Input and Predefined Functions

Chapter 3

I/O: sequence of bytes (stream of bytes) from source to destination

• Bytes are usually characters unless the program requires other types of information
• Stream: sequence of characters (or bytes) from source to destination
• Input stream: sequence of characters (or bytes) from an input device to the computer
• Output stream: sequence of characters (or bytes) from the computer to an output device

Use iostream header file to receive data from the keyboard and send output to the screen.

• The header defines two stream types:
  • istream: input stream
  • ostream: output stream
• It also declares commonly used stream objects (variables):
  • cin: the standard input stream object
  • cout: the standard output stream object
• Do not redeclare cin or cout in your program; simply include the header #include <iostream> at the top of your file to use these objects.

Input

cin and the extraction operator, >>

• >> is a binary operator.

• When extracting with >>, the operator first skips any leading whitespace (spaces, tabs, newlines) and then reads characters for the next token until the following whitespace. It therefore reads a single whitespace-delimited token.

• Examples:

  • Reading into a string with >> reads one word (stops at the first whitespace).
  • Reading into a char with >> reads the next non-whitespace character.
  • Reading into an int or double with >> parses a numeric token (digits and an optional sign/decimal) and converts it to the requested type.

Data Type	Valid Input
char	One printable character except the blank
int	An integer (whole number), possibly preceded by a + or – sign
double	A decimal number, possibly preceded by a + or – sign. If the actual data input is an integer, the input is converted to a decimal number with the decimal part equal to 0.

Exploring User Input and Input Errors

Try the following code snippet from the video.

Try the following code snippets with:

• letter
• word
• positive number
• negative number
• multi-digit number
• decimal number
• combination (with and without spaces)

/*
 * Test program to see what happens with the user input does not match the
 * variables' types
 *
 * by Sean T. Hayes
 */

#include <iostream>
#include <string>
using namespace std;

int main()
{
	// Variables to receive input
	char ch1 = ' '; // 1 character
	char ch2 = ' '; // a second character
	int int1 = -8888; // whole number
	double doub1 = -999.999; // decimal (floating point) number
	string text;

	// Get input from the user
	cout << "Input a char, floating-point number, char, int, string: ";
	cin >> ch1 >> doub1 >> ch2 >> int1;
	getline(cin, text);

	// Display the values for each variable
	cout << endl;
	cout << "    Double: " << doub1 << '\n'
		<< "Characters: '" << ch1 << "', '" << ch2 << "'\n"
		<< "   Integer: " << int1 << endl
		<< "    String: \"" << text << "\"" << endl;

	return 0; // exit with success code.
}

If the user inputs a character or something erroneous like that – say, “Hello there” for an integer, the system enters what is known as a “fail state”. The integer is not set to anything, and the computer no longer takes in any other input.

Predefined Functions

• Function (subprogram): set of instructions
  • When activated, it accomplishes a task
• main executes when a program is run
• Other functions execute only when called
• C++ includes a wealth of functions
  • Predefined functions are organized as a collection of libraries accessible through header files.

To use pow() (which is short for power), include cmath.

• Two numeric parameters

• Syntax: pow(x, y) means

  • and are the arguments or parameters

• In pow(2, 3), the parameters are 2 and 3

• Note: pow() typically returns a floating-point value (for example, double). Using pow() for integer powers can introduce rounding or precision errors. If you need an exact integer result for small integer exponents, prefer repeated integer multiplication or cast/round the result carefully.

Input including Whitespace

The cin.get() function inputs the next character (including whitespace).

• Stores in memory location indicated by its argument
• cin.get(varChar);
  • varChar is a char variable
  • is an argument (or parameter) of the function
• cin and get are two separate identifiers separated by a dot
  • The dot separates the input stream variable name from the member (e.g., function name).
  • In C++, dot is the member-access operator.

The getline() function gets a whole line of user input including spaces as one string variable.

string userInput;
getline(cin, userInput); // read until a newline character.

In the previous code, if the user inputs: Hello world !!!, then userInput will contain, "Hello world !!!".

Adding a third parameter changes getline() to stop at that character instead of a newline ('\n'). For example,

string userInput;
getline(cin, userInput, '$'); // read until a $ character.

Ignoring Characters

The cin.ignore() function discards a portion of the input.

• cin.ignore(intExp, chExp);
• Ignore the next intExp characters unless we reach a chExp.
• Often '\n' is a great value to pass to the second parameter to ignore everything on the current line of input.
• Examples:
  • ignore(); // ignore the next character.
  • ignore(100); // ignore the next 100 characters.
  • ignore(INT_MAX, '\n'); // ignore until newline.
    • INT_MAX is a constant defined in the climits header that holds the maximum value for an integer.

Mixing cin >> and getline() and cin.ignore()

When you use cin >> to read a value (for example an int) the extraction operator leaves the newline character on the input stream. If you call getline() immediately after, it will read that leftover newline and return an empty string. A common pattern is to discard the remainder of the line before calling getline().

#include <iostream>
#include <climits>
#include <string>
using namespace std;

int main() {
    int num;
    string line;
    cout << "Enter a number: ";
    cin >> num;                         // reads number but leaves '\n'
    cout << '\n';
    
    cin.ignore(INT_MAX, '\n');       // discard rest of the line including '\n'

    cout << "Enter a full line: ";
    getline(cin, line);               // now reads the user's full line
    cout << '\n'
    
    cout << "You entered: " << n << " and \"" << line << "\"\n";
    return 0;
}

This ignore(INT_MAX, '\n') call discards characters up to the next newline, so the following getline() reads the intended input.

Quick Reference: Input functions

Function	Behavior	Short example
getline(cin, s)	Reads a whole line (up to \n) into s (a string)	getline(cin, line);
cin.get(ch)	Reads the next character (including whitespace) into ch (a char)	cin.get(c);
cin >> var	Extracts the next whitespace-delimited token and converts to var's type	cin >> number;
cin.ignore(n, ch)	Discards up to n characters or until ch is found	cin.ignore(INT_MAX, '\n');
cin.peek()	Returns next character without removing it from stream	int c = cin.peek();
cin.putback(ch)	Puts a character back onto the stream so it will be read again	cin.putback('x');

Keeping Values within the Input Stream

The cin.putback() function places the previous character extracted by the get function from an input stream back to that stream.

The cin.peek() function returns the next character from the input stream.

• It does not remove the character from that stream.

Handling Input Errors

The cin.clear() function restores the input stream to a working state. Useful after an input error.

• It does not remove the current contents of the stream. For that, ignore() with clear().

Self-Check Questions

1. What is a stream of data?
2. How would you read a string of data with spaces?

Practice Exercise

Write a short program that:

• Prompts the user to enter their age (an integer) on one line.
• Then prompts the user to enter their full name (which may include spaces) on the next line.
• Finally, prints the name and age on one line.

Example interaction:

Enter age: 21
Enter full name: Ada Lovelace
Name: Ada Lovelace, Age: 21

Hint: use cin >> to read the number, then discard the rest of that line before calling getline() to read the full name.