Introductory Programming Lecture Notes: C++ output

Basic output in C++

To use cout you must include the following directive at the start of your program
```
#include <iostream>
using namespace std::cout;
```
The basic form for writing output to the screen is
```
   cout <<  ...something... ;
```
For variables, simply put the variable name.
```
   cout << MyVariableName;
```

For text, use the double quotes

  cout << "This will appear on the monitor";

For newlines, use the "\n" special character, e.g.
```
  cout << "\n";
```

Multiple items can be run together, e.g.

  int X = 17;   int Y = 23;
  cout << "The value of variable X is " << X << "\n";
  cout << "... and Y is" << Y << "\n";

This would produce output

The value of variable X is 17
... and Y is23

Special characters

There are a variety of characters in C++ that are used for special purposes, such as single-quotes, double-quotes, backslashes, etc.

To include one of these as a "regular" character in an output stream, or to assign them as a character value, we have to let the compiler know that we don't want the "special" use of the character.

To do so, we use an escape sequence - which usually simply means preceding the desired character with a backslash \

Some of the common escape sequences are listed below:

Escape sequence Meaning

\b backspace

\n newline

\t tab

\g bell

\\ backslash

\' single quote

\" double quote

\nnn use ASCII value

For example:

   cout << "Here comes a double-quote: \" there it went ... \n";

The ASCII value for a character is an integer interpretation of the bit pattern used to store the character.

For example, the ASCII values for the characters 'A' through 'Z' are 65 through 90.

Formatting output

Formatting output makes it clearer
The <iomanip> library contains formatting aids

The awkward way to get data to line up is to manually put in the right number of blanks:

   cout << "blah value       " << blah << "\n";
   cout << "longerblah value " << longerblah << "\n";

An easier way is to use a library routine to pad out the right number of blanks:
std::setw(x) sets the width of the next output field, padding with as many blanks as necessary
```
   cout << "Boo" << std::setw(5) << "zoom\n";
   cout << std::setw(6) << 3650;
```
Pads zoom (on the left) with one space (i.e. " zoom")
and pads 3650 with two spaces, producing:
```
Boo  zoom
  3650
```
If the number is too large for the std::setw field it simply ignores the std::setw, e.g.
```
  cout << "Boo " << std::setw(3) << 1365 << ".";
```
produces
```
Boo 1365.
```

Formatting real numbers

For real numbers we often want to specify both how wide the whole number is (std::setw works for this), and how many decimal places of accuracy are displayed.

The latter requires a two-step process:

first we indicate the desire to use fixed width fields, then we set the width for specific fields individually:

   cout << std::setiosflags(ios::fixed);
   cout << std::setprecision(3) << std::setw(7) << 765.432 << "\n";
   cout << std::setprecision(2) << std::setw(7) << 023.14 << "\n";
   cout << std::setprecision(2) << std::setw(7) << 341.2 << "\n";
   cout << std::setprecision(4) << std::setw(7) << 0.00456 << "\n";

produces output

Escape sequence	Meaning
\b	backspace
\n	newline
\t	tab
\g	bell
\\	backslash
\'	single quote
\"	double quote
\nnn	use ASCII value