Transcript Chapter 3 Input/Output

CHAPTER 3
INPUT/OUTPUT
Input/Output Streams
• I/O is a sequence of bytes, called a stream of bytes,
from the source to the destination.
• The bytes are usually characters, unless the program
requires other types of information such as a graphic
image or digital speech.
• A stream is a sequence of characters from the source to
the destination.
Input Stream: A sequence of characters from an input
device to the computer.
Output Stream: A sequence of characters from the
computer to an output device.
I/O STREAMS AND STANDARD I/O DEVICES
 To extract (that is, receive) data from keyboard and send output to the
screen, every C++ program must use the header file iostream.
 This header file, iostream, contains definitions of two data types,
istream (input stream) and ostream (output stream).
 The header file, iostream, contains the declaration of two
variables, cin (stands for common input), pronounced see-in, and
cout (stands for common output), pronounced see-out, and the
declaration is similar to the following C++ statements:
istream cin;
ostream cout;
 To use cin and cout every C++ program must use the
preprocessor directive
#include <iostream>
 Variables of the type istream are called input stream
variables.
 Variables of the type ostream are called output stream
variables.
 A stream variable is either an input stream variable or an
output stream variable.
cin and the Extraction Operator (>>)
Consider the following C++ statement:
cin>>payRate;
If you type 15.50, the value stored in payRate after the execution
of this statement is 15.50.
 The extraction operator >> is binary. The left-hand operand is an
input stream variable such as cin. The right-hand operand is a
variable of a simple data type.
 The purpose of an input statement is to read and store values in a
memory location and only variables refer to memory locations, the
items (that is, right hand operand in the case of the extraction
operator, >>) must be a variable in an input statement.
• The syntax of an input statement using cin and the
extraction operator >> is
cin>>variable>>variable...;
 Every occurrence of >> extracts the next data item from the
input stream.
 You can read both payRate and hoursWorked via a single
cin statement by using the following code:
cin>>payRate>>hoursWorked;
 There is no difference between the preceding cin statement
and the following two cin statements.
cin>>payRate;
cin>>hoursWorked;
 When scanning for the next input, >> skips all whitespaces.
 Whitespace characters consist of blanks and certain
nonprintable characters, such as tabs and the newline
character.
Whether the input is
15.50 48.30
or
15.50 48.30
or
15.50
48.30
The input statement:
cin>>payRate>>hoursWorked;
would store 15.50 in payRate and 48.30 in hoursWorked.
 Suppose the input is 2. How does >> distinguish between character 2
and the number 2?
 This is distinguished by the right hand operand of >>.
 If the right hand operand, that is, variable, is of type char, input 2 is
treated as character 2 (recall that in this case, the ASCII value of 2
will be stored). And if the right-hand operand is of the type int (or
double), input 2 is treated as the number 2.
 Now consider the input 25 and the statement:
cin>>a;
where a is a variable of some simple data type.
 If a is of the data type char, then only the single character 2 will be
stored in a.
 If a is of the data type, say int, then 25 will be stored in a.
 If a is of the type double, then the input 25 is converted to a
decimal number with zero decimal part.
Consider the statement
cin>>a;
where a is a variable of some simple data type.
 When reading data into a char variable, after skipping
any leading whitespace characters, the extraction operator
>> finds and stores only the next character; reading stops
after a single character.
 To read data into an int or double variable, after
skipping all leading whitespace characters and reading the
plus or minus sign (if any), the extraction operator >>
reads the digits of the number, including the decimal point
for floating-point variables, and stops when it finds a
whitespace character or a character other than a digit.
Example 3-1
int a,b;
double z;
char ch,ch1,ch2;
1
2
Statement
cin>>ch;
cin>>ch;
Input
A
AB
3
4
cin>>a;
cin>>a;
48
46.35
5
6
7
8
9
cin>>z;
cin>>z;
cin>>z>>a;
cin>>a>>b;
cin>>a>>ch>>z;
74.35
39
65.78 38
4 60
57 A 26.9
Value Stored in Memory
ch='A'
ch='A', 'B' is held
for later input
a=48
a=46, .35 is held
for later input
z=74.35
z=39.0
z=65.78, a=38
a=4, b=60
a=57, ch='A', z=26.9
16
cin>>a>>z;
46 32.4 68 a=46, z=32.4, 68 is
held for later input
17
cin>>ch>>a;
256
ch='2', a=56
18
cin>>a>>ch;
256
19
cin>>ch1>>ch2;
A B
a=256, computer
waits for the input
value for ch.
ch1 = 'A', ch2 = 'B’
• During program execution, when entering character data such
as letters, you do not enter the single quotes around the
character.
• Entering a char value into an int or double variable causes
serious errors, called input failure.
cin and the get Function
Consider the declaration:
char ch1, ch2;
int num;
and the input
A 25
Now consider the statement:
cin>>ch1>>ch2>>num;
• When the computer executes this statement, A is stored in ch1, blank
is skipped by >>, 2 is stored in ch2, and 5 is stored in num.
• If we intended to store A in ch1, blank in ch2 and 25 in num? It is
clear that we can not use the extraction operator >>.

The get function inputs the very next character (including
whitespaces) from the input stream and stores in the memory location
indicated by its argument.

The syntax of cin together with the get function to read a character
is:
cin.get(varChar);
where varChar is a char variable.


varChar is called the argument or parameter of the function.
The next input character is stored in varChar.
Now, again, consider the input
A 25
We can effectively use the get function as follows:
cin.get(ch1);
cin.get(ch2);
cin>>num;
to store A in ch1, blank in ch2, and 25 in num.
The above set of statements is equivalent to the following:
cin>>ch1;
cin.get(ch2);
cin>>num;
cin and the ignore Function
 To process partial data, say with in a line, we can effectively use the
ignore function to discard some portion of the input.
 The syntax to use the function ignore is:
cin.ignore(intExp,chExp);
where intExp is an integer expression yielding an integer value
and chExp is a char expression.
 Suppose intExp yields a value, say m. This statement says, ignore
the next m characters or until the character specified by chExp,
whichever comes first.
Consider the following statement:
cin.ignore(100,'\n');
The execution of this statement will ignore the next 100 characters or
until the newline character is found whichever comes first.
The execution of the statement:
cin.ignore(100,'A');
will result in ignoring the first 100 characters or until the character
'A' is found, whichever comes first.
Example 3-2
int a,b;
Suppose the input is:
25 67 89 43 72
12 78 34
Consider the statements:
cin>>a;
cin.ignore(100,'\n');
cin>>b;
• The first statement cin>>a; stores 25 in a.
• The second statement, cin.ignore(100,'\n'); discards all of
the remaining numbers in the first line.
• The third statement cin>>b; stores 12 (from the next line) in b.
Example 3-3
char ch1,ch2;
Suppose the input is
Hello there. My name is Mickey.
Now consider the statements:
cin>>ch1;
cin.ignore(100,'.');
cin>>ch2;
• The first statement cin>>ch1; stores 'H' in ch1.
• The second statement, cin.ignore(100,'.' ); results in
ignoring all characters until '.' (period).
• The third statement cin>>ch2; stores 'M' (from the same line) in
ch2.
Consider the following statements
int a, b, c;
double x;
If the input is
W 54
then the statement
cin>>a>>b;
would result in an input failure because you are trying to input the
character 'W' into the int variable a.
OUTPUT AND FORMATTING OUTPUT
Syntax of cout when used together with the insertion operator << is
cout<<expression or manipulator
<<expression or manipulator...;
• expression is evaluated, its value is printed, and
manipulator is used to format the output.
• The simplest manipulator that you have used so far is endl, which
is used to move the cursor to the beginning of the next line.
setprecision
The general form of setprecision is:
setprecision(n)
where n is the number of decimal places.
The statement
cout<<setprecision(2);
will output all decimal numbers up to two decimal places until it is
reset.
To use the manipulator setprecision, the program must include
the header file iomanip.
#include <iomanip>
fixed
• To output floating-point numbers in a fixed decimal format, you use
the manipulator fixed.
• The following statement sets the output of floating-point numbers in
a fixed decimal format on the standard output device:
cout<<fixed;
• After the preceding statement executes, all floating-point numbers
are displayed in the fixed-decimal format until the manipulator
fixed is disabled.
• You can disable the manipulator fixed by using the stream member
function unsetf.
• The following statement disables the manipulator fixed on the
standard output device, you use:
cout.unsetf(ios::fixed);
• After the manipulator fixed is disabled, the output of the floatingpoint numbers return to their default settings.
• The manipulator scientific is used to output floating-point
numbers in scientific format.
showpoint
• If the decimal part of a decimal number is zero, then when you
instruct the computer to output the decimal number in a fixed decimal
format, the output may not show the decimal point and the decimal
part.
• To force the output to show the decimal point and trailing zeros, you
use the manipulator showpoint.
• The following statement sets the output of decimal numbers with a
decimal point and trailing zeros on the standard input device:
cout<<showpoint;
• The following statement sets the output of a floating-point number in
a fixed decimal format with the decimal point and trailing zeros on the
standard output device:
cout<<fixed<<showpoint;
Example 3-7
//Example: setprecision, fixed, showpoint
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
double x,y,z;
x = 15.674;
//Line 1
y = 235.73;
//Line 2
z = 9525.9864;
//Line 3
cout<<fixed<<showpoint;
//Line 4
cout<<setprecision(2)
<<"Line 5: setprecision(2)"<<endl;
//Line 5
cout<<"Line 6: x = "<<x<<endl;
//Line 6
cout<<"Line 7: y = "<<y<<endl;
//Line 7
cout<<"Line 8: z = "<<z<<endl;
//Line 8
cout<<setprecision(3)
<<"Line 9: setprecision(3)"<<endl;
//Line 9
cout<<"Line 10: x = "<<x<<endl;
//Line 10
cout<<"Line 11: y = "<<y<<endl;
//Line 11
cout<<"Line 12: z = "<<z<<endl;
//Line 12
cout<<setprecision(4)
<<"Line 13: setprecision(4)"<<endl;
//Line 13
cout<<"Line 14: x = "<<x<<endl;
//Line 14
cout<<"Line 15: y = "<<y<<endl;
//Line 15
cout<<"Line 16: z = "<<z<<endl;
//Line 16
cout<<"Line 17: "
<<setprecision(3)<<x<<"
"
<<setprecision(2)<<y<<"
"
<<setprecision(4)<<z<<endl;
return 0;
}
//Line 17
Output:
Line 5: setprecision(2)
Line 6: x = 15.67
Line 7: y = 235.73
Line 8: z = 9525.99
Line 9: setprecision(3)
Line 10: x = 15.674
Line 11: y = 235.730
Line 12: z = 9525.986
Line 13: setprecision(4)
Line 14: x = 15.6740
Line 15: y = 235.7300
Line 16: z = 9525.9864
Line 17: 15.674
235.73
9525.9864
Input/Output and the string Type
• You can use an input stream variable, such as cin, and the extraction
operator >> to read a string into a variable of the data type string.
• If the input is the string "Shelly", the following code stores this
input into the string variable name:
string name; // declaration
cin>>name;
// input statement
• The extraction operator skips any leading whitespace characters and
that reading stops at a whitespace character.
• You cannot use the extraction operator to read strings that contain
blanks.
• If the input is
Alice Wonderland
the value of the variable name after the following statement executes
is "Alice":
cin>>name;
• To read a string containing blanks, you can use the function
getline .
• The syntax to use the function getline is
getline(istreamVar, strVar);
where istreamVar is an input stream variable and strVar is a
variable of the type string. The reading is delimited by the newline
character, '\n'.
• The function getline reads until it reaches the end of the current
line.
• The newline character is also read but not stored in the string
variable.
• Consider the following statement:
string myString;
• If the input is 29 characters,
bbbbHello there. How are you?
where b represents a blank, after the statement
getline(cin,myString);
the value of myString is
myString =
"
Hello there. How are you?"
File Input/Output
File: An area in secondary storage used to hold information.
For file I/O, the following steps are necessary.
1. Include the header file fstream in the program. So the following
statement is needed.
#include <fstream>
2. Declare file (fstream) variables. For example the statements:
ifstream inData;
ofstream outData;
declare the variable inData for input and outData for output. That is,
inData is an input (file) stream variable and outData is an output
(file) stream variable.
3. Open Files
The general syntax for opening a file is
fileStreamVariable.open(sourceName,
fileOpeningMode);
Here fileStreamVariable is a file stream variable,
sourceName is the name of the input/output file, and
fileOpeningMode specifies the mode in which the file is to be
opened.
• Suppose declaration of Step 2.
• Suppose that the input data is stored in a file called prog.dat on a
floppy disk in drive A:.
• Save the output in a file called prog.out on a floppy disk in drive
A:.
• The following statements associate inData with prog.dat and
outData with prog.out.
inData.open("A:prog.dat"); //open input file
outData.open("A:prog.out"); //open output file
Step 4:
 Use the (file) stream variable together with >> or << or with other
functions for input/Output.
 The syntax for using >> or << with file variables is exactly similar to
the syntax of cin and cout.
 The statement
inData>>payRate;
reads the data from the file prog.dat and stores it in the variable
payRate
 The statement
outData<<"The pay check is: "<<pay<<endl;
stores the output line, which is: The pay check is: 565.78,
in the file prog.out. Here we are assuming that the pay was
calculated as 565.78.
 Close File
inData.close();
outData.close();
#include <fstream>
//Add additional header files you use
using namespace std;
int main()
{
//Declare file stream variables such as the following
ifstream inData;
ofstream outData;
...
//Open files
inData.open("A:prog.dat"); //open input file
outData.open("A:prog.out"); // open output file
//Code for data manipulation
//Close files
inData.close();
outData.close();
return 0;
}
PROGRAMMING EXAMPLE: STUDENT GRADE
Write a program that reads a student ID followed by five test scores.
The program should output the student ID, the five test scores, and the
average test score. Output the average test score with two decimal
places. Assume that the student ID is a character.
The data to be read is stored in a file called test.txt, and the file is
stored on a floppy disk in drive A:. The output should be stored in a file
called testavg.out, and the output file should be stored on the
floppy disk in drive A:.
Input: A file containing the student ID and five test scores.
Output: The student ID, five test scores and the average of the five test
scores. The output is to be saved in a file.
Problem Analysis and Algorithm Design
To find the average of five test scores, first we add the five test scores
and then divide the sum by 5. Now, the input data is in the form:
student ID followed by five test scores. Therefore, we first read the
student ID and then the five test scores. This discussion translates in
the following algorithm:
1. Read student ID and the five test score.
2. Output student ID and five test scores.
3. Calculate the average.
4. Output the average.
We will output the average test score in the fixed decimal format with
two decimal places.
Variables:
ifstream inFile;
//input file stream variable
ofstream outFile;
//output file stream variable
int test1, test2, test3, test4, test5; // variables
//to read five test scores
double average;
//variable to store average
//test score
char studentId;
//variable to store
//student ID
Main Algorithm
1.
2.
3.
4.
Declare the variables.
Open the input file.
Open the output file.
To output the floating-point numbers in a fixed decimal format with
a decimal point and trailing zeros, set the manipulators fixed and
showpoint. Also, to output the floating-point numbers with two
decimal places, set the precision to two decimal places.
5. Read the student ID.
6. Output the student ID.
7. Read the five test scores.
8. Output the five test scores.
9. Find the average test score.
10. Output the average test score.
11. Close the input and output files.
//Program to calculate average test score.
#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
int main()
{
//Declare variables; Step 1
ifstream inFile;
//input file stream variable
ofstream outFile; //output file stream variable
int test1, test2, test3, test4, test5;
double average;
char studentId;
inFile.open("a:test.txt");
//Step 2
outFile.open("a:testavg.out");
//Step 3
outFile<<fixed<<showpoint;
//Step 4
outFile<<setprecision(2);
//Step 4
cout<<"Processing data"<<endl;
inFile>>studentId;
//Step 5
outFile<<"Student ID: "<<studentId
<<endl;
//Step 6
inFile>>test1>>test2>>test3
>>test4>>test5;
//Step 7
outFile<<"Test scores: "<<setw(4)<<test1
<<setw(4)<<test2<<setw(4)<<test3
<<setw(4)<<test4
<<setw(4)<<test5<<endl;
//Step 8
average = static_cast<double>(test1+test2+test3+
test4+test5)/5.0;
//Step 9
outFile<<"Average test score: "<<setw(6)
<<average<<endl;
inFile.close();
//Step 11
outFile.close();
//Step 11
return 0;
}
//Step 10
Sample Run:
Input File: (contents of the file a:test.txt):
T 87 89 65 37 98
Output File: (contents of the file a:testavg.out)
Student ID: T
Test scores:
87
Average test score:
89
65
75.20
37
98