The study of strings is useful to further tie in the relationship between pointers and arrays.
It also makes it easy to illustrate how some of the standard C string functions can be
implemented. Finally it illustrates how and when pointers can and should be passed to
functions.
In C, strings are arrays of characters. This is not necessarily true in other languages. In
BASIC, Pascal, Fortran and various other languages, a string has its own data type. But in
C it does not. In C a string is an array of characters terminated with a binary zero
character (written as '\0'). To start off our discussion we will write some code which,
while preferred for illustrative purposes, you would probably never write in an actual
program. Consider, for example:
char my_string[40];
my_string[0] = 'T';
my_string[1] = 'e';
my_string[2] = 'd':
my_string[3] = '\0';
While one would never build a string like this, the end result is a string in that it is an
array of characters terminated with a nul character. By definition, in C, a string is an
array of characters terminated with the nul character. Be aware that "nul" is not the same
as "NULL". The nul refers to a zero as defined by the escape sequence '\0'. That is it
occupies one byte of memory. NULL, on the other hand, is the name of the macro used to
initialize null pointers. NULL is #defined in a header file in your C compiler, nul may not
be #defined at all.
Since writing the above code would be very time consuming, C permits two alternate
ways of achieving the same thing. First, one might write:
char my_string[40] = {'T', 'e', 'd', '\0',};
But this also takes more typing than is convenient. So, C permits:
char my_string[40] = "Ted";
When the double quotes are used, instead of the single quotes as was done in the previous
examples, the nul character ( '\0') is automatically appended to the end of the string.
The compiler sets aside an contiguous
block of memory 40 bytes long to hold characters and initialized it such that the first 4
characters are Ted\0.
