to minutes and seconds.
  • The minutes and the seconds will be separate numbers.
  • (quot x y) will give you the whole number part of x divided by y.
  • (rem x y) will give you the remainder of x divided by y.

Return to the first slide, or go to the curriculum outline.