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Strings

So far we've learnt about the following types of Lisp object:

  • Numbers, like 2, 4, and 17.
  • Procedure names, like + and list
  • Lists, like (1 2 3 4) and (+ 3 6).

We also learnt that Lisp uses lists for both data and programs. The first list (1 2 3 4) is data, because 1 isn't the name of a procedure. The second list, (+ 3 6), is a procedure call to add 3 and 6 (or it could be data that just happens to look like that procedure call).

Strings

The new type is a string: a sequence of any characters enclosed in double-quotes. So here are four strings:

"cat"
"dog"
"Once upon a time there was a prince"
""

The last one is the empty string, which contains no characters, but its still a valid and useful string. You can include a double-quote in a string by prefixing it with a backslash:

"He shouted \"Help!\" and ran away."

Now some procedures that work with strings (note that most of these also work with lists):

Finding the length of a string: length

We've already met length with lists. It can also be used to find the length of a string; for example:

CL-USER > (length "Lisp")
4

Reversing a string: reverse

The procedure reverse also works for strings:

CL-USER > (reverse "dog")
"god"

Joining two (or more) strings together: concatenate

The second argument has to be 'string to tell the procedure what type of thing you're concatenating:

CL-USER > (concatenate 'string "band" "age")
"bandage"

The concatenate procedure can take an arbitrary number of strings and they will all be joined into one long string.

Getting a subsequence from a string: subseq

This procedure extracts part of a string. It takes three parameters:

  • the string
  • the number of the first character you want
  • the number of the character one after the last character you want

The characters are counted starting at 0. For example:

CL-USER > (subseq "averylongword" 5 9)
"long"

If you leave out the third parameter you get the rest of the string to the end.

Writing a procedure using strings

Now we're ready to write a procedure using strings. Let's write a simple piglatin program. It takes the first letter of the word, concatenates it to the word with the first letter removed, and adds "ay" on the end:

(defun piglatin (word)
(concatenate 'string (subseq word 1) (subseq word 0 1) "ay"))

Try it out:

CL-USER 16 : 2 > (piglatin "pig")
"igpay"

Note that it doesn't work perfectly. Try:

(piglatin "ant")

Later you'll learn about the if construct that will enable you to make a piglatin function that works perfectly.

Exercises

1. Reverse the middle letters of a word

Write a procedure midverse to reverse all but the first and last letters of a word. For example:

(midverse "retinues")

becomes "reunites". Check that

(midverse (midverse word))

leaves the word unchanged.

2. Rotate a string n places to the left

Write a procedure rotate that rotates a string n characters to the left. So, for example:

(rotate "mousetrap" 5)

should give:

"trapmouse"

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