loops concept

concepts/loops/.meta/config.json

@@ -0,0 +1,5 @@
+{
+  "authors": ["colinleach"],
+  "contributors": [],
+  "blurb": "R provides the usual looping constructs, though they are needed less often than in most languages."
+}

concepts/loops/about.md

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+# About
+
+## Iterating over a vector
+
+This is often unnecessary in R. 
+As discussed in other concepts, many functions will operate on entire vectors.
+
+However, explicit loops are sometimes unavoidable.
+This is especially true when the loop body has side effects such as printing or file I/O.
+
+```R
+> words <- c("This", "is", "a", "loop")
+> for (w in words) { print(w) } # the braces are optional here
+[1] "This"
+[1] "is"
+[1] "a"
+[1] "loop"
+```
+
+If the numerical index is needed, use `seq_along()`.
+
+```R
+> v <- LETTERS[1:3]
+> v
+[1] "A" "B" "C"
+
+> for (i in seq_along(v)) { print(sprintf("%s%i", v[i], i)) }
+[1] "A1"
+[1] "B2"
+[1] "C3"
+```
+
+Using `i in 1:length(v)` is not recommended, as it will cause problems with length-zero vectors: the range `1:0` is equivalent to `c(1, 0)`, so the loop body will execute and probably fail. 
+`seq_along` is designed to handle this case correctly.
+
+
+## `while` and `repeat`
+
+These work much as you might guess, based on many C-family languages.
+If necessary, use `break` to exit a loop completely and `continue` to exit the current iteration.
+
+These three variants are equivalent and all end with `x == 0.4444...`
+
+```R
+x <- 12
+while (x > 1) {
+  x <- x / 3
+}
+
+x <- 12
+while (TRUE) {
+  x <- x/3
+  if (x <= 1) break
+}
+
+x <- 12
+repeat {  # no boolean clause 
+  x <- x/3
+  if (x <= 1) break
+}
+```

concepts/loops/introduction.md

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+# Introduction
+
+## Iterating over a vector
+
+This is often unnecessary in R. 
+As discussed in other concepts, many functions will operate on entire vectors.
+
+However, explicit loops are sometimes unavoidable.
+This is especially true when the loop body has side effects such as printing or file I/O.
+
+```R
+> words <- c("This", "is", "a", "loop")
+> for (w in words) { print(w) } # the braces are optional here
+[1] "This"
+[1] "is"
+[1] "a"
+[1] "loop"
+```
+
+If the numerical index is needed, use `seq_along()`.
+
+```R
+> v <- LETTERS[1:3]
+> v
+[1] "A" "B" "C"
+
+> for (i in seq_along(v)) {print(sprintf("%s%i", v[i], i))}
+[1] "A1"
+[1] "B2"
+[1] "C3"
+```
+
+Using `i in 1:length(v)` is not recommended, as it will cause problems with length-zero vectors. 
+`seq_along` is designed to handle this case correctly.
+
+## `while` and `repeat`
+
+These work much as you might guess, based on many C-family languages.
+If necessary, use `break` to exit a loop completely and `continue` to exit the current iteration.
+
+These three variants are equivalent and all end with `x == 0.4444...`
+
+```R
+x <- 12
+while (x > 1) {
+  x <- x / 3
+}
+
+x <- 12
+while (TRUE) {
+  x <- x/3
+  if (x <= 1) break
+}
+
+x <- 12
+repeat {  # no boolean clause 
+  x <- x/3
+  if (x <= 1) break
+}
+```

concepts/loops/links.json

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+[
+    {
+      "url": "https://intro2r.com/loops.html",
+      "description": "An Introduction to R: Loops"
+    }
+  ]

config.json

@@ -569,6 +569,11 @@
       "uuid": "2751b6f2-7d71-4397-b063-9bf927a57756",
       "slug": "booleans",
       "name": "Booleans"
+    },
+    {
+      "uuid": "d48dec37-b0fd-4193-954a-f005c72f8eb6",
+      "slug": "loops",
+      "name": "Loops"
     }
   ],
   "key_features": [