CS1101S S5

Extra questions:

4. Write a Source function every_second that takes a list and returns another list containing the elements at odd indexes. For example:

   // returns ["x", ["y", ["z", null]]]
   every_second(list("a", "x", "b", "y", "c", "z", "d"));
   

5. Write a Source function sums that takes a list of numbers and returns a list, whose first item is the sum of all even indexed items, and the second item is the sum of all odd indexed items.

   // returns: [9, [6, null]]
   sums(list(1, 2, 3, 4, 5));
   

6. Strings can be represented as lists of characters. This allows us to perform list operations on strings. Write a function lexico that takes in two lists of characters xs and ys and returns true if xs > ys and false otherwise. The relation > here is the lexicographic order, i.e. the way words are sorted in dictionaries. So if xs comes after ys in a dictionary, return true.

   // returns: true
   lexico(list("a", "b", "c"), list("a", "b", "b"));
   // returns: false
   lexico(list("a", "b", "c"), list("a", "b", "c"));
   

7. Write a function substr that takes in lists of characters xs and ys and returns true iff ys is a substring of xs. For example:

   // returns: true
   substr(list("h", "e", "l", "l", "o"), list("h", "e", "l", "l"));
   

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