lapply-sapply concept

concepts/lapply-sapply/.meta/config.json

@@ -0,0 +1,5 @@
+{
+  "authors": ["colinleach"],
+  "contributors": [],
+  "blurb": "lapply() and sapply() will apply an unvectorized function to a vector or list"
+}

concepts/lapply-sapply/about.md

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+# About
+
+We saw previously that many functions in R are "vectorized": they will operate across entire vectors, with no need to write explicit loops. 
+
+Functions using only vectorized functions are themselves likely to be automatically vectorized.
+In a trivial example:
+
+```R
+> triple <- function(x) return(3 * x)
+> triple(1:10)
+ [1]  3  6  9 12 15 18 21 24 27 30
+ ```
+
+However, more complex function bodies may not be fully vectorized, for example if they contain loops or if-else blocks. 
+Then we need a way to apply them to vectors (similar to `map()` in some other languages).
+
+Because R has a rich variety of data structures in the core language, it also has a whole family of `*apply()` functions to operate on them.
+For now, consider `lapply()` and `sapply()`.
+
+## `lapply()`
+
+"List-apply" is designed to work on list inputs, but vectors will be silently coerced to lists as necessary.
+
+The output is a list, so this may need unpacking to get a vector:
+
+```R
+h <- function(size) {
+    switch(size,
+           "small" = 6,
+           "medium" = 8,
+           "large" = 10
+    )
+}
+
+> v <- c("small", "large", "medium")
+
+> h(v) # bad use of an unvectorized function
+Error in switch(size, small = 6, medium = 8, large = 10) : 
+  EXPR must be a length 1 vector
+
+> lapply(v, h)  # output is a list
+[[1]]
+[1] 6
+
+[[2]]
+[1] 10
+
+[[3]]
+[1] 8
+
+> unlist(lapply(v, h)) # output is a vector
+[1]  6 10  8
+```
+
+## `sapply()`
+
+"Simplified-apply" will do something similar, but in this case returns a named vector. 
+Depending on context, use of `unname()` may be necessary:
+
+```R
+> sapply(v, h)
+ small  large medium 
+     6     10      8 
+
+> unname(sapply(v, h))
+[1]  6 10  8
+```
+
+## Advanced Topics: Alternative Approaches
+
+Base R also has a `vapply()` function. 
+This is similar to `sapply()` but also has a required parameter to specify the output format.
+
+Some developers prefer this extra level of predictability when writing library code.
+Using it interactively or in simple functions is probably less useful.
+
+Outside Exercism, there is also the `purrr` package, which provides several additional options such as `map()`. 
+Less constrained by backwards compatibility than base R, this package is currently closer to modern functional programming in other languages.

concepts/lapply-sapply/introduction.md

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+# Introduction
+
+We saw previously that many functions in R are "vectorized": they will operate across entire vectors, with no need to write explicit loops. 
+
+Functions using only vectorized functions are themselves likely to be automatically vectorized.
+In a trivial example:
+
+```R
+> triple <- function(x) return(3 * x)
+> triple(1:10)
+ [1]  3  6  9 12 15 18 21 24 27 30
+ ```
+
+However, more complex function bodies may not be fully vectorized, for example if they contain loops or if-else blocks. 
+Then we need a way to apply them to vectors (similar to `map()` in some other languages).
+
+Because R has a rich variety of data structures in the core language, it also has a whole family of `*apply()` functions to operate on them.
+For now, consider `lapply()` and `sapply()`.
+
+## `lapply()`
+
+"List-apply" is designed to work on list inputs, but vectors will be silently coerced to lists as necessary.
+
+The output is a list, so this may need unpacking to get a vector:
+
+```R
+h <- function(size) {
+    switch(size,
+           "small" = 6,
+           "medium" = 8,
+           "large" = 10
+    )
+}
+
+> v <- c("small", "large", "medium")
+
+> h(v) # bad use of an unvectorized function
+Error in switch(size, small = 6, medium = 8, large = 10) : 
+  EXPR must be a length 1 vector
+
+> lapply(v, h)  # output is a list
+[[1]]
+[1] 6
+
+[[2]]
+[1] 10
+
+[[3]]
+[1] 8
+
+> unlist(lapply(v, h)) # output is a vector
+[1]  6 10  8
+```
+
+## `sapply()`
+
+"Simplified-apply" will do something similar, but returns a named vector. 
+Depending on context, use of `unname()` may be necessary:
+
+```R
+> sapply(v, h)
+ small  large medium 
+     6     10      8 
+
+> unname(sapply(v, h))
+[1]  6 10  8
+```

concepts/lapply-sapply/links.json

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+[
+  {
+    "url": "https://www.rdocumentation.org/packages/base/versions/3.6.2/topics/lapply",
+    "description": "lapply: Apply a Function over a List or Vector"
+  },
+  {
+    "url": "https://www.dataquest.io/blog/apply-functions-in-r-sapply-lapply-tapply/",
+    "description": "Apply Functions in R with Examples"
+  },
+  {
+    "url": "https://www.learnbyexample.org/r-apply-functions/",
+    "description": "R Apply Family"
+  },
+  {
+    "url": "https://adv-r.hadley.nz/functionals.html",
+    "description": "Advanced R: Functionals"
+  }
+]

config.json

@@ -586,6 +586,11 @@
       "uuid": "2751b6f2-7d71-4397-b063-9bf927a57756",
       "slug": "booleans",
       "name": "Booleans"
+    },
+    {
+      "uuid": "137411b7-5af8-4a78-9225-5b85353ba81e",
+      "slug": "lapply-sapply",
+      "name": "Lapply Sapply"
     }
   ],
   "key_features": [