new concept booleans

concepts/booleans/.meta/config.json

@@ -0,0 +1,9 @@
+{
+  "authors": [
+    "colinleach"
+  ],
+  "contributors": [
+    "SaschaMann"
+  ],
+  "blurb": "Julia has boolean values true and false, operators ! (not), && (and), || (or)."
+}

concepts/booleans/about.md

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+# About
+
+## Booleans in Julia
+
+True or false values are represented by the `Bool` type.
+It contains only two values: `true` and `false`.
+
+```julia-repl
+julia> true
+true
+
+julia> false
+false
+
+julia> typeof(true)
+Bool
+```
+
+## Boolean logic
+
+Imagine we have the following Boolean expressions in Julia: `5 > x` and `x != 0`.
+If `x` was 3 they would both be `true`.
+We can express statements like "is x less than 5 and not equal to y?" using [Boolean operators][boolean-operators]: `!` (not), `&&` (and), `||` (or).
+
+In Julia (and many other programming languages), `&&` has a [higher precedence][operator-precedence] than `||` (in the same way that `*` is applied before `+`).
+This means that `true || false && true` evaluates to `true` because it is parsed as `(true || false) && true`.
+It is common to include explicit brackets anyway so that the reader doesn't need to think about this.
+
+### Logical _not_
+
+`!` represents the logical "not" operation in  Julia.
+Not is also called negation.
+
+```julia-repl
+julia> !false
+true
+
+julia> false != true
+true
+
+julia> 3 != "apple"
+true
+
+julia> !(false == true)
+true
+
+julia> !(1 < 7)
+false
+```
+
+### Logical _and_
+
+`&&` (two ampersands) represents logical "and" in Julia.
+
+```julia-repl
+julia> 5 > 3
+true
+
+julia> 1 != 0
+true
+
+julia> (5 > 3) && (1 != 0)
+true
+```
+
+Parentheses are optional and can make the code easier to read.
+
+### Logical _or_
+
+`||` (two pipe characters) represents logical "or" in Julia.
+
+```julia-repl
+julia> 5 * 5 == 25
+true
+
+julia> 2 < 1
+false
+
+julia> (5 * 5 == 25) || (2 < 1)
+true
+```
+
+### Short-circuit evaluation
+
+When `&&` or `||` are used to link Boolean expressions, Julia evaluates from left to right, and stops when it finds an unambiguous answer.
+
+Consider this expression:
+
+```julia-repl
+julia> (5 > 12) && (1 != 0)
+true
+```
+
+Because `5 > 12` is `false`, clearly the combined expression must be `false`.
+The `1 != 0` expression is not relevant and is never evaluated.
+
+This is quite often used as a shortcut to trap runtime problems and edge cases, in the form:
+
+```julia
+all_ok || do_something()
+```
+
+For example, the `do_something` might be an early `return` from the function if `all_ok` is `false`, or assigning a default value to a variable before continuing.
+
+
+[operator-precedence]: https://docs.julialang.org/en/v1/manual/mathematical-operations/#Operator-Precedence-and-Associativity
+[boolean-operators]: https://docs.julialang.org/en/

concepts/booleans/introduction.md

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+# Introduction
+
+## Booleans in Julia
+
+True or false values are represented by the `Bool` type.
+It contains only two values: `true` and `false`.
+
+```julia-repl
+julia> true
+true
+
+julia> false
+false
+
+julia> typeof(true)
+Bool
+```
+
+## Boolean logic
+
+Imagine we have the following Boolean expressions in Julia: `5 > x` and `x != 0`.
+If `x` was 3 they would both be `true`.
+We can express statements like "is x less than 5 and not equal to y?" using [Boolean operators][boolean-operators]: `!` (not), `&&` (and), `||` (or).
+
+In Julia (and many other programming languages), `&&` has a [higher precedence][operator-precedence] than `||` (in the same way that `*` is applied before `+`).
+This means that `true || false && true` evaluates to `true` because it is parsed as `(true || false) && true`.
+It is common to include explicit brackets anyway so that the reader doesn't need to think about this.
+
+### Logical _not_
+
+`!` represents the logical "not" operation in  Julia.
+Not is also called negation.
+
+```julia-repl
+julia> !false
+true
+
+julia> false != true
+true
+
+julia> 3 != "apple"
+true
+
+julia> !(false == true)
+true
+
+julia> !(1 < 7)
+false
+```
+
+### Logical _and_
+
+`&&` (two ampersands) represents logical "and" in Julia.
+
+```julia-repl
+julia> 5 > 3
+true
+
+julia> 1 != 0
+true
+
+julia> (5 > 3) && (1 != 0)
+true
+```
+
+Parentheses are optional and can make the code easier to read.
+
+### Logical _or_
+
+`||` (two pipe characters) represents logical "or" in Julia.
+
+```julia-repl
+julia> 5 * 5 == 25
+true
+
+julia> 2 < 1
+false
+
+julia> (5 * 5 == 25) || (2 < 1)
+true
+```
+
+[operator-precedence]: https://docs.julialang.org/en/v1/manual/mathematical-operations/#Operator-Precedence-and-Associativity
+[boolean-operators]: https://docs.julialang.org/en/

concepts/booleans/links.json

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+[
+  {
+    "url": "https://docs.julialang.org/en/v1/manual/mathematical-operations/#Boolean-Operators",
+    "description": "Boolean Operators, section in the manual"
+  }
+]

config.json

@@ -1058,6 +1058,11 @@
       "slug": "functions",
       "name": "Functions"
     },
+    {
+      "uuid": "813fb2fa-0068-4167-93d6-e8b8e7b7b55c",
+      "slug": "booleans",
+      "name": "Booleans"
+    },
     {
       "uuid": "006ebce8-87cd-4695-87e6-8a7b8dc2f239",
       "slug": "integer-introduction",