Computer Programming (11) - Strings

administrivia

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

  int iarray[] = {1,2,3,4,5}; char string[] = "abcde"

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
    • date (25)
    • month (December)
    • 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:
  • convert the numerals of the date and year from text into numbers
  • extract the month name, probably into another string
• 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

• what sorts of things would we have to do?
  • scan a string for patterns
    • to find the date, month name, 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

#include <stdio.h>

int main (void)
{
  char string[80];
  int length;

  printf ("Enter a string: ");
  scanf ("%s", string);
  length = strlen (string);
  printf ("You entered %d characters.\n", length);

  return 0;
}

what does this do?

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

  scanf ("%s", string);

  • 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

int FindMonthNum (const char *monthName)
{
  if (strcmp (monthName, "January") == 0)
    return 1;
  else if (strcmp (monthName, "February") == 0)
    return 2;
  else if (strcmp (monthName, "March") == 0)
    return 3;
  else if (strcmp (monthName, "April") == 0)
    return 4;
  else if (strcmp (monthName, "May") == 0)
    return 5;
  else if (strcmp (monthName, "June") == 0)
    return 6;
  else if (strcmp (monthName, "July") == 0)
    return 7;
etc.

example 2

int FindMonthNum (const char * monthName)
{
  int i;
  char *month[] = {
    "January", "February", "March", "April", "May", "June",
    "July", "August", "September", "October", "November", "December"
  };

  for (i = 0; i < numMonths; ++i)
    if (strcmp (monthName, month[i]) == 0)
      return i+1;
  return 0;
}

those two examples...

• Either of them would work
  • but most people would probably prefer the shorter one.

Character Input and Output

• printf - format and print data
• scanf - scan and format input
• getchar - read a character

  int getchar(void);

• putchar - write a character

  int putchar(int ch);

getchar

• most basic way of reading input
• reads one character from the "standard input"
  • usually the user's keyboard, but
  • can sometimes be redirected by the operating system.
• returns (rather obviously) the character it reads, or,
  • if there are no more characters, the special value EOF "end of file"

getchar example

• reads one character
• as long as the character read is not EOF prints it and reads another in a loop

  main() { int c; c = getchar(); while(c != EOF) { putchar(c); c = getchar(); } return 0; }

• a char variable can hold integers corresponding to character set values
• an int can hold integers of more arbitrary values (up to +-32767)
• most character sets contain a few hundred characters (nowhere near 32767)
  • an int variable can in general comfortably hold all char values
• there's nothing wrong with declaring c as an int
• it's important to do so, because getchar can return every character value, plus that special, non-character value EOF
  • otherwise, possibly mix it up with some actual character value

• how to stop the program above?
• it stops when it receives end-of-file (EOF),
  • EOF indicates the absence of any more characters to read
• but how do you send EOF?
  • operating system dependent
    • Unix and Unix-related systems - almost always control-D.
    • MS-DOS machines - control-Z followed by the RETURN key.
    • Think C on the Macintosh - control-D, just like Unix.
    • other systems require some research to learn how to send EOF.

• the character you type to generate an end-of-file is not the same as the special EOF value returned by getchar.
  • the EOF value returned by getchar is a code indicating that the input system has detected an end-of-file condition
    • in a disk file there is no EOF

• if a program is running forever waiting for input
  • stop it by sending it an end-of-file, as above
• if it's running forever not waiting for something
  • you'll have to take more drastic measures.
    • under Unix, control-C (or, occasionally, the DELETE key) will terminate the current program, almost no matter what.
    • under MS-DOS, control-C or control-BREAK will sometimes terminate the current program
    • on the Mac, try command-period or command-option-ESCAPE.

shorter getchar example

while((c = getchar()) != EOF) putchar(c);

• an assignment expression has a value, remember?
• the extra parentheses around

  (c = getchar())

  are important, because the precedence of the != operator is higher than that of the = operator

getline

getline