# Laboratory: Simple Lists

Summary: We 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 map1 and reduce.

## Useful procedures and notation

### Standard list notation

'(val1 val2 ... valn) - a list of n values.

### 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.

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

### Manipulating lists

(map1 fun lst) - apply the function to each element of the list. (map1 fun (list val1 val2 ... valn)) gives you (list (fun val1) (fun val2) ... (fun valn)).

(reduce fun lst) - reduce the list to a single value by repeatedly replacing each pair of neighboring values with the result of applying fun to that pair of values.

(map fun lst1 lst2) - create a new list by applying the function to corresponding pairs of elements from the two lists.

### Sorting

(sort nums <) - reorder the elements in a list of real numbers from smallest to largest.

(sort nums >) - reorder the elements a list of real numbers from largest to smallest.

(sort strings string-ci<?) - reorder the elements in a list of strings from alphabetically first to alphabetically last.

(sort strings string-ci>?) - reorder the elements in a list of strings from alphabetically last to alphabetically first.

### 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.

(index-of val lst) - Determine how many values need to be dropped from lst to reach val.

### Combining procedures

(o f g) - Create a new procedure that applies g and then applies f to the result.

(o f g h) - Create a new procedure that applies h, then g to the result of h, then f to the result of g.

## Preparation

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

b. We will be updating the csc151 library throughout the semester. Using the Install Package or Package Manager menu item, make sure that you update our library to the latest version. Use https://github.com/grinnell-cs/csc151.git.

c. In your definitions window, require the relevant portions of the csc151 library.

(require csc151/lists)
(require csc151/hop)
(require csc151/numbers)


; A list of movie ratings for The Large Illness
(define large-illness-ratings
(list 7 4 0.2 -3 6 5.5 5 6 -1.5 54/7 0))

; Some words
(define story-beginning
(list "Once" "upon" "a" "time" "in" "a" "land" "not" "so" "far" "away"))

; Some names
(define names
(list 'millicent 'octavius))

; Tuitions at various schools
(define tuitions
(list 40000 10000 23000 50000 42000 55000 2000 30000 20000 100000))


e. Add the names of about six other people (e.g., your group and the two groups nearest you) to the list of names.

## Exercises

### Exercise 1: Reviewing the self check

a. Predict the results of evaluating each of the following expressions.

(list 2 1)
(make-list 1 2)
(make-list -1 2)
(map - (iota 2))
(map - (iota 2) (list 2 1))
(map iota (list 2 1))


b. Check your predictions with DrRacket.

### Exercise 2: Averaging ratings

Write a set of instructions that allow you to compute the average rating given The Large Illness.

### Exercise 3: Rounding ratings

The ratings associated with The Large Illness include both whole numbers and fractional numbers. Suppose that “those on high” decide that only whole-number ratings are permitted.

a. Write an instruction that rounds each rating to the nearest integer. (Hint: You’ll want to use map1 and round.)

b. Write an instruction that rounds each rating down.

c. Write an instruction that rounds each rating up.

d. Determine the effects of each strategy on the average rating.

### Exercise 4: Restoring ratings

a. Suppose we discover that a jealous competitor has artificially reduced the ratings of The Large Illness by a bit. Write an expression that lets us update the ratings by adding 1 to each rating.

b. Perhaps the ratings were more serverely impacted. Write an expression that lets us update the ratings by adding 2 to each rating.

c. Or perhaps it’s in between those two. Write an expression that let us update the ratings by adding 1.5 to each rating.

Hint: In the prior two instances, you could use map1 and a single list because you could use the increment procedure from csc151/numbers (or the composition of that procedure with itself). But you do not yet know a procedure that adds 1.5 to its parameter. What can we do? Use map with two lists, rather than map1 with one list. (You will need to figure out what the second list is.)

d. Suppose that we also discover that the jealous competitor has added negative signs to some ratings. Write an expression that first removes those negatives signs (e.g., by computing the absolute value) and then adds 1 to each score. Note that you can use increment (from csc151/numbers) to add 1 to a value.

e. If you used more than one call to map1 in the your solution to the prior sub-exercise, come up with a solution that uses only one call to map1.

### Exercise 5: Rethinking ratings

As you may have noted, some of the ratings are very strange, indeed. Write an expression that converts any rating less than 0 to 0 and any rating greater than 6 to 6.

How will you do that? If you think back to the previous lab, we found that (min (max val lower) upper) gives us val if it is between lower and upper and otherwise bounds smaller values. You’ll need to convert that to an appropriate sequence of calls to map or something similar.

### Exercise 6: Sorting out sorting

a. Write an expression to sort the ratings from lowest to highest.

b. Write an expression to sort the ratings from highest to lowest.

c. Write an expression to alphabetically sort the words in the story beginning.

d. Write an expression to alphabetically sort the names. Note that the names are symbols, so you will need to do some conversion.

### Exercise 7: Removing ratings

a. Using sort, index-of, and drop, write an expression to remove the ratings that are less than 0. You can assume that at least one rating is 0.

b. Do the same thing, but without the assumption that at least one rating is 0. (Hint: You can use append to make the list more like the one we want.)

### Exercise 8: Average tuitions

a. Write an expression to compute the average tuition at schools that charge more than $25,000 per year. Use tuitions for the list of tuitions. Your code should work no matter what values are in tuitions. b. Suppose tuitions are going up by 10%. Write an expression to compute the average tuition at schools that charge more than$25,000 after the increase.

## For Those with Extra Time

### Extra 1: Inconsistent subtraction, revisited

Consider the following list.

(define ones (make-list 10 1))


a. How many different values do you think you can get from (reduce - ones)?