# Atoms and Lists

You may want to refer to the corresponding reading as you work on this lab.

## Exercises

Start DrScheme

### Exercise 1: A simple list

Call the `cons` procedure to create a list that has the number 1 as its first and only element. The result of your call should be `(1)`.

### Exercise 2: Interpreting compound `cons`es

```(cons 'alpha (cons 'beta (cons 'gamma (cons 'delta null))))
```

b. Check your answer by asking DrScheme to evaluate this expression.

### Exercise 3: Using the `list` procedure

Start Scheme and call the procedure `list`, supplying the numerals `17` and `43` as operands. Describe the value returned by the procedure.

### Exercise 4: Building a list of symbols

a. How would you call the `list` procedure to create a list containing the symbols `alpha`, `beta`, and `gamma`, in that order?

b. Verify your answer by entering the code in DrScheme.

### Exercise 5: Empty lists

How would you invoke the `list` procedure to create an empty list?

### Exercise 6: Repeated elements

Determine by experiment whether it is possible to create a list in which the same element occurs more than once?

### Exercise 7: A small `cdr`

a. What is the `cdr` of a one-element list?

### Exercise 8: Extracing information from empty lists

It makes no sense to apply the `car` and `cdr` procedures to an empty list, because there's no way to split off the ``first element'' of a list that has no elements. What happens if you try it anyway? Find out by having DrScheme evaluate a deliberately incorrect procedure call.

### Exercise 9: Extracting information from symbols

a. Does it make sense to apply `car` and `cdr` to values other than lists? Why or why not?

b. Determine what happens if you apply these procedures to symbolic values and numeric values.

### Exercise 10: Compound lists

a. Create the list `(e)`

b. Create the list `(d (e))`

c. Create the lsit `(b c)`

d. Create the list `(a (b c) (d (e)))`

### Exercise 11: It's all Greek to me

Use Scheme to give the name `Greek-letters` to the list constructed by the expression ```(list 'alpha 'beta (list 'gamma-1 'gamma-2) 'delta)```. Then call the `length` procedure to confirm that it has four elements.

### Exercise 12: How long is emptiness?

Determine the length of the empty list.

### Exercise 13: Checking your own length

a. Create a list of length 5. I don't care what's in the list.

b. Check your answer by having Scheme compute the length of that list.

### Exercise 14: Length of compound lists

a. Create the list `(a (b c) (d (e)))`

b. What do you think the length of this list should be?

c. Experimentally determine the length of this list.

d. Explain the result.

### Exercise 15: Reversing lists

Use Scheme to compute the reversal of the list whose elements are the symbols `senior`, `junior`, `sophomore`, and `freshling`, in that order.

### Exercise 16: Reversing compound lists

a. If a list has another list as one of its elements, should `reverse` reverse that inner list as well as the outer one?

b. Find out by experiment what Scheme does.

### Exercise 17: Joining lists

Use Scheme to find the result of stringing together a list with the symbols `alpha` and `beta` as its elements and a list with the numbers 1, 2, and 3 as its elements. How many elements does the resulting list have?

### Exercise 18: Listing lists

a. Invoke the procedure `list`, applying it to the two lists that you strung together in the previous exercise: a list with the symbols `alpha` and `beta` as its elements and a list with the numbers 1, 2, and 3 as its elements.

b. How many elements does the resulting list have?

c. The answer to this question is different from the answer to the question at the end of the previous exercise -- why?

### Exercise 19: Consing lists

a. Write a call to the procedure `cons`, applying it to our favorite two lists: a list with the symbols `alpha` and `beta` as its elements and a list with the numbers 1, 2, and 3 as its elements.

b. How many elements does the resulting list have?

c. Why is the answer to this question different from the answers to the questions at the end of the previous two exercises.

### Exercise 20: Extracing elements

Write a call to the `list-ref` procedure that will extract the fourth element of the list `(38 72 apple -1/3 sample)` -- namely, the number -1/3.

### Exercise 21: Wrapup

Quit DrScheme and log out of the workstation.

## Notes

Today's lab currently lacks notes.

## History

2 Septemer 1997 (John Stone)

• Created

31 March 2000 (John Stone)

• Last revised

Friday, 1 September 2000 (Samuel A. Rebelsky)

Disclaimer Often, these pages were created "on the fly" with little, if any, proofreading. Any or all of the information on the pages may be incorrect. Please contact me if you notice errors.

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