Console I/O

Table of Contents

• Output
  • Escape Codes
• I/O Manipulators
  • iomanip
• Input
  • Input Functions
  • Input Errors

As you work with code, outputting information is critical. The console (aka: terminal, command line, command prompt, or shell) is the easiest way to do this. C++ has a standard syntax for streaming I/O - including the console and files.

Output

You can output any single value by using the << operator.

#include <iostream> //enables console I/O
using namespace std;

int main() {
    int x = 10;
    cout << "The value of x is: " << x; //outputs the value of x to the console
    //Final Output:
    //  The value of x is: 10
}

Note

using namespace std; is important here. The namespace feature of C++ exists in order to prevent name conflicts. If that line is omitted, every instace of something within the std namespace must be prefixed with std::. For example, std::cout << x;. For now, this is fairly insignificant, but namespaces become important when working with multiple files.

Any operation that yields a simple data type (boolean, integer, float) will be evaluated before it is output, so, the cout << 2 + 2; will output 4. If you want a statement to be output literally (such as 2 + 2 instead of 4), place the text in double quotes (cout << "2 + 2";).

Another feature worth noting is that the char datatype outputs the corresponding character. The value of the char variable is output as its corresponding character on the ASCII chart. In order to output the raw value of a character, use `static_cast<int>('a')`.

Note

Single and double quotes are completely different. Single quotes allow only one character which is treated as a char constant (const char ...). Double quotes allow for entire strings of text and are much more complex.

Escape Codes

Escape codes are single characters that allow you to output nonprinting characters. For example, in C++, strings are always enclosed in quotes. Placing a quote prematurely will terminate the string. Escape codes fix this.

Escape Sequence	Output
\'	single quote
\"	double quote
\?	question mark
\\	backslash
\0	null character, this is often used to denote the end of a transmission or string
\a	audible bell - sends a system beep sound
\b	backspace
\f	form feed - new page
\n	new line, this is very useful
\r	carriage return
\t	horizontal tab
\v	vertical tab
\unnnn	outputs a unicode character

"This string \"has a quotation" and makes the computer beep. "

I/O Manipulators

There are also several operations that you can run on the stream itslef. Assumes that the line using namespace std; is at the top of the file.

Command	Action	Example
boolalpha & noboolalpha	toggles the numeric and text format for bool values	cout << boolalpha << true; will output true insetad of 1.
showpoint & noshowpoint	toggles the decimal point on x.0 values	cout << showpoint << 0; outputs 0.0 instead of 0
showpos & noshowpos	shows the + sign in front of positive numbers	cout << showpos << 3.14; outputs +3.14
fixed & scientific	outputs floating point numbers in either scientific or fixed notation	cout << scientific << 0.01; outputs 1.000000e-02
left	sets the alignment of fixed width output to left	cout << left ...;
right	sets the alignment of fixed width output to right	cout << right ...;
internal	sets the alignment of fixed width output with multiple components to have whitespace in the middle	cout << internal << scientific << 0.01 outputs 1.000000*****e-2 where * is set as the fill character (see iomanip)
uppercase & nouppercase	toggles uppercase letters for values expressed in scientific notation	cout << uppercase << scientific << 23.145; outputs 2.3145E-1
flush	outputs the buffer early, which be used for long processes	cout << "this is output first" << flush << "and this is output later";
endl	creates a new line and runs flush	cout << a << endl << b; outputs a and b on separate lines assuming they are defined

iomanip

Including iomanip (#include <iomanip) allows for even finer control of the output. Here are the most notable features.

Command	Action	Example
setfill	Sets the fill character for numeric output (often used in combination with setw).	cout << setfill('\*'); sets the fill character to be an asterisk, note that the argument is a character (enclosed in single quotes) and not a string (enclosed in double quotes).
setprecision	Sets the precision of floating point numbers, estimates	cout << setprecision(2) << 1.25; outputs 1.3.
setwidth	Sets the width of the next input or output field	cout << setfill('0') << setw(5) << 2; outputs 00002.

Input

Input is very similar to output, but it has some quirks. With the following code segment...

int x, y, z;
cin >> x, y, z;

The input should be formatted like this

1 2 3

The data can then be used for any purpose later in the program.

Note

cin is whitespace delimited. There can be any number of tabs, spaces, or new lines in the input. Once all of the values have been entered, the user must press enter one last time to send the data.

Finally, the user is also able to input data before it is even asked. See the example below.

#include <iostream>
//because, I am not using namespace std, I need to put std:: in front of anything included from iostream

//This program is a simple calculator. I will expand this later. Right now, it can only add.

int main() {
    int a, b;
    std::cout << "Enter the first operand: ";
    std::cin >> a;
    std::cout << "Enter the second operand: ";
    std::cin >> b;
    std::cout << "The answer is: " << a + b;
}

Note

<< is known as the insertion operator and >> is the extraction operator. They are evaluated after other operations.

There will not be an error if the user inputs both operands at the first cin statement. The input is a stream (hence, iostream). Once the first value is read in, the second is stored until it is requested by the program. If it is not needed, it is simply discarded.

Input Functions

In addition to cin, which is whitespace-delimited, there are also several functions that can be used with the input stream.

Note

Functions are named chunks of code.
They are called (run) as follows: SomeFunction();.
They can be passed data: SomeFunction(x);.
They can also return data, which can be placed into a variable as follows: a = SomeFunction(x, y);.

Example	Action
cin.get();	Reads one value from the buffer (stream) and discards it.
cin.get(x);	Reads one value into x.
cin.ignore();	Ignores the next character in the buffer.
cin.ignore(100);	Ignores the next 100 characters in the buffer.
cin.ignore(100,'\n');	Ignores the next 100 characters, stops early if a newline is found.
z=cin.peek();	Copies the value at the top of the stream into z without discarding it from the buffer. cin.get(). removes the value from the stream, cin.peek() does not.
cin.putback(x);	Puts x back into the stream.

Input Errors

If invalid data is provided when reading into cin, the stream will enter a "fail" state and stop reading data. The .clear() function will return the stream to a usable state. You'll then need to clear the buffer using the .ignore() function.

cin.clear();
cin.ignore();

An input buffer enters the fail state if the user provides the wrong type of data. For instance, if a float is supposed to be an int, the stream will enter a fail state and not read data until the problem is fixed.

The following code will check to make sure that cin is not in the fail state...

if (!cin) //checks to see if cin is in a fail state
{
    cin.clear();
    cin.ignore();