In this class, we focus on writing programs that manipulate strings. String-manipulation in programs are incredibly important, so we will spend some time learning how to write 'em.

The mysterious empty string

What's the length of the string "abc"? Three of course? How about "ab"? Two! How about "a"? One. Well is there a string of length zero? Yes! We define an empty string like this: "". It is a string of no characters — of length zero. Why would I want a string of nothing? Well ... European mathematicians of long ago couldn't see the value in having a "number" zero? Why would I want a number for nothing? Actually, we'll see a very common use of the empty string below, when we create a variable that's initially set to the empty string so that we can add on to it later.

A useful program: random password generation

There are two basic string operations in Javascript that are really important:
  1. .length — whenever you have a string, you can stick a .length at the end of it to find out how long it is. So, for example, "foo".length is 3. If z is a variable of type string, z.length tells you its length.
  2. [ ] — you can pick individual characters out of a string using [ ]'s. The only trick is that the index you use in the [ ]'s starts at 0, not 1. Thus, "abcd"[0] → "a", "abcd"[1] → "b", "abcd"[2] → "c" and "abcd"[3] → "d". Note that the last valid index into a string is its length minus one!
The more "random" a password is, the more secure it is. Let's use Javacript's Math.random() function to generate random passwords of six characters.
ProgramMessages
Exercise: Modify the program so the user specifies the length of the password.
Exercise: Modify the program so punctuation and mathematical characters can appear in the password.
An interesting exercise is to generate a truly random 7 or 8 character password and see whether you can remember it for a few minutes. I think you'll find that we humans don't do a good job of remembering random strings.
Try this from your command prompt 
ssh m159999@rona.cs.usna.edu "pwgen -0 -A 8"
This should generate a pronoucable 8-character password. It should be much easier to remember!

Manipulating ASCII values in Javascript

Javascript has two string-related functions for dealing with ASCII values:
  1. .charCodeAt( i ) — whenever you have a string, you can stick a .charCodeAt( i ) at the end of it, where i is an index, starting at zero just like with [ ]'s. So, for example, "abcd".charCodeAt(2) → 99, because the ASCII code for c is 99.
  2. String.fromCharCode( i ) — returns a string consisting of a single character, namely the one with ASCII code i. So, for example, String.fromCharCode(99) → "c"
A very traditional use of functions like these is changing a string to all lower case. Let's start with something easier. We'll just replace all upper-case letters with a period. If you look at the ASCII table, you see that A-Z are consecutive in the table, as are a-z. That makes it easy to test whether something is indeed an upper-case letter.
ProgramMessages
The last piece of the puzzle is to take the variable code, when we know it refers to an upper-case letter, and come up with the equivalent lower-case letter. The key is that the letter a comes 32 places after A in the ASCII table. So adding 32 to the ASCII value of an upper-case letter gives you the ASCII value of the corresponding lower-case letter. So we finish off the program by replacing res = res + "."; with: 
var lower = String.fromCharCode(code + 32);
res = res + lower;
You should try putting the pieces together to make the finished program.

Morse Code: -- --- .-. ... .

A more interesting example of what we can do with even our simple knowledge of strings is shown in the following program, which converts a string of uppercase letters (no space, punctuation, etc) into morse code:
ProgramMessages
This is actually the kind of thing we do all the time with programs: we take data and simply change the way it is represented. When we do it for secrecy, it's cryptography, which we'll talk a lot more about later. When we do it to save memory space it's compression — which is what .zip files are all about.