CPS120: Introduction to Computer Science

INPUT/OUTPUT

Input/Output Structures

In our pseudocode algorithms we have used the expressions Read and Write or Prompt

High-level languages view input data as a stream of characters divided into lines

Input/Output Structures

The key to the processing is in the data type that determines how characters are to be converted to a bit pattern (input) and how a bit pattern is to be converted to characters (output)

Console I/O

console I/O refers to using the keyboard and screen as the standard input and output devices, respectively.

C-In (cin)

cin is a stream of data flowing from an input device such as the keyboard (considered to be the standard input device) into your program.

Input OperationsThe operator >> is known as the input operator. It is also known as the extraction operator

You use the input operator in statements like,

cin >> numItems;

which would allow the user to input a value to be stored in the variable numItems.

Multiple Input

cin to allows the user to input several items with one C++ statement. The user however must type one or more spaces between each separate inputted value Example:

cin >> value1 >> value2 >> value3; Inputs need to have spaces or tabs between them (can’t use a comma to delimit C++)However, this prevents you from properly giving the user individual prompt messages

Output Operations

The operator << is known as the output operator. It is also known as the insertion operator

You use the output operator in statements like:

cout << "Hello world";

C-Out (cout)

cout is a stream which represents data flowing from one place to another. The statement:

cout << "Hello world"; causes data to flow from your program to the screen. The stream, cout, leads to what your computer considers to be its standard output device (usually the screen)

Menus and Prompting

// Prompt for values

cout << "What was your beginning mileage? ";

cin >> StartingMileage;

cout << "What was your ending mileage? ";

cin >> EndingMileage;

cout << "How many gallons of gas did you use? ";

cin >> GallonsOfGas;

cout << "How much did one gallon of gas cost? ";

cin >> PriceOfGas;

Inputting Words>> can be used but once it hits a space or a tab, the rest of the string is ignored

Use the get function for strings that contain ‘white space’String ends when you press enter

Complexities of Word InputSome things are done automatically with >>

get does not skip over line breaks and spaces

If the user enters a string longer than the length specified in the call to the get function, the remaining characters are left in the input streamGet always ignores the new line character (‘\n’) and leaves it in the streamUse the ignore function to flush the contents of the input stream

cin.ignore(80, ‘\n’);

Line Spacing

In order to end a line in an output statement you may use the new line character, \n, instead of endl. Examples:cout << "Hello world" << '\n';cout << "Hello world" << "\n";cout << "Hello world\n";

These are practically equivalent to: cout << "Hello world" << endl;

Escape Sequences

Other useful "escape sequences" (since the \ is the escape operator) are: \t to generate a tab

\\ to print a backslash

\' to print a single quote

\" to print a double quote

Using setf and unsetfEach stream has format options that can be changed

OPTION DESCRIPTIONleft Left-justifies the output

right Right-justifies the output

showpoint Displays decimal point and trailing zeros for floats

uppercase Displays e in scientific as E

showpos Displays a leading plus sign

scientific Displays floating point number scientifically

fixed Displays floating-point in normal notation

Using Format Options

Format options are set immediately prior to the COUT statement

float x = 24.0;

cout << x << ‘\n’; // displays 24

cout.setf(ios::showpoint);

cout << x << ‘\n’; // displays 24.00000

cout.unsetf(ios::showpoint);

cout << x << ‘\n’; // displays 24

Using Manipulators

You must include the <iomanip.h> header file at the top of your program in order to use the setprecision, setw, and other manipulators. You must use place the following compiler directive at the top of your program.#include <iomanip.h>

I/O manipulators are placed directly in the output statementcout << setprecision(2) << price << ‘\n’;

Setting Precision

The setprecision manipulator allows you to limit the number of digits that are displayed when a numeric data type is displayed:

cout << setprecision(2) << price << '\n';

only allows the leading two digits of the value stored in the variable, price, to be displayed

More Precisely

If the fixed format was set previously with the statement:

cout.setf(ios::fixed);then the setprecision(2) manipulator would have the effect of rounding or truncating price (and all future floating-point values in the cout stream) to the hundredths place

Field Width

The setw manipulator controls the width of the field when displaying a value. The statement:

cout << setw(10) << umEndow << endl;

sets the width of the field allocated for the variable, umEndow, to 10 characters

Formatted Output

cout.setf(ios : : fixed)

Print “fixed point” form, not in exponential form

cout.setf(ios : : showpoint)

Says to always print the decimal point

cout.precison(2)

Says to print out the two most significant decimal digits, after rounding to this precision