Input/Output

#include <iostream> for cin, cout
#include <iomanip> for manipulators (setw, setprecision, etc.)
#include <limits> for numeric_limits (used with cin.ignore)

Reading Input(cin)


    int x;
    cin >> x;

>> is extraction operator
Takes two operands:

left operand: an input stream expression (cin, ifstream object)
right operand: variable to store input

Automatically skips leading whitespace
Stops reading at whitespace for strings.

cin.clear()

It clears the error state flags of cin.

If user enters the wrong type (ex. letters for an int), cin fails
When cin fails:

It enters a failed state
It stops accepting more input

cin.clear() resets the error state (makes cin usable again)
cin.ignore(removes the bad leftover input from the buffer)
if (cin >> x) checks if input was succesful (false if it failed)

cin >> x returns the stream itself.

Example:

If invalid input is entered...

cin enters a fail state
further operations stop working

cin.clear() does NOT remove bad input from the buffer. It only resets the stream state.

Output (cout)


    cout << "hello" << endl;

<< is the insertion operator

The left operand is an output stream expression (like cout or an ofstream object)
The right operand is an expression of a simple type, string, or manipulator

endl prints newline AND flushes buffer
'\n' prints newline only(faster)

Printf() C-style Output

C-style Output

Uses printf() function
requires #include <cstdio> or <stdio.h>
Uses format specifiers like %d, %f, %s
not type-safe (compiler doesn't strictly check format vs variable type)
procedural style

C++ Output

Uses cout object
Requires #include <iostream>
uses the << insertion operator
type-safe (compiler checks data types automatically)
object-oriented / stream-based style

General printf Format Specifier: %[flags][width][.precision][length]specifier

Specifier	Type	Example Format	Example Output (value = 12.3456 or 42)	Description
%d	Integer (int)	printf("%d", 42);	42	Prints a signed decimal integer
%f	Floating point (double/float)	printf("%f", 12.3456);	12.345600	Default: 6 decimal places
%.2f	Floating point	printf("%.2f", 12.3456);	12.35	Sets precision to 2 decimal places
%10d	Integer	printf("%10d", 42);	________42	Field width of 10 (right-justified by default)
%-10d	Integer	printf("%-10d", 42);	42________	Left-justified in field width of 10
%10.2f	Floating point	printf("%10.2f", 12.3456);	_____12.35	Field width 10, precision 2, right-justified
%s	String	printf("%s", "Hello");	Hello	Prints a string
%c	Character	printf("%c", 'A');	A	Prints a single character

Manipulators

In C++, I/O manipulators are special functions or objects that you can insert into your cin or cout streams to control formatting.
Examples:

Allow for a "field width" to align text
Change how values are displayed

Decimal, Hex, etc

Manipulators modify how data is displayed or read without changing the actual value.


    #include 
    #include 
    using namespace std;

    double pi = 3.14159;
    cout << fixed << setprecision(2) << pi << endl; // output: 3.14

fixed

Type: persistent manipulator
What is does: Forces floating point numbers to be printed in fixed-point notation rather than scientific(1.234e+00)
Effect: Ensures the number is displayed as a regular decimal number.
Without fixed, it might show scientific notation for very large/small numbers.

setprecision(2)

Type: persistent manipulator
What is does: Controls the number of digits printed.

Without fixed: setprecision(n) controls the total number of significant digits, not decimal places.
With fixed: setprecision(n) now controls the number of digits after the decimal point

So it rounds pi to 2 decimal places because we are using fixed.

Each manipulator (fixed, setprecision(2)) modifies the stream's behavior for everything that comes after it.
Order matters. Fixed must come before setprecision(2) to ensure it controls decimal places instead of significant digits.

Manipulators in iostream and iomanip

Manipulator	Header / Source	Example Code	Persistence	Description / Notes
endl	<iostream>	cout << "Hello" << endl;	N/A	Terminates the output line (like "\n") and flushes the output buffer.
fixed	<iomanip>	cout << fixed << 3.14159;	Persistent	Displays floating-point numbers in decimal format; works with setprecision() to control decimals.
showpoint	<iomanip>	cout << showpoint << 3;	Persistent	Always shows the decimal point for floating-point numbers, even whole numbers (e.g., 3 → 3.000000).
setw(n)	<iomanip>	cout << setw(10) << 42;	Temporary (next output only)	Sets the width of the next output to 'n' characters; right-justified by default. Useful for aligning columns.
setprecision(n)	<iomanip>	cout << fixed << setprecision(2) << 3.14159;	Persistent	Controls decimal places if fixed/scientific, or significant digits otherwise. Default is 6.
scientific	<iomanip>	cout << scientific << 31415.9;	Persistent	Displays floating-point numbers in scientific notation (e.g., 3.141590e+04).
hex	<iomanip>	cout << hex << 255;	Persistent	Displays integers in hexadecimal. Use dec to revert to decimal output.
dec	<iomanip>	cout << dec << 255;	Persistent	Displays integers in decimal format. Useful after hex or oct manipulators.
setfill(c)	<iomanip>	cout << setfill('0') << setw(5) << 42;	Persistent	Fills unused field width with the specified character (default is space). Useful for leading zeros.
left	<iomanip>	cout << left << setw(10) << 42;	Persistent	Left-justifies all subsequent output within the field width.
right	<iomanip>	cout << right << setw(10) << 42;	Persistent	Right-justifies all subsequent output within the field width (default).

Input/Output

Reading Input(cin)

cin.clear()

Example:

Output (cout)

Printf() C-style Output

C-style Output

C++ Output

General printf Format Specifier: %[flags][width][.precision][length]specifier

Manipulators

Manipulators in iostream and iomanip

setw() Quirks & Other Examples