Author : Christopher Sawtell
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for the number to go negative. Also if the word in memory is
going
to be used as a bit pattern or a mask and not a number the use
of
unsigned is strongly urged. If it is possible for the sign bit
in
the bit pattern to be set and the program calls for the bit
pattern
to be shifted to the right, then you should be aware that the
sign
bit will be extended if the variable is not declared unsigned.
The default for the "int" types is always "signed", and, as
discussed
above that of the "char" is machine dependent.
This completes the discussion on the allocation of data types, except to
say that we can, of course, allocate arrays of the simple types simply by
adding a pair of square brackets enclosing a number which is the size of
the array after the variable's name:
char client_surname[31];
This declaration reserves storage for a string of 30 characters plus the
NULL character of value zero which terminates the string.
Structures.
Data elements which are logically connected, for example - to use the
example alluded to above - the dimensions and other details about a
sea
going ship, can be collected together as a single data unit called a
struct. One possible way of laying out the struct in the source code
is:
struct ship /* The word "ship" is known as the
structure's "tag". */
{
char name[30];
double displacement; /* in grammes */
float length_of_water_line; /* in meters */
unsigned short int number_of_passengers;
unsigned short int number_of_crew;
};
Note very well that the above fragment of program text does NOT
allocate any storage, it merely provides a named template to
the
compiler so that it knows how much storage is needed for the
structure. The actual allocation of memory is done either like
this:
struct ship cunarder;
Or by putting the name of the struct variable between the "}"
and
the ";" on the last line of the definition. Personally I don't
use this method as I find that the letters of the name tend to
get
"lost" in the - shall we say - amorphous mass of characters
which
make up the definition itself.
The individual members of the struct can have values assigned to
them in this fashion:
cunarder.displacement = 97500000000.0;
cunarder.length_of_water_line = 750.0
cunarder.number_of_passengers = 3575;
cunarder.number_of_crew = 4592;
These are a couple of files called demo1.c & demo1a.c which contain
small 'C' programs for you to compile. So, please cut them out
of the
news posting file and do so.
----------------------------------------------------------------------
#ident demo1.c /* If your compiler complains about this line,
chop it out */
#include <stdio.h>
struct ship
{
char name[31];
double displacement; /* in grammes */
float length_of_water_line; /* in meters */
unsigned short int number_of_passengers;
unsigned short int number_of_crew;
};
char *format = "\
Name of Vessel: %-30s\n\
Displacement: %13.3f\n\
Water Line: %5.1f\n\
Passengers: %4d\n\
Crew: %4d\n\n";
main()
{
struct ship cunarder;
cunarder.name = "Queen Mary"; /* This is the bad line. */
cunarder.displacement = 97500000000.0;
cunarder.length_of_water_line = 750.0
cunarder.number_of_passengers = 3575;
cunarder.number_of_crew = 4592;
printf ( format,
cunarder.name,
cunarder.displacement,
cunarder.length_of_water_line,
cunarder.number_of_passengers,
cunarder.number_of_crew
);
}
----------------------------------------------------------------------
Why is the compiler complaining at line 21?
Well C is a small language and doesn't have the ability to allocate
strings to variables within the program text at run-time. This
program shows the the correct way to copy the string "Queen
Mary",
using a library routine, into the structure.
----------------------------------------------------------------------
#ident demo1a.c /* If your compiler complains about this line,
chop it out */
#include <stdio.h>
/*
** This is the template which is used by the compiler so that
** it 'knows' how to put your data into a named area of memory.
*/
struct ship
{
char name[31];
double displacement; /* in grammes */
float length_of_water_line; /* in meters */
unsigned short int number_of_passengers;
unsigned short int number_of_crew;
};
/*
** This character string tells the printf() function how it is to output
** the data onto the screen. Note the use of the \ character at the end
** of each line. It is the 'continue the string on the next line' flag
** or escape character. It MUST be the last character on the line.
** This technique allows you to produce nicely formatted reports with all the
** ':' characters under each other, without having to count the characters
** in each character field.
*/
char *format = "\n\
Name of Vessel: %-30s\n\
Displacement: %13.1f grammes\n\
Water Line: %5.1f metres\n\
Passengers: %4d\n\
Crew: %4d\n\n";
main()
{
struct ship cunarder;
strcpy ( cunarder.name, "Queen Mary" ); /* The corrected line */
cunarder.displacement = 97500000000.0;
cunarder.length_of_water_line = 750.0;
cunarder.number_of_passengers = 3575;
cunarder.number_of_crew = 4592;
printf ( format,
cunarder.name,
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