Topic13

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Topic 13 –
Various Other Topics
Enumerated Types
Enumerated Types
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We have seen a way to construct new
datatypes that are composed of a
combination of other datatypes. This is
called a structure.
The C language also allows the creation
of new datatypes that are “simple”
datatypes, similar to ints, chars, etc…
CISC105 – Topic 13
Enumerated Types
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An enumerated type is a datatype with
its own, programmer-defined, list of
possible values.
This is useful when a datatype is
necessary that can have one of a set of
some possible values.
An an example, consider a new
datatype for the manufacturer of cars.
CISC105 – Topic 13
Enumerated Types
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This datatype definition could look like:
typedef enum
{ GM, Honda, Ford, Nissan,
Toyota, Chrysler }
automaker;
This definition defines a new datatype,
automaker. A variable of this datatype can
have any of the six values listed in the
definition.
CISC105 – Topic 13
Enumerated Types
typedef enum
{
GM, Honda, Ford, Nissan,
Toyota, Chrysler }
automaker;
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This definition goes in the same location in a
C file as a structure definition, before the
function prototypes.
Once this definition is present, any function is
free to declare variables of this new datatype.
CISC105 – Topic 13
Enumerated Types
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This allows us to use a variable of this
new datatype in the same ways as a
variable of other datatypes:
automaker car1, car2 = Ford;
car1 = Nissan;
if (car1 == Nissan)
printf(“It’s a Nissan!”);
if (car2 == Ford)
printf(“It’s a Ford!”);
CISC105 – Topic 13
Enumerated Types
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So, why is this called an enumerated
type?
The computer does not know what a
“Ford” is. Therefore, the enumerated
type definition uses integers that
correspond to the enumerated values.
CISC105 – Topic 13
Enumerated Types
typedef enum
{
GM, Honda, Ford, Nissan,
Toyota, Chrysler }
automaker;

Thus, GM has a value of 0, Honda has a
value of 1, Ford has a value of 2, Nissan
has a value of 3, Toyota has a value of
4, and Chrysler has a value of 5.
CISC105 – Topic 13
Enumerated Types
typedef enum
{
GM, Honda, Ford, Nissan,
Toyota, Chrysler }
automaker;

We can see that GM (0) is less than Honda
(1), which is less than Ford (2), etc… Thus,
we know that (for example):
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Honda < Chrysler
Ford != Nissan
Toyota >= GM
CISC105 – Topic 13
A Further Example
void display_carmake(automaker carmake)
{
switch (carmake) {
case GM:
printf(“The make is GM.”); break;
case Honda:
printf(“The make is Honda.”); break;
case Ford:
printf(“This make is Ford.”); break;
case Nissan:
printf(“The make is Nissan.”); break;
case Toyota:
printf(“The make is Toyota.”); break;
default: printf(“The make is Chrysler”);
}
}
CISC105 – Topic 13
Text File Input/Output
Input Redirection
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We have previously seen how to use a text
file to substitute for user input in a program
(batch mode).
The program still expects input using scanf
statements.
The text file must contain one value for each
value expected in the program.
This is done using the input redirection
operator at the UNIX (or Windows) prompt:
a.out < input_data
CISC105 – Topic 13
Input Redirection
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It is important to realize that the program has
no knowledge it will be receiving its input
data from a file.
This operator simply instructs the operating
system to change where the standard input
device is. It replaces the normal input device
(the keyboard) with the file specified.
Thus, when the scanf statement attempts to
read data from the standard input device, it
reads from the file instead of the keyboard.
CISC105 – Topic 13
Program-Controlled File I/O
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It is possible to write a C program such
that it directly works with a file, instead
of altering the standard input device.
The first thing that must be done is to
declare a file pointer variable.
This is a variable that stores the
information necessary to access a file.
File pointers are of type FILE *.
CISC105 – Topic 13
Program-Controlled File I/O
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One file pointer is required for each file that
will be accessed.
Once a file pointer has been declared, it must
be assigned to a file and that file must be
opened.
This is done using a fopen function call. This
function takes two parameters, a string with
the file name, and a string with the type of
access desired (such as read, write, etc…)
CISC105 – Topic 13
Program-Controlled File I/O
Once the file is opened, it can be
Here, the file “datafile” is
Here, theaccessed
file pointers by
are declared.
using
the
file
pointer
Here,
the
file(“r”)
“output”
is
opened
for
read
access.
They are not yet associated with
opened
for write
(“w”)
access.
associated
with
it.
A
pointer
to
this
file
is
any file.

A pointer
this file is
assigned
to to
input_file.
to output_file.
that we can
use this file
FILE *inp, *outp;Noteassigned
Note that
we can
file
pointer
to READ
theuse
file this
only!
pointer to WRITE the file only!
inp = fopen(“datafile”,”r”);
outp = fopen(“output”,”w”);
CISC105 – Topic 13
Errors in Opening Files
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When the operating system is unable to
open a file, for whatever reason, the
pointer returned (the file pointer) is set
to NULL.
Thus, to see if a file is opened
successfully, test the file pointer to see
if it is NULL after the call to fopen.
CISC105 – Topic 13
Errors in Opening Files
FILE *inp, *outp;
inp = fopen(“input_file”,”r”);
outp = fopen(“output_file”,”w”);
if (inp == NULL || outp == NULL)
printf(“Error opening one of the files.”);
else
{
/* files were both opened
correctly and we can access them */
}
CISC105 – Topic 13
File Input

Once the file pointer is ready, input
from a file opened with “read” access
can be performed using the fscanf
function. This function works just like
scanf except that it takes one
additional parameter, the file pointer:
fscanf(inp,”%d”,&x);
CISC105 – Topic 13
Closing File Pointers
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Once all access to an open file is
completed, the file must be closed.
This is done with a call to fclose, which
takes one parameter, the name of the
file pointer that controls the file the
program has finished accessing.
CISC105 – Topic 13
Closing File Pointers
FILE *inp, *outp;
inp = fopen(“input_file”,”r”);
outp = fopen(“output_file”,”w”);
if (inp == NULL || outp == NULL)
printf(“Error opening one of the files.”);
else
{
/* do something with files! */
fclose(inp); fclose(outp);
}
CISC105 – Topic 13
File Input
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Using this syntax has the same effect as
using scanf and redirecting the input using
the “<“ operator.
The file that is being read must contain the
data the program is expecting, and it must be
in order.
In addition to fscanf, there is also the fgets
function which reads in an entire line from
the file and stores that line in a string
variable.
CISC105 – Topic 13
File Input
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fgets takes three parameters, the
string to store the line being read, a
maximum number of characters to read
in, and the file pointer to read from.
This second parameter can be used to
prevent an overflow of the string.
As an example,
CISC105 – Topic 13
File Input
FILE * inp;
char buffer[20];
inp = fopen(“input_file”,”r”);
fgets(buffer,19,inp);
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This last statement reads in a line from
the file that inp points to,
input_file. It stores the first 19
characters in the string buffer.
CISC105 – Topic 13
File Output
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Just like we can read from a file that
has been opened with read access, we
can write to a file that has been opened
with write access.
This is done with the fprintf function.
This function behaves just like printf
except it takes one addition parameter,
the file pointer to write to.
CISC105 – Topic 13
File Output
FILE * outp;
int linenum = 1;
outp = fopen(“output_file”,”w”);
fprintf(outp,“This string will go as the
%d st line in the file.”, linenum);
It outputs the string
“This string will go as the 1 st line in the file.”
 What
does
thisto call
to fprintf
do?
to the file
pointed
by outp,
output_file.
Since this happens immediately after the program opens
the file for writing, this string will be the 1st line of
the file, output_file.
CISC105 – Topic 13

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