Interactive Output in C Example Program

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Interactive Output in C Example Program

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printf ( control-string , argument1 , argument2 , ... constants (e.g., 100, C', 'a string' - string constants in C) variables (e.g., days, years - in example program) ... – PowerPoint PPT presentation

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Title: Interactive Output in C Example Program

1
Interactive Output in C Example Program

/ Example program computes prints users age
in days
Course CS 2308-252/253, Spring 1998
Author ...
Date ... /
include ltstdio.hgt
int main()
int years, days
printf("How old are you in years?\n")
scanf("d", years)
days years 365
printf("Your age is about d days\n", days)
return(0)

2
Interactive Output in C printf Function

Allows writing output to computers standard
output device (usually the monitor or terminal)
One of many functions provided in C standard
library
Declared in standard library header file stdio.h,
which must be included
General format
printf ( control-string , argument1 , argument2 ,
... )
control string - string containing formatting
info
arguments - represent individual output items

3
Interactive Output in C printf Function (contd)

Control String
Delimited by double quotes, which is how C
recognizes strings
Composed of individual groups of characters -
each character group begin with
Each character group specifies output format for
each output item - they correspond in the order
in which they appear
Character group (formally called conversion
control sequence)
Simplest form - followed by a conversion
character indicating data type of corresponding
output item (e.g., d)
Character groups can be contiguous or separated
by other characters (including spaces) - other
characters (if any) are simply transferred
directly to output device

4
Interactive Output in C printf Function (contd)

Some Commonly Used Conversion Characters
Conversion
Character Meaning
c display as character
d display as decimal integer
e display as floating-point with exponent
f display as floating-point with no exponent
s display as string

5
Interactive Output in C printf Function (contd)

Arguments (Output Items)
Can be written as
constants (e.g., 100, C, a string - string
constants in C)
variables (e.g., days, years - in example
program)
expressions (e.g., years365 - could have been
used in place of years in 2nd printf statement in
example program)
function references (e.g. sqrt(5) - needs
math.h)
Type compatibility between argument (output item)
conversion control sequence - your
responsibility
printf("d, f\n", 1, 1) - whats wrong? how
should it be?

6
Interactive Output in C printf Function (contd)

More Examples
printf("How old are you in years?\n")
printf("Your age is about d days\n", days)
printf("\nYou are d years (about d days)
old\n",
years, days)
printf("Bytes allocated to char is d\n",
sizeof(char))
printf("Bytes allocated to int is d\n",
sizeof(int))
printf("Bytes allocated to float is d\n",
sizeof(float))
printf("c d e f s\n", '1', 1, 1.0, 1.0, "1")

7
Interactive Output in C printf Function (contd)

More Examples (Formatted Output)
Additional formatting characters can be placed
between and conversion character
Examples - only a taste of some of the flavors
printf("Sum of3d and4d is5d.", 6, 5, 21)
Sum of 6 and 15 is 21.
printf("-10d", 59) / left justified /
59
printf("10d", 59) / signed /
59
printf("-10d", 59) / signed left
justified /
59
printf("5.2f", 1234.6789) / whats the deal?
/
1234.67

8
Interactive Input in C scanf Function

/ Example program computes prints users age
in days
Course CS 2308-001
Author ...
Date ... /
include ltstdio.hgt
int main()
int years, days
printf("How old are you in years?\n")
scanf("d", years)
days years 365
printf("Your age is about d days\n", days)
return(0)

9
Interactive Input in C scanf Function (contd)

Allows input data to be entered into computer
from standard input device (usually monitor or
terminal)
One of many functions provided in C standard
library
Declared in standard library header file stdio.h,
which must be included
General format
scanf ( control-string, argument1, argument2,
... )
control string string containing formatting info
arguments represent individual input data items
- actually addresses of where items are stored
in computers memory

10
Interactive Input in C scanf Function (contd)

scanf("d", years)
Puts computer into wait state for as long as it
takes user to enter data
User signals end of data entry by pressing
Enter/Return
Entered value is stored in variable years, whose
address was passed to scanf, computer is taken
out of wait state
Program execution continues with the next
statement
Another example scanf("f f", num1, num2)
entering 0.3 3.1 would result in 0.3 and 3.1
being stored in num1 and num2, respectively
spaces between values matter only if char
variable is involved

11
File Input/Output in C

Declaring Files
Within a C program, a file is always referenced
by a variable name that must be declared within
the program (like any other variables)
For files, the variable is actually a pointer to
a special file structure
Syntax FILE file-pointer-name
Example FILE inputFilePtr
inputFilePtr is pointer name selected by you -
name you want to use in your program to reference
the file - need not be the same as the external
name of the file
FILE is tag name of a special data structure used
by C (declared in stdio.h, which must be
included) for storing information about the file
Declaration for file variable follows the same
format as for pointer variables

12
File Input/Output in C

Opening Files
Two things to accomplish
Establish physical communication link between
program file
Equate files external name (used by
computer/operating system) to files pointer name
you want to use internally in your program
Accomplished using fopen - one of standard C
library functions - declared in stdio.h, which
must be included in the program
Syntax file-pointer-name fopen (external-name
, desired-mode)
Example inputFilePtr fopen (
"c\work\input.dat" , "r" )
inputFilePtr is pointer name for file previously
declared
c\work\input.dat is path name of external
file - as string
r is mode in which file is to be used - as
string - more next

13
File Input/Output in C

File Modes (by no means exhaustive)
Mode Meaning existing or new? how will it be
used?
r Existing file, to read - specified external
file must exist
w New file, to write - if specified external
file exists, it will be destroyed new file
created in its place
a Existing file, to append (add new stuff at
the end) - if specified external file doesnt
exist, new file is created
r Existing file, to read write - specified
external file must exist
w New file, to read write - if specified
external file exists, it will be destroyed
new file created in its place
a Existing file, to read append - if
specified external file doesnt exist, new file
is created

14
File Input/Output in C

Test for Successful File Opening
include ltstdio.hgt
include ltstdlib.hgt
int main()
FILE inFilePtr
char inFileName21
printf("\nEnter file name (up to 20
characters) ")
gets(inFileName)
inFilePtr fopen(inFileName, "r")
if (inFilePtr NULL)
printf("\nError opening file s!",
inFileName)
exit(1)
...

15
File Input/Output in C

Test for Successful File Opening
include ltstdio.hgt
include ltstdlib.hgt
int main()
FILE inFilePtr
char inFileName21
printf("\nEnter file name (up to 20
characters) ")
gets(inFileName)
if ( (inFilePtr fopen(inFileName, "r"))
NULL )
printf("\nError opening file s!",
inFileName)
exit(1)
...

16
File Input/Output in C

Closing Files
Two things to accomplish
Break link between file's external internal
names
Release internal file pointer name - can then
be re-used
Accomplished using fclose - one of standard C
library functions - declared in stdio.h, which
must be included in the program
Syntax fclose ( file-pointer-name )
Example fclose ( inputFilePtr )
inputFilePtr is pointer name of file you want to
close
All computers have limit on number of files that
can be opened at one time - closing files that
you no longer need makes good sense
All opened files are automatically closed by
operating system upon normal termination of
program

17
File Input/Output in C

Reading Writing Files
Files must be open before reading /or writing
Involve standard library functions similar to
scanf printf
With addition of file (pointer) name to indicate
the file from which to read or to which to write
Syntax
fscanf ( file-pointer-name, control-string,
argument1, argument2, ... )
fprintf ( file-pointer-name, control-string,
argument1, argument2, ... )
Arguments for fscanf must be addresses (same as
for scanf )
E.g. fscanf ( inputFilePtr, "d f", intVar,
floatVar )
E.g. fprintf ( outputFilePtr, "You are d days
old.\n", days )
Must know how data appears in file to correctly
read into variables

18
Dynamic Memory Allocation in C

Two functions that can be used for dynamic memory
allocation in C (needs stdlib.h) malloc
free
malloc
Reserves requested number of bytes as indicated
by argument passed to it
Returns address of first reserved location or
NULL if sufficient memory is not available
free
Releases block of bytes previously reserved
(through malloc)
Receives address of first reserved location as
argument

19
Dynamic Memory Allocation in C

In requesting storage space to be allocated using
malloc, user must indicate amount of storage
needed
One may specify specific number of bytes
More usually, one would request enough space for
particular data type using sizeof operator
For example,
malloc(10 sizeof(int))
requests enough memory to store 10 integers
Address (of first byte of storage reserved)
returned is declared (in stdlib.h) as pointer to
void, regardless of data type requested
Returned address must always be reinterpreted
(re-cast) as pointer to desired data type (will
see how this is done)

20
Dynamic Memory Allocation in C Example

include ltstdio.hgt
include ltstdlib.hgt
int main()
int i, num, grades
printf("\nNumber of grades ")
scanf("d", num)
grades (int ) malloc(num sizeof(int))
if (grades NULL)
printf("\nFailed to allocate grades
array.\n")
exit(1)
(Continued)

21
Dynamic Memory Allocation in C Example (contd)

for (i 0 i lt num i)
printf("Enter a grade ")
scanf("d", gradesi)
printf("\nAn array was created for d
integers", num)
printf("\nValues stored in the array are\n")
for (i 0 i lt num i)
printf(" d\n", gradesi)
free(grades)
return(0)

22
Command Line Arguments

In C/C, arguments can be passed to any function
Have seen how arguments are passed to functions
other than main
Arguments passed to main are called command
line arguments
Common uses of command line arguments
Printing the arguments
Passing options to program
Passing filenames to program
Example program tcopy that copies one file
called in into another file called out one
character at a time
tcopy in out

23
Command Line Arguments

include ltstdio.hgt
int main(int argc, char argv)
FILE inFilePtr, outFilePtr
char c
if (argc ! 3)
printf("Usage tcopy infile outfile\n")
else
if ((inFilePtr fopen(argv1, "r")) !
NULL)
if ((outFilePtr fopen(argv2, "w"))
! NULL)
while ((c fgetc(inFilePtr)) ! EOF)
fputc(c, outFilePtr)
else
printf("File s could not be
opened\n", argv2)
else
printf("File s could not be opened\n",
argv1)
return(0)

24
Command Line Arguments

include ltiostream.hgt
include ltfstream.hgt
int main(int argc, char argv)
char c
if (argc ! 3)
cout ltlt "Usage tcopy infile outfile\n"
else
ifstream inStream(argv1, iosin)
ofstream outStream(argv2, iosout)
if (!(inStream.fail()))
if (!(outStream.fail()))
while ( (c inStream.get()) ! EOF)
outStream.put(c)
else
cout ltlt "Error opening file " ltlt
argv2
else
cout ltlt "Error opening file " ltlt
argv1

25
Command Line Arguments

Command line indicates that file in is to be
copied into file out
When program is run
If argc is not 3 (notice that tcopy counts
as one argument), program prints error message
terminates
Otherwise, array argv contains strings "tcopy",
"in", "out"
2nd argument in 3rd argument out on
command line are used as file names by program
Files are opened using usual methods in C or in
C
If both files are opened successfully, characters
are read from file in written to file out
until EOF for file in is reached
Copying is then successfully completed program
terminates

26
Command Line Arguments

Command line tcopy in out
Upon encountering command line, operating system
stores it as sequence of three strings as shown
below, assuming each character used one byte of
storage

t
c
o
p
y
\0
i
n
\0
o
u
t
\0
27
Command Line Arguments

Arguments passed to main, like all function
arguments, must be declared as part of function
definition
To standardize argument passing to main, only
two items are allowed a number an array
The number is an integer variable, which must be
named argc (short for argument counter),
holds the total number of items on the command
line (3 in our tcopy example)
The array is a 1D array, which must be named
argv (short for argument values), holds a list
of pointers containing the starting addresses of
each string typed on command line