PC World Interactive 7/PC World Interactive 7.iso/program/ctutor.exe/TEXT/CHAP07.TXT

Text File | 1994-05-15 | 20KB | 406 lines

                                                        Chapter 7
                                               STRINGS AND ARRAYS

WHAT IS A STRING?
-----------------------------------------------------------------
A string is a group of characters, usually letters of the
alphabet.  In order to format your printout in such a way that it
looks nice, has meaningful names and titles, and is aesthetically
pleasing to you and the people using the output of your program,
you need the ability to output text data.  Actually you have
already been using strings, because the second program in this
tutorial, way back in Chapter 2, output a message that was
handled internally as a string.  A complete definition of a
string is a series of char type data terminated by a NULL
character.

When C is going to use a string of data in some way, either to
compare it with another string, output it, copy it to another
string, or whatever, the functions are set up to do what they are
called to do until a NULL, which is a zero, is detected.  Such a
string is often called an ASCII-Z string.  We will use a few
ASCII-Z strings in this chapter.


WHAT IS AN ARRAY?
-----------------------------------------------------------------
An array is a series of homogeneous pieces of data that are all
identical in type, but the type can be quite complex as we will
see when we get to the chapter of this tutorial discussing
structures.  A string is simply a special case of an array, a
series of char type data.

The best way to see these principles is by use  =================
of an example, so load the program CHRSTRG.C        CHRSTRG.C
and display it on your monitor.  The first      =================
thing new is in line 6 which defines a char
type of data entity.  The square brackets define an array
subscript in C, and in the case of the data definition statement,
the 5 in the brackets defines 5 data fields of type char all
defined as part of the variable.  In the C language, all
subscripts start at 0 and increase by 1 each step up to the
maximum which in this case is 4.  We therefore have 5 char type
variables named, name[0], name[1], name[2], name[3], and name[4].
You must keep in mind that in C, the subscripts actually go from
0 to one less than the number defined in the definition
statement.  This is due to the original definition of C and
these limits cannot be changed or redefined by the programmer.






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                                   Chapter 7 - Strings and Arrays

HOW DO WE USE THE STRING?
-----------------------------------------------------------------
The variable name is therefore a string which can hold up to 5
characters, but since we need room for the NULL terminating
character which counts as one of the five characters, there are
actually only four usable characters.  To load something useful
into the string, we have 5 assignment statements, each of which
assigns one alphabetical character to one of the string
characters.  Finally, the last place in the string is filled with
the numeral 0 as the end indicator and the string is complete.
(A define would allow us to use NULL instead of a zero, and this
would add greatly to the clarity of the program. It would be very
obvious that this was a NULL and not simply a zero for some other
purpose.)  Now that we have the string, we will print it out with
some other string data in the output statement, line 14.

The %s in the format portion of the printf() statement is the
output definition used to output a string.  The system will
output characters starting with the first one in the string name
until it comes to the NULL character, where it will quit.  Notice
that in the printf() statement, only the variable name which
happens to be name needs to be given, with no subscript since we
are interested in starting at the beginning.  (There is actually
another reason that only the variable name is given without
brackets.  The discussion of that topic will be given in the next
chapter.)  It is important to realize that name by itself refers
to the entire string, but name[] with some value in the square
braces refers to only a single character in the string.


OUTPUTTING PART OF A STRING
-----------------------------------------------------------------
The printf() in line 15 illustrates that we can output any single
character of the string by using the %c and naming the particular
character of the variable name we want by including the subscript.
Notice that the term with the square brackets refers to only a
single character in the string, so we output it with the %c
notation which is used to format and output a single character.
The last printf() illustrates how we can output part of the
string by stating the starting point by using a subscript.  The &
specifies the address of name[1].  We will study this in the next
chapter but I thought you would benefit from a little glimpse
ahead, so don't worry about this construct yet.  You will notice,
however, that we can print only part of the string by starting
later in the string and printing until we reach the terminating
null.

This example may make you feel that strings are rather cumbersome
to use since you have to set up each character one at a time.
Strings would be very difficult to use if they had to be defined
like we defined the string in this program, but we only did this
so you could see the internal structure of the string.  The next


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                                   Chapter 7 - Strings and Arrays

example program will illustrate that strings are very easy to
use.  Be sure to compile and run this program.


SOME STRING FUNCTIONS
-----------------------------------------------------------------
Load the example program STRINGS.C for an       =================
example of some ways to use strings.  First we      STRINGS.C
define four strings in lines 7 and 8.  Next we  =================
come to a new function that you will find very
useful, the strcpy() function, or string copy.  It copies from
one string to another until it comes to the NULL character in the
source string.  Remember that the NULL is actually a zero and is
added to the character string by the system.  It is easy to
remember which one gets copied to which if you think of them like
an assignment statement.  Thus if you were to say, for example,
x = 23;, the data is copied from the right entity to the left
one.  In the strcpy() function, the data is also copied from the
right entity to the left, so that after execution of the first
statement, the string variable name1 will contain the string
"Rosalinda", but without the double quotes, they are the
compiler's way of knowing that you are defining a string.  The
term "Rosalinda" is actually a string constant in exactly the
same way that 23 is an integer constant as used in the expression
index = 23.  Line 10 is copying a string constant into a string
variable.

Likewise, the string "Zeke" is copied into name2 in line 11, then
the title string is copied into the string named title.  The
title and both names are then printed out.  Note that it is not
necessary for the destination string to be exactly the same size
as the string it will be called upon to store, only that it is at
least as long as the source string plus one more character for
the terminating NULL.


ALPHABETICAL SORTING OF STRINGS
-----------------------------------------------------------------
The next function we will look at is the strcmp() or the string
compare function illustrated in line 18.  It will return a 1 if
the first string is larger than the second, zero if they are the
same length and have the same characters, and -1 if the first
string is smaller than the second.  One of the strings, depending
on the result of the compare is copied into the string variable
mixed, and the largest name alphabetically is printed out.  It
should come as no surprise to you that Zeke wins because it is
alphabetically larger, length doesn't matter, only relative
position in the alphabet.  It might be wise to mention that the
result would also depend on whether the letters were upper or
lower case.  There are also functions available with your C
compiler to change the case of a string to all upper or all
lower case if you desire.  These will be used in an example
program later in this tutorial.

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                                   Chapter 7 - Strings and Arrays

COMBINING STRINGS
-----------------------------------------------------------------
Lines 25 through 28 illustrate another new feature, the strcat(),
or string concatenation function.  This function simply adds the
characters from one string onto the end of another string taking
care to adjust the NULL so everything is still all right.  In
this case, name1 is copied into mixed, then two blanks are
concatenated to mixed, and finally name2 is concatenated to the
combination.  The result is printed out with both names in the
one string variable mixed.

Strings are not difficult to use and are extremely useful, but
they so require some care in their use.  It is possible to copy a
long string into a string that has been defined as shorter, but
this is an error and will overwrite some portion of your program.
There is no way for the compiler to warn you of this.

A quick check of the documentation for one compiler revealed
about 24 string functions available for use.  Some are used for
copying strings with upper limits on how many characters can be
copied.  There are string functions to search for certain
characters in a string, and others for adding characters to the
front, middle, or end of a string.  And of course you can remove
characters from anywhere also.  It would pay you to read your
compiler documentation to see just what string functions are
available for your use.  It could greatly simplify something you
will be doing in the near future if you know what is available.
You should spend some time getting familiar with strings before
proceeding on to the next topic.  We included the file named
string.h in line 3 because it contains prototypes for all of the
string functions.  A little time spent examining this file would
be time well spent.

Compile and run this program and observe the results for
compliance with this definition.


AN ARRAY OF INTEGERS
-----------------------------------------------------------------
Load the file INTARRAY.C and display it on your  ================
monitor for an example of an array of integers.     INTARRAY.C
Notice that the array is defined in much the     ================
same way we defined an array of char in order
to do the string manipulations in the last example program.  We
have 12 integer variables to work with, plus one more named
index.  The names of the variables are values[0], values[1], ...
, and values[11].  In lines 9 and 10 we have a loop to assign
nonsense, but well defined, data to each of the 12 variables,
then print all 12 out in lines 12 and 13.  Note carefully that
each element of the array is simply an int type variable capable
of storing an integer value.  The only difference between the
variables index and values[2], for example, is in the way you
address them.  You should have no trouble following this program,

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                                   Chapter 7 - Strings and Arrays

but be sure you understand it.  Compile and run it to see if it
does what you expect it to do.


AN ARRAY OF FLOATING POINT DATA
-----------------------------------------------------------------
Load and display the program named BIGARRAY.C    ================
for an example of a program with an array of        BIGARRAY.C
float type data.  This program has an extra      ================
feature to illustrate how strings can be
initialized.  Line 4 of the program illustrates how to initialize
a string of characters.  Notice that the square brackets are
empty leaving it up to the compiler to count the characters and
allocate enough space for our string plus the terminating NULL.
Another string is initialized in line 11 of the body of the
program but it must be declared static here. This prevents it
from being allocated as an automatic variable and allows it to
retain the string once the program is started.  There is nothing
else new here, the variables are assigned nonsense data and the
results of all the nonsense are printed out along with a header.
This program should also be easy for you to follow, so study it
until you are sure of what it is doing before going on to the
next topic.  Once again, the float array can corrupt a program if
it is used to write past the end of the array.


GETTING DATA BACK FROM A FUNCTION
-----------------------------------------------------------------
Back in chapter 5 when we studied functions, I   ================
hinted to you that there was a way to get data      PASSBACK.C
back from a function by using an array, and      ================
that is true.  Examine the program PASSBACK.C
for an example of doing that.  In this program, we define an
array of 20 variables named matrix in line 8, then assign some
nonsense data to the variables, and print out the first five.  In
line 16 we call the function dosome() taking along the entire
array by putting the name of the array in the parentheses.

The function dosome() beginning in line 22 has a name in its
parentheses also but it prefers to call the array list
internally.  The function needs to be told that it is really
getting an array passed to it and that the array is of type int.
Line 22 does that by defining list as an integer type variable
and including the square brackets to indicate an array.  It is
not necessary to tell the function how many elements are in the
array.  Generally a function works with an array until some
end-of-data marker is found, such as a NULL for a string, or
some other previously defined data or pattern.  Many times,
another piece of data is passed to the function with a count of
how many elements to work with.  In our present illustration,
we will use a fixed number of elements to keep it simple.



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                                   Chapter 7 - Strings and Arrays

So far nothing is different from the previous functions we have
called except that we have passed more data points to the
function this time than we ever have before, having passed 20
integer values, the entire array.  We print out the first 5
again in lines 26 and 27 to see if they did indeed get passed
here.  In lines 29 and 30 we add ten to each of the elements and
print out the new values.  Finally we return to the main program
and print out the same 5 data points.  We find that we have
indeed modified the data stored in the calling program from
within the function, and when we returned to the main program,
we brought the changes back.  Compile and run this program to
verify this conclusion.


ARRAYS PASS DATA BOTH WAYS
-----------------------------------------------------------------
We stated during our study of functions that when we passed data
to a function, the system made a copy to use in the function
which was thrown away when we returned.  This is not the case
with arrays.  The actual array is passed to the function and the
function can modify it any way it wishes to.  The result of the
modifications will be available back in the calling program.
This may seem strange to you that arrays are handled differently
from single point data, but they are.  It really does make sense,
but you will have to wait until we get to pointers to understand
it.


A HINT AT A FUTURE LESSON
-----------------------------------------------------------------
Another way of getting data back from a function to the calling
program is by using a pointer which we will discuss in the next
chapter.  When we get there we will find that an array is in
reality a pointer to a list of values.  Don't let that worry you
now, it will make sense when we get there.  In the meantime,
concentrate on arrays and understand the basics of them because
when we get to the study of structures we will be able to define
some pretty elaborate arrays.


MULTI-DIMENSIONAL ARRAYS
-----------------------------------------------------------------
Load and display the file named MULTIARY.C for   ================
an example of a program with doubly dimensioned     MULTIARY.C
arrays.  The variable big is an 8 by 8 array     ================
that contains 8 times 8 or 64 elements total.
The first element is big[0][0], and the last is big[7][7].
Another array named large is also defined which is not square to
illustrate that the array need not be square.  Both are filled
with data, one representing a multiplication table, and the other
being formed into an addition table.



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                                   Chapter 7 - Strings and Arrays

To illustrate that individual elements can be modified at will,
one of the elements of big is assigned the value from one of the
elements of large after being multiplied by 22 in line 17.  Next
big[2][2] is assigned the arbitrary value of 5, and this value is
used for the subscripts of the assignment statement in line 19.
The assignment statement in line 19 is in reality big[5][5] = 177;
because each of the subscripts contain the value 5.  This is only
done to illustrate that any valid expression can be used for a
subscript.  It must only meet two conditions, it must be an
integer (although a char will work just as well), and it must be
within the range of the subscript it is being used for.

The entire matrix variable big is printed out in a square form in
lines 21 through 25 so you can check the values to see if they
did get set the way you expected them to.


PROGRAMMING EXERCISES
-----------------------------------------------------------------
1.  Write a program with three short strings, about 6 characters
    each, and use strcpy() to copy the string literals "one",
    "two", and "three" into them. Concatenate the three strings
    into one larger string defined with 30 characters and print
    the result out 10 times.

2.  Define two integer arrays, each 10 elements long, called
    array1 and array2. Using a loop, put some kind of nonsense
    data in each and add them term for term into another 10
    element array named arrays.  Finally, print all results in a
    table with an index number.

    1       2 + 10 = 12
    2       4 + 20 = 24
    3       6 + 30 = 36   etc.

    Hint; The print statement will be similar to;
    printf("%4d %4d + %4d = %4d\n", index,
                 array1[index], array2[index], arrays[index]);

3.  Define a string of some selected length, assign a word or
    phrase to it, and print it out as a string, then print it out
    as individual characters.  Finally, print it out backwards by
    using a for loop with a decrementing third term.  A useful
    function for the first term of the for loop is strlen() which
    returns the length of the string by counting characters up
    to, but not including, the terminating null.








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