Lab: List basics

Assigned

Friday, 21 February 2025

Summary

We further explore Scheme’s list structures. Lists permit us to group data and process those data as a group. We also explore the procedures that we can use with lists, such as range, map, and apply

Useful procedures and notation

Standard list notation

'(val1 val2 ... valn) - a list of n values. Note that the single-quote mark (', also called “tick”) means “take this verbatim”, which means that no expressions inside the parentheses will be evaluated.

(list val1 val2 ... valn) - another way that DrRacket sometimes shows lists. (We don’t know why it sometimes chooses one notation and other times another; we’ll alo use both.)

Creating lists

(list exp1 exp2 ... expn) - create a list by evaluating each of the expressions and then joining together their values.

(make-list n val) - make a list of n copies of val.

(range n) - create a list of all the natural numbers strictly less than n (starting with 0).

(range s n) - create a list of all the natural numbers between s (inclusive) and n (exclusive).

(range s n i) - create a list of all the natural numbers between s (inclusive) and n (exclusive), incrementing by i each time.

Manipulating lists

(map fun lst) - apply the function to each element of the list. (map fun (list val1 val2 ... valn)) gives you (list (fun val1) (fun val2) ... (fun valn)).

(apply fun lst) - apply the function to all the elements of the list, en masse. (apply fun (list val1 val2 ... valn)) gives you (fun val1 val2 ... valn).

(map fun lst1 lst2) - create a new list by applying the function to corresponding pairs of elements from the two lists. You can also use map with more than two lists.

Other list operations

(length lst) - Determine how many elements are in a list.

(reverse lst) - Create a new list with the elements in the opposite order.

(append lst1 lst2) - Join two lists together.

(take lst n) - Build a new list consisting of the first n elements of lst.

(drop lst n) - Build a new list consisting of all but the first n elements of lst.

(list-ref lst n) - Extract element n of the list. (Remember that lists start with element 0.)

(index-of val lst) - Determine the position of val in lst. (It turns out the position is how many values need to be dropped from lst to reach val.)

(indexes-of lst val) - Find all the indices of the value in the list.

Preparation

a. Review the list of procedures above.

b. If you have not done so already, you may want to open a separate tab or window in your browser for the reading on list basics and the reading on transforming lists.

c. Load the lab. Remember that the person closer to the board is Side A and the person further from the board is Side B.

• list-basics.rkt

d. Get started!