WIP draft loops concept

concepts.wip/loops/.meta/config.json

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+{
+  "authors": [
+    "colinleach"
+  ],
+  "contributors": [],
+  "blurb": "It is possible to either loop until a condition is met, or loop over all elements in a collection."
+}

concepts.wip/loops/about.md

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+# About
+
+There are basically two types of loops:
+
+1. Loop until a condition is satisfied.
+2. Loop over the elements in a collection.
+
+Both are [possible in Julia][loops], though the second may be more common.
+
+## The `while` loop
+
+For open-ended problems where the number of times round the loop is unknown in advance, Julia has the `while` loop.
+
+The basic form is fairly simple:
+
+```julia
+while condition
+    do_something()
+end
+```
+
+In this case, the program will keep going round the loop until `condition` is no longer `true`.
+
+Two ways to exit the loop early are available:
+
+- A `break` causes the loop to exit, with execution continuing at the next line after the loop `end`.
+- A `return x` stops execution of the current function, passing the return value `x` back to the caller.
+
+With these options available, it can sometimes be convenient to create an "infinite" loop with `while true ... end`, then rely on finding a stopping condition within the loop body to trigger a `break` or `return`.
+
+## Loop over a collection
+
+The simplest illustration is to loop over a range.
+
+If we want to do something 10 times:
+
+```julia
+for n in 1:10
+    do_something(n)
+end
+```
+
+If the current iteration fails to satisfy some condition, it is possible to skip immediately to the next iteration with a `continue`:
+
+```julia
+for n in 1:10
+    if is_useless(n)
+        continue
+    end
+
+    # we decided this iteration could be useful
+    do_something_slow(n)
+end
+```
+
+In a shorter form, the `if` block could be replaced by `is_useless(n) && continue`.
+
+Many other collection types can be looped over: elements in an array, characters in a string, keys in a dictionary...
+
+The requirement is that the collection must be `iterable`.
+This will be explained in more detail in a later Concept, but in essence the collection must have a way to yield the next item when asked, plus a way to indicate the end is reached and there are no more items.
+
+The examples so far loop over the range `1:10`, where the value is also the loop index.
+
+More generally, the index may be needed and not just the value.
+For this, the [`eachindex()`][eachindex] function is used, for example `for i in eachindex(my_array) ... end`.
+
+## Other options
+
+As a modern, mostly-functional language, Julia (of course) has a variety of ways to operate on collections, not just an explicit loop.
+
+Later concepts will look at alternatives such as comprehensions, broadcasting, and higher-order functions.
+
+[loops]: https://docs.julialang.org/en/v1/manual/control-flow/#man-loops
+[eachindex]: https://docs.julialang.org/en/v1/base/arrays/#Base.eachindex

concepts.wip/loops/introduction.md

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+# Introduction

concepts.wip/loops/links.json

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+[
+  {
+    "url": "https://docs.julialang.org/en/v1/manual/control-flow/#man-loops",
+    "description": "Loops section in the Julia manual."
+  }
+]