Lecture #27: More Strings

December 8, 1999

Bob Pitha

bpitha@cs.ucsd.edu

http://www-cse.ucsd.edu/classes/fa99/cse5a

so what else can we do with strings?

• Whatever we want

– we just need functions to do it

• As far as C is concerned, strings are just arrays of chars

– they follow all the same rules as other arrays

– the only possibly exception is initialization

• C gives us a convenient way to initialize a string

but we need to do more

• Say I asked you to write a program that let the user enter a date like this:

December 8, 1999

and it should tell me what day of the week it is

– compared to what we’ve done so far, this is a pretty complex problem

– we don’t have to figure this out right now, but let’s look at it a little

our date program

• Given a date like December 8, 1999

– first we have to divide it into pieces

• month (December)

• date (8)

• year (1999)

– that, in itself, is a challenge

• the month name is everything up to the first space

• the date is whatever comes next, up to the comma

– but if it’s not numerals, we have an error

• the year comes last

so what do we do?

• Given a string containing the date, we must:

– extract the month name, probably into another string

– convert the numerals of the date and year from text into numbers

• Then once we have the month name, we have to figure out:

– whether it’s valid

– and if so, what month it is

parsing the string

• Breaking the string down into its parts is sometimes called parsing

– it’s sometimes a very complex task

– this is pretty straightforward as text parsing goes, but it’s still pretty complex compared to what we’ve been doing

– part of that complexity comes from the fact that we can’t assume someone’s entered the date correctly

• we have to deal with things like:

– Dec ember 8, 1999

– December 8 1999

– December8, 1999

• at least to recognize that they’re incorrect

string operations

• So what sorts of things would we have to do?

– scan a string for patterns

• to find the month name, date, and year

– copy out parts of the string into separate strings

• so we can convert the date and year into numbers

• and so we can process the month name

– compare strings to other strings to recognize words

• to figure out which month the month name is

– convert digits into numbers

But…

• None of these operations are part of the C language!

– remember, C just thinks of strings as any other array

• just a way to handle a bunch of values of the same type

– these operations all involve processing the contents of the string

– these are all done using useful string functions

string functions

• C comes complete with a whole set of useful functions

– we’ve already met printf and scanf

– there are a lot more

– including a bunch of useful string functions

let’s talk about a simple one…

• Very often you need to find the length of a string

– remember that a string is a bunch of chars lined up in memory with a null character at the end

– so to find the length of the string, we just need to count from the beginning until we reach the end.

example

what does this do?

• This is a very simple program, although it uses one small detail we haven’t talked about

– the call to scanf uses a control string with %s in it

– %s tells scanf to read a string

• it will read from the keyboard up to the first whitespace character

– whitespace is a space, tab, or carriage return

• it expects a corresponding pointer argument, which tells it where to store the string

• So this program reads in a string, and prints its length

strlen()

• The function strlen is one of the string functions profided in the C Standard Library

– it takes a single argument: a pointer to a string

– it returns the length of that string

strlen()

• How does it work?

– this function looks at memory starting at the location specified by its argument

– it then scans forward through memory, counting bytes until it finds a null character

what other functions are there?

• There are a lot… some of the more useful are:

– strcpy – copy the contents of one string into another

– strcat – concatenate two strings (add one string onto the end of another)

– strcmp – compare two strings

– strstr – find a substring within a string

how might we use them?

• Here’s an example…

– earlier we talked about parsing a date and calculating the day of the week it falls on

– we’re not going to get into how to figure out days of the week for arbitrary dates

• but let’s take a specific piece of this problem…

• let’s look at how we recognize the month name

• we need to take a month name (i.e., “January”) and figure out which number month it is

– because the algorithm for figuring out what day of the week it is requires the month as a number

so how can we do this?

• What we basically need to do is:

– compare the entered month name with each month name that we know

– when we find a match, we know which number it is.

– we have a few different ways we can do this:

• I’ll show you with an if statement, and a loop

example 1

example 2

those two examples…

• Either of them would work

– but most people would probably prefer the shorter one.