0. Introduction to C++ computers and programming
CMPT 129
© Janice Regan, CMPT

1. C++ Basic Input and Output (I/O)
To print to your computer’s screen (console) or the read data typed into your keyboard you will need to use objects from the C++ iostream library.
To access the objects in the iostream library you must use a #include pre-processor directive to include their declarations:
#include <iostream>
This directive tells C++ to use appropriate library so we can use the I/O objects cin, cout, cerr, endl needed for simple input and output
© Janice Regan, CMPT

2. C++ Spaces inside “ “ cout << fixed << setprecision(2)
<< "The balance is $" << balance << endl;
cout << “the interest is “ << percent << “ % ” ;
cout << setprecision(5) << percent/100 << endl;
PRINTS
The balance is $78.50
The interest is 12.45%
Thee is no space between $ and the final quote in code, there is a space between % and the final “ in the code. Therefore, in the output $ is immediately followed by the value (no space) but % is followed by a space.
© Janice Regan, CMPT

3. C++ endl When your code executes the instruction
Consider the output window (the window in which the output from your program, and the input from your keyboard is displayed.)
When your code executes the instruction
cout << endl;
The cursor in the output window is moved to
the beginning of the next line
© Janice Regan, CMPT

4. C++: Moving to a new output line
C++ style: endl
C++ can also use C style: "\n"
escape sequence for the char "newline“
cout << "Hello World" << endl;
cout << "Hello World\n";
In C++ both the statements print string literal
"Hello World" to display, then move to next line
© Janice Regan, CMPT

5. C++ Output Window Output
What can you print to your computer’s output window using the insertion operator <<?
string name=“John”;
const double scale_factor = 2.2;
cout << scale_factor // Prints 2.2, the value of constant scale_factor
cout << name; // Prints John, the name stored in variable name
cout << “This is a string literal”; // Prints This is a string literal
Each cout above prints the value of one variable or constant
© Janice Regan, CMPT

6. C++ Output window Output
You can print the values of more than one variable or constant in a single cout statement
cout << “the number of games was ” << numGames << endl
<< “our team won “ << numWin << “ games” << endl;
Given that the variables have the values
numGames = 23 and numWin = 15
This single cout statement prints
the number of games was 23
our team won 15 games
to the output window
© Janice Regan, CMPT

7. C++ 3 Equivalent Outputs
cout << “the number of games was ” << numGames <<
endl << “ out team won “ << numWin << “ games” << endl;
// One cout statement printing many values in succession
cout << “the number of games was ” << numGames << endl;
cout << “ out team won “ << numWin << “ games” << endl;
// Two cout statements, each printing half the values
cout << “the number of games was ” ;
cout << numGames;
cout << endl;
cout << “ out team won “ ;
cout << numWin ;
cout << “ games” ;
cout << endl; // each value printed using a single cout statement
© Janice Regan, CMPT

8. Number of digits after decimal
There are two ways in C++ to specify how many digits to print after the decimal point
Method 1 uses setprecision() and
Uses <iostream> library only
Uses “magic formula” to set number of digits after the decimal point
The other method uses manipulators
Uses <iomanip> and <iostream> libraries
Syntax is simpler to remember
© Janice Regan, CMPT

9. Explain the “magic formula”
cout.setf(ios::fixed);
//Tells C++ to display output in fixed format xxx.yyy
cout.setf(ios::showpoint);
//Tells C++ to always print the decimal point
//Tells C++ to print trailing 0’s
cout.precision(3);
//Tells C++ to print 3 digits after the decimal point
Sets your program so all floating point numbers printed after these statements will print in fixed format with 3 digits after the decimal point
Adding another cout.precision(N) statement later in your code will cause all floating point numbers after that statement to print with N digits after the decimal point …
© Janice Regan, CMPT

10. Extend the “magic formula”
cout.unsetf(ios::fixed);
//Tells C++ to stop displaying in fixed format xxx.yyy
cout.setf(ios::scientific);
//Tell C++ to start displaying in scientific notation xxx.yyy
You can switch from fixed point notation to scientific notation (or from scientific notation to fixed point notation)
First you must unset the flag telling C++ to print using floating point. Then you must set the flag to tell C++ to print using scientific notation. (Or unset scientific and set fixed)
Results will not be predictable if you set both flags simultaneously
© Janice Regan, CMPT

11. What else can you specify?
The programmer can explicitly specify how C++ should format the numbers output by their programs using the <iomanip> library
In particular you can specify
The number of digits printed
The number of digits printed after the decimal point
width of field (how many spaces to leave for a value)
Fixed point or scientific notation E2
© Janice Regan, CMPT

12. C++ Manipulators fixed scientific setw() setprecision() left right
Used to control how output is formatted
Require user to include <iomanip> library
fixed
scientific
setw()
setprecision()
left
right
© Janice Regan, CMPT

13. Manipulators: fixed + setprecision()
fixed, prints number as fixed point, xx.yyy
setprecision(2) indicates 2 digits after the decimal point
Continues to use precision 2 and fixed until told to change in another cout command cout << "$" << fixed << setprecision(2)
<< 10.3 << " "<< "$" << << endl;
Prints $ $20.51
© Janice Regan, CMPT

14. Manipulators: scientific + setprecision()
scientific and setprecision() manipulators:
setprecision(4) indicates 4 digits after the decimal point
cout << “population is " << scientific
<< setprecision(4) << mypop << endl;
If mypop has value cout statement prints:
population is e+005
scientific prints number in scientific notation, xxx.yy Ezz
© Janice Regan, CMPT

15. Changing Format: Example
cout << fixed << setprecision(2) << "the interest is "
<< << "% or $”<< dollars << endl;
cout << scientific << setprecision(4)
<< "The amount is " << << endl;
Format changes after scientific is used. The code above print the following when dollars has value 33.12:
the interest is 12.34% or $33.12
The amount is e+002
© Janice Regan, CMPT

16. Manipulator: setw() setw() Note: setw() affects only NEXT value output
Sets the width in characters of the output field
By default output will be right justified in the output field
If the output has the same number of characters as the number or spaces available within the field it will fill the field
If the output has fewer characters as the number or spaces available within the field it will by default be right justified within the field
Note: setw() affects only NEXT value output
Must include setw() manipulator before each item output
© Janice Regan, CMPT

17. Manipulator Example: setw()
setw() manipulator:
cout << “xxxxxxxxxxxxxxxxxxx”; cout << endl<<"Start" << setw(5) << 10 << setw(4) << 20 << setw(6) << 30;
Prints:
xxxxxxxxxxxxxxxxxxxx Start
© Janice Regan, CMPT

18. Manipulator Example: left, right
left and right manipulators:
cout << fixed << left << setw(20) << interest << " %";
cout << endl << setw(20) << balance << endl;
cout << setw(20) << right << ;
Prints:
xxxxxxxxxxxxxxxxxxxxxx %
© Janice Regan, CMPT

19. Input Using cin Differences: cin >> num;
">>" (extraction operator) points opposite
Think of it as "pointing toward where the data goes"
Object name "cin" used instead of "cout"
No literals allowed for cin
Must input "to a variable"
cin >> num;
Waits on-screen for keyboard entry
Value entered at keyboard is "assigned" to num
© Janice Regan, CMPT

20. Prompting for Input:
When using console output and keyboard input always "prompt" user for input
cout << "Enter number of dragons: "; cin >> numOfDragons;
No endl in cout means that the prompt "waits" on same line for keyboard input as follows: Enter number of dragons:
Waits here for input
© Janice Regan, CMPT

21. Prompting for Input:
When using console output and keyboard input always "prompt" user for input
cout << "Enter number of dragons: “
<< endl; cin >> numOfDragons;
endl in cout means that the prompt "waits" on next line for keyboard input as follows:
Enter number of dragons:
Waits here for input
© Janice Regan, CMPT

22. Variable types and input
Be careful to give the correct type of data when responding to the prompt in a program.
Items from the keyboard will be converted but this may still not give the results you expect
Because of overflow
Because of conversion
© Janice Regan, CMPT

23. Input the correct type of data
Think of all the information you type in as a continuous stream of characters
If you are reading an integer and you read a decimal point you will stop reading at the decimal point because the decimal point is not a part of an integer
The next time you read you will begin with the decimal point left over from the last input
© Janice Regan, CMPT

24. Incorrect input examples
int one; double two; Int three; cout << "enter an integer"; cin >> one; cout << "enter a double "; cin >> two; cin >> three; cout << one << “ “ << two << “ " << three;
enter an integer4
enter a double 2.4
enter an integer7
enter an integer5.6
enter a double enter an integer6
enter an integer12
enter a double 44
enter an integer88
© Janice Regan, CMPT

25. Incorrect input examples
int one;
int two;
int three;
cout << "enter an integer ";
cin >> one;
cout << "enter integer 2";
cin >> two;
cout<<"enter integer 3";
cin>>three;
cout << "XX" << endl
<< one << " "<< two
<<three;
enter an integer 4.44
enter integer 2enter integer 3XX
enter an integer hello
© Janice Regan, CMPT

26. C++ output by example
Consider the code sampleIO.cpp posted with this set of notes.
Lets discuss the examples given in the code
© Janice Regan, CMPT