printf

The printf command (format printing) translates internal values into characters for sending to the standard output. The simplest form of this command echos back the text pattern it is passed.

> printf('The simplest form of this command echos');

However, we can use the text pattern to control the exact format we would like for the output through the use of conversion patterns and cursor control characters. For example,

> printf('\nFirst a carriage return & new line\n');
First a carriage return & new line
> printf('then print an integer %d', 101);
then print an integer 101
> printf('\nstep\vstep\vstep\vstep\vstep\vstep\vstep\n');
step
    step
        step
            step
                step
                    step
                        step

A text pattern is an ordinary name with the addition of zero or more conversion patterns. Each conversion pattern begins with a percent symbol (%) and is followed by

• a minus symbol (-) indicating the argument should be left justified,
• a number that specifies the minimum field width,
• a period which separates the field width from the precision,
• a number indicating the precision for a real value (this is the number of digits after the decimal point),
• a character from the set {a, c, d, e, f, g, l, o, s, u, x, %}. (Table  contains a description of the function for each.)

Table: Conversion characters for the printf command.

1.1 

Table: Conversion characters for the printf command.

Character	Description
a	prints any Darwin value including lists and sets
c	prints a single character
d	prints an integer
e	prints a number in exponential notation
f	prints a real number
g	prints a real number in the same manner as the
	e character if the exponent is greater than 8.
	Otherwise equivalent to the f or d characters.
o	prints the octal conversion of an integer
s	prints out an item of type name
u	prints an unsigned integer
x	prints the hexadecimal conversion of an integer
%	prints a percent sign %

 

> printf('%a, %a', ['L', 'I', 'S', 'T'], 'a means any structure');
[L, I, S, T], a means any structure
> int := 1234:
> printf('\n|%d|%10d|%-10d|', int, int, int, int);         # integers
|1234|      1234|1234      |
> printf('\n|%11s|%12s|%12s|%12s|', 'normal', 'field of 12', '5 decimal', 'left flush'); 
|     normal| field of 12|   5 decimal|  left flush|
> r := 1234.567:
> printf('\n|%f|%12f|%12.5f|%-12.5f|', r, r, r, r); 
|1234.567000| 1234.567000|  1234.56700|1234.56700  |

The backslash symbol (\) is treated as a special symbol in a text pattern. Followed by a letter from Table , it can be used to control the position of the cursor.

> printf('     <- The number five goes here\r 5\n');
 5   <- The number five goes here
> printf('A newline\vfollowed by a single quote \''');
A tab
     followed by a single quote '

Table: The cursor control sequences for the printf command.

1.1 

Table: The cursor control sequences for the printf command.

Character	Description
\b	backspace
\n	carriage return
	and newline
\t	tab
\v	newline
\r	carriage return
\\	\
\''	'

 

The syntax for a printf command is as follows:
printf( textpattern, $exp_{1}, exp_{2}, \ldots$)
The value of expression exp_i is placed in the i^th conversion pattern found in the string item textpattern according to the type of this conversion pattern. If the number of expressions is greater than the number of conversion patterns, the extraneous ones are ignored. If the number of conversion patterns is greater than the number of expressions, Darwin responds with an error. One must take care to match correct conversion patterns with the type of the corresponding expression. In some cases, Darwin will not respond with an error and the resulting text may appear nonsensical.

The best way to become familiar with the printf command in Darwin is through examples and experimentation.

> printf('\n%e \b %g \b %g' , 1234567, 1234567, 123456);   
1.234567e+06 1.23457e+06 123456
> printf('\nOctal: %o, Hexidecimal: %x',888, 888); 
Octal: 1570, Hexidecimal: 378
> printf('\nA character %c followed by a string %s', 'z', 'hello'); 
A character z followed by a string hello
> printf('\nA percent sign %% followed by a \'' symbol.');
A percent sign % followed by a ' symbol.
> printf('\nfirst a new line and carriage return\vnow just a newline');
first a new line and carriage return
                                    now just a newline