1. Chapter 3
Input output

2. Objectives In this chapter, you will:
Discover how to input data into memory using input statements
Explore how to read data from the standard input device
Learn how to debug a program using cout statements
Learn how to debug a program by understanding error messages

3. Objectives (continued)
Explore how to use the input stream functions get, ignore, and clear
Become familiar with input failure
Learn how to perform input and output operations with the string data type
Learn how to format output using the manipulators fixed, showpoint, setprecision, setw, left, and right
Become familiar with file input and output

4. Input (Read) Statement
The syntax of an input (read) statement using cin and the stream extraction operator >> is:
During programming execution, if more than one value is entered in a line, these values must be separated by at least one blank or tab
Or, one value per line can be entered

5. Input (Read) Statement (continued)
Must be included to use cin
So we can refer to cin without using std:: 5

6. Variable Initialization
C++ does not automatically initialize variables when they are declared
Ways to initialize a variable
By using the assignment statement
feet = 35;
By using a read statement
cin >> feet;
More versatile 6

7. 7

8. Programming Style and Form (Revisited)
In Chapter 1, you learned about syntax, semantics, errors, and how to format a program properly
In this section, we extend our capabilities to include prompt lines in a program 8

9. Prompt Lines
Prompt lines: executable statements that tell the user what to do to interact with the program
cout << "Please enter a number between 1 and 10 and then "
<< " press Enter." << endl;
cin >> num;
Prompt lines should include sufficient information about what input is acceptable 9

10. Debugging – Sprinkling with couts
Walk-throughs may fail to reveal bugs
Inserting temporary strategically located cout statements can aid in the debugging process
int fahrenheit = 32;
int celsius = 5 / 9 * (fahrenheit - 32);
cout << fahrenheit << " degrees "
<< "Fahrenheit is " << celsius
<< " degrees " << "Celsius. " << endl;
cout << "5 / 9 is " << 5 / 9
<< " and fahrenheit - 32 is "
<< fahrenheit - 32 << endl; 10

11. Debugging – Understanding Error Messages
The C++ compiler will find syntactic errors in your program and will provide messages describing the errors
Provide a good indication of where to look for the problem(s)
Look carefully in the immediate vicinity of the reported error
Finding and reporting errors is like peeling away layers of an onion 11

12. More on Input
>> distinguishes between character 2 and number 2 by the right-side operand of >>
Entering a char value into an int or double variable causes serious errors; called input failure 12

13. More on Input (continued)
When reading data into a char variable:
>> skips leading whitespace; finds and stores only the next character
Reading stops after a single character
To read data into an int or double variable:
>> skips leading whitespace; reads + or - sign (if any); reads the digits (including decimal)
Reading stops on whitespace non-digit character

14. More on Input (continued)

15. More on Input (continued)15

16. cin and the get Function
Inputs next character (including whitespace)
Stores in memory location indicated by its argument
The syntax of cin and the get function is:
where varChar :
Is a char variable
Is the argument (parameter) of the function

17. cin and the ignore Function
ignore: discards a portion of the input
The syntax to use the function ignore is:
intExp is an integer expression
chExp is a char expression
If intExp is a value m, the statement says to ignore the next m characters or all characters until the character specified by chExp

18. cin and the ignore Function (continued)18

19. cin and the ignore Function (continued)19

20. Input Failure Things can go wrong during execution
If input data does not match corresponding variables, program may run into problems
Trying to read a letter into an int or double variable will result in an input failure
If an error occurs when reading data:
Input stream enters the fail state

21. Input Failure (continued)21

22. Input Failure (continued)
Sample run 1
Line 9: Enter four integers: 34 K 67 28
Line 12: The numbers you entered are:
Line 13: a = 34, b = 20, c = 30, d = 40
Sample run 2
Line 9: Enter four integers:
Line 13: a = 37, b = 653, c = 30, d = 40 22

23. The clear Function
Once in a fail state, all further I/O statements using that stream are ignored
The program continues to execute with whatever values are stored in variables
This causes incorrect results
The clear function restores input stream to a working state

24. 24

25. The clear Function (continued)
Sample run
Line 7: Enter a number followed by a character: 78 d
Line 9: a = 78, b = 34
Line 12: Enter two numbers: 65 88
Line 14: a = 65, b = 88 25

26. Input/Output and the string Type
An input stream variable (cin) and >> operator can read a string into a variable of the data type string
Extraction operator
Skips any leading whitespace characters and reading stops at a whitespace character
The function getline
Reads until end of the current line 26

27. Formatting Output
Other than writing efficient programs, generating the desired output is one of a programmer’s highest priorities
In this section, you will learn about various output functions and manipulators that allow you to format your output in a desired way

28. setprecision Manipulator
Syntax is:
Outputs decimal numbers with up to n decimal places
Must include the header file iomanip:
#include <iomanip>

29. fixed Manipulator
fixed outputs floating-point numbers in a fixed decimal format
Disable by using the stream member function unsetf
The manipulator scientific is used to output floating-point numbers in scientific format

30. showpoint Manipulator
showpoint forces output to show the decimal point and trailing zeros
Example
cout << fixed << showpoint;

31. 31

32. showpoint Manipulator (continued)
Sample run
Line 10: setprecision(2)
Line 11: x = 15.67
Line 12: y =
Line 13: z =
Line 14: setprecision(3)
Line 15: x =
Line 16: y =
Line 17: z =
Line 18: setprecision(4)
Line 19: x =
Line 20: y =
Line 21: z = 32

33. setw Outputs the value of an expression in specific columns
cout << setw(5) << x << endl;
If number of columns exceeds the number of columns required by the expression:
Output of the expression is right-justified
Unused columns to the left are filled with spaces
Must include the header file iomanip

34. 34

35. setw (continued) Sample run 12345678901234567890 19 345 Hi 19
34519 35

36. left and right Manipulators
left: left-justifies the output
right: right-justifies the output

37. left and right Manipulators (continued)37

38. left and right Manipulators (continued)
Sample run
Warm
Warm 38

39. File Input/Output File: area in secondary storage to hold info
File I/O is a five-step process
Include fstream header
Declare file stream variables
Associate the file stream variables with the input/output sources (requires opening the files)
Use the file stream variables with >>, <<, or other input/output functions
Close the files File Input/Output

40. File Input/Output (continued)40

41. Programming Example: Student Grade
Write a program that reads a student name followed by five test scores
The program should output the student name, the five test scores, and the average test score
Output the average test score with two decimal places
The data to be read is stored in a file called test.txt
The output should be stored in a file called testavg.out

42. Programming Example: I/O
Input
A file containing the student name and the five test scores
Example: Andrew Miller
Output
The student name, the five test scores, and the average of the five test scores, saved to a file

43. Programming Example: Problem Analysis
The program needs to:
Read the student name and the five test scores
Output the student name and the five test scores
Calculate the average
Output the average

44. Programming Example: Variables
ifstream inFile;
ofstream outFile;
double test1, test2, test3, test4, test5;
double average;
string firstName;
string lastName;

45. Programming Example: Main Algorithm
Declare the variables
Open the input file
Open the output file
Set the manipulators fixed and showpoint
Also, set the precision to two decimal places
Read the student name

46. Main Algorithm (continued)
Output the student name
Read the five test scores
Output the five test scores
Find the average test score
Output the average test score
Close the input and output files

47. Summary
Input from the standard input device is accomplished by using cin and >>
>> is the stream extraction operator
When data is input in a program, the data items are usually separated by blanks, lines, or tabs
When inputting data into a variable, >> skips all leading whitespace characters
To use cin, include the header file iostream
Prompt lines: executable statements that tell user what to do

48. Summary (continued) get reads data character by character
ignore skips data in a line
clear is used to restore the input stream (from input failure) to a working state
After attempting to read invalid data into a variable
setprecision, fixed, showpoint, setw, left, and right can be used to format output
Include iomanip to use setprecision and setw
Header fstream contains the definitions of ifstream and ofstream