Chapter 3 Input and Output
C Programming for Scientists &
Engineers with Applications
by Reddy & Ziegler
3.1 Input and Output Functions
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Formatted Input and Output Functions
Standard Input Function
Standard Output Function
Standard Input Function
For input from the keyboard:
scanf(“format control string”, &var1, &var2, ...,
&varn);
or
For input from an input data file:
fscanf(stdin, “format control string”, &var1,
&var2,…, &varn);
Programming Rule: The addresses of the input variables must be specified in the
scanf() function.
Reading Data From the Keyboard
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scanf (“%f %lf”, &income, &expense);
Pass these auguments to scanf
 Convert the first keyboard data to a float type value and
store the value in memory cell reserved for the variable
income.
 Conversion specifications
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Note that %lf is for a double value
The addresses of the variables, income and expense
 Use the “address of” operator, &, to pass the addresses of
variables to functions
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Using scanf() for input
And then saved in a float
type variable income
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And then saved in a double
type variable expense
scanf(“ %f %lf ”, &income, &expense );
Will be converted
to float type
using %f
First keyboard
input
Will be converted
to double type
using %lf
Second keyboard
input
Double type variable
Address of double
type variable
Operation of scanf function
Keyboard
input
Value expressed
as int or float
value
scanf
scanf statement
In source code
Addresses of memory
cell and way to
store the value
Value in correct
memory cell
Standard Output Function
For output to the monitor or printer:
printf(“format control string”, var1, var2, var3,…,
varn);
or
For output to an output data file
fprintf(stdout, “ format control string”, var1,
var2, var3,…, varn);
Sample C Program
2.5.1 sample code
/*******************************************************/
/*
*/
/* THIS IS THE FIRST COMPLETE C PROGRAM
*/
/*
*/
/*******************************************************/
1. #include <stdio.h>
2. int main()
3. {
4.
int x, y, sum;
5.
x = 5;
6.
y = 6;
7.
sum = x + y;
8.
printf("x = %d y = %d sum = %d\n", x, y, sum);
9.
return 0;
10.}
For input and output the format control
specifications for the built-in data types are:
For data type int :
For data type float :
For data type double :
For data type char :
For data type character string :
%d
%f
%lf
or %e
%c
%s
3.2 File Input and Output
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Declaration of File Pointers
open and close Statements
Input from a Data File
Output to a Data File
Declaration of File Pointers
A file pointer for input and output files must be
declared. The declaration of the file pointers are
as follows:
FILE *inptr, *outptr;
The word FILE must be in uppercase letters; you
cannot use lowercase letters.
The file pointer name must be preceded by an
asterisk(*).
Input and output data files
For input:
inptr = fopen(“inputfile.dat” , “r”);
For output:
outptr = fopen(“outputfile.dat” , “w”);
Close data files
fclose(inptr);
fclose(outptr);
There are several options other than “r” and “w”
for file operations listed in the text.
Input from an input data file
fscanf(inptr , “format control string”, &var1, &var2,
&var3, ……..,&varn);
Instead of scanf for standard input, use fscanf for
input from a data file. The first parameter must
be the input file pointer.
Reading Data From A File
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What function is most commonly used to read
data from a file?
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fscanf(file_pointer, format_string, argument_list);
Read the content of the file by file_pointer according to
the conversion specifications in format_string
 The contents read are put into the addresses given by the
argument_list.
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fscanf(inptr, “%d %lf”, &kk, &xx);
 The functions scanf and fscanf are closely related.
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Reading Data From A File
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What is a file pointer?
A file pointer is a variable whose memory cell
contains an address, which gives the key to accessing
the file stored on disk.
 FILE *inptr;
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What is the convention for naming file pointers?
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The same as naming conventions for C identifiers
Reading Data From A File
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What is FILE?
A type set up for holding information about disk
files
 C derived data type in the header file stdio.h, which
must be included to use FILE
 Use the data type FILE to declare a file_pointer and
use this file_pointer to handle your file
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FILE
file_pointer
 FILE “L3_6.IN”; (Wrong!!)
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actual file
Reading Data From A File
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How to open and close the file?
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Use the C library function fopen( ) to create a link
between a disk file and a file pointer.
file_pointer = fopen(file_name, access_mode);
 inptr = fopen(“C3_6.IN”, “r”);
 “r” means the file is opened for reading in text mode.
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Use fclose( ) function to close a file manually.
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fclose(file_pointer);
Run source code
Source Code
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#include <stdio.h>
void main(void)
{
double xx ;
int ii, kk;
FILE *inptr;
inptr = fopen ("C3_6.IN","r");
fscanf(inptr, "%d", &ii);
fscanf(inptr, "%d %lf", &kk, &xx);
fclose(inptr);
printf("ii = %5d\n kk = %5d\n xx = %9.3lf\n",ii, kk, xx);
}
Output to an output data file
fprintf(outptr , “format control string”, var1,
var2, var3, ……..,varn);
Instead of printf for standard output, use fprintf
for output to a data file. The first parameter
must be the output file pointer as shown
Writing Output to A File
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Topics
Writing data to a file
 Using the fprintf( ) function
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Previous programs have displayed all their
output on the screen.
May be convenient
 Once the screen scrolls or cleans, the output is lost
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Writing Output to A File
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An output file,
Can have any acceptable file name
 Must be linked with a file pointer before it’s used.
 Must be open before it’s used
 Should be closed after it’s used
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Writing Output to A File
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What function is used to write data to a file?
fprintf(file_pointer, format_string, argument_list)
 The fprintf( ) function writes the values of
argument_list using the given format_string to a file
that is linked to the program with the file pointer of
file_pointer.
 fprintf(myfile, “week=%5d\n year=%5d\n”, week,
year);
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The function fprintf is closely related to printf
(like fscanf being related to scanf)
Operations of fprintf Function
Call to fprintf in
source code
File pointer
Indicating
disk file
Conversion
specification
Variable
names
My file
%7.2lf
income
Variable
Value with
formatting
fprintf
Disk file
indicated
by file pointer
Writing Output to A File
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What function is used to open an output file and
to create a link between the file pointer and file?
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Before data can be written to an external file, the file
must be opened using the fopen( ) function
file_pointer = fopen(file_name, access_mode);
 myfile = fopen(“L3_8.OUT”, “w”);
 Use the fclose( ) function to close an output file
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Run source code
Writing Output to A File
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#include <stdio.h>
void main(void)
{
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double income=123.45, expenses=987.65;
int week=7, year=1996;
FILE *myfile;
myfile = fopen("L3_8.OUT","w");
fprintf(myfile,"Week = %5d\n Year = %5d\n",week,year);
fprintf(myfile,"Income = %7.2lf\n Expenses = %8.3lf\n",
income,expenses);
fclose(myfile);
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}
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3.3 Filed Width Specification
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Input Field Width Specification
Output Field Width Specification
Field Width Specification
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In the format control string, the field width for
each of the control fields can be specified for
both input and output.
The specification of field width to align the
output data properly is very important, because
the alignment increases the readability of
output displayed on the monitor, printed, or
written to a data file.
%nd for integers
%nd for integers, where n must be a numeric literal
which specifies the format field width for
locating integers to input.
Examples are as follows:
%7d %10d %12d
These specifications provide 7 spaces, 10 spaces,
and 12 spaces, respectively, for three integer
numbers.
%nf for real numbers
%nf for real numbers, n must be a numeric literal
which specifies the number of total decimal
digits to be input possibly including a leading
sign and a decimal point.
%8f
%10f
These specifications mean the decimal numbers
with sign and decimal point are found within the
next 8 and 10 positions
example
With the field width specification %8.2f, the
spaces are allocated as follows:
2 spaces for the fractional digits
1 space for the decimal point
1 space for the sign
4 spaces for the whole digits
3.4 Input and Output of Characters
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Standard Input and Output
File Input and Output
Unformatted Input and Output
Input and Output of Characters
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Characters are stored internally in a code such as ASCII.
The ASCII code for characters is presented in
Appendix A.
Each character is allocated one byte of storage.
One byte of memory can hold decimal numbers in the
range of 0 to 255.
Therefore one byte of storage is enough to store 256
different character codes.
Character variables are declared using the basic data
type char
Standard input of characters
scanf(“%c%c%c”, &chr1, &chr2, &chr3);
assume data is xyz
There are no blank spaces in XYZ because there are no
blank spaces in the format control string in the input
statement.
Notice that %c is the format control string for character
data.
standard output of characters
printf(“%c
%c %c”, chr1, chr2, chr3);
The output is as follows: X Y Z
If the format string is %d instead %c:
printf(“%d %d %d”, chr1, chr2, chr3);
The output is as follows: 88 89 90
Input from a data file
FILE *inptr;
char chr1, chr2, chr3, chr4;
inptr = fopen(“infile.dat”, “r”);
fscanf(inptr, “%c%c%c%c”, &chr1, &chr2, &chr3,
&chr4);
The input data is as follows: 3 XY stored in memory as:
Output of character data to an output
data file
FILE *outptr;
outptr = fopen(“outoutfile.dat”, “w”):
char chr1 = ‘3’, chr2 = ‘ ‘, chr3 = ‘X’, chr4 = ‘Y’;
fprintf(outptr, “%c%c%c%c”, chr1, chr2, chr3, chr4);
The output is as follows: 3 XY
3.5 Sample Programs
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Conversion from Polar to Cartesian Coordinates
Cost of a Steel Cage
Convection Heat Transfer
Air Conditioners Sales Report
Wavelength of Electron