map-index example

config.json

@@ -1306,7 +1306,8 @@
           "unicode",
           "arrays",
           "windows",
-          "locals"
+          "locals",
+          "while"
         ],
         "difficulty": 5
       },
@@ -1469,7 +1470,8 @@
           "sequences",
           "dynamic-variables",
           "mutation",
-          "bitwise-operations"
+          "bitwise-operations",
+          "while"
         ],
         "difficulty": 7
       },
@@ -1501,7 +1503,8 @@
           "higher-order-sequences",
           "sequences",
           "strings",
-          "locals"
+          "locals",
+          "while"
         ],
         "difficulty": 7
       },
@@ -1556,7 +1559,8 @@
           "combinators",
           "errors",
           "mutation",
-          "locals"
+          "locals",
+          "while"
         ],
         "difficulty": 8
       },

exercises/concept/backyard-birdwatcher/.docs/introduction.md

@@ -61,6 +61,19 @@ index-or-length ( seq n -- seq n' )
 { 10 20 } 3 index-or-length head .            ! => { 10 20 }
 ```
 
+The starred variants `head*` and `tail*` count from the end —
+`head*` drops the last `n`, `tail*` keeps the last `n`:
+
+```
+head* ( seq n -- headseq )   ! everything except the last n
+tail* ( seq n -- tailseq )   ! the last n elements
+```
+
+```factor
+{ 1 2 3 4 5 } 2 head* .   ! => { 1 2 3 }
+{ 1 2 3 4 5 } 2 tail* .   ! => { 4 5 }
+```
+
 For an arbitrary slice between two indices, `subseq` takes the
 *start* (inclusive) and *end* (exclusive):
 

exercises/concept/lasagna-luminary/.docs/introduction.md

@@ -106,6 +106,18 @@ H{ { "a" 1 } { "b" 2 } }
 ! => H{ { "a" -1 } { "b" -2 } }
 ```
 
+The same shape works for `map-index` (in [`sequences`][sequences]),
+whose quotation receives the element *and* its index. The names
+make clear which operand plays which role:
+
+```factor
+USING: locals sequences ;
+
+! Divide each element by its 1-based position:
+{ 10 80 270 } [| elt i | elt i 1 + / ] map-index .
+! => { 10 40 90 }
+```
+
 The lambda's stack effect is determined by the input list and what
 its body produces.