Perl printf Function
last modified April 4, 2025
The Perl printf function produces formatted output based on a
format string. It's similar to C's printf and provides precise control.
printf takes a format string followed by values to interpolate.
The format specifies how each value should be displayed, including width,
precision, and alignment.
Basic printf Usage
The simplest printf formats a string with placeholders.
#!/usr/bin/perl
use strict;
use warnings;
use v5.34.0;
my $name = "Alice";
my $age = 28;
printf("Name: %s, Age: %d\n", $name, $age);
This example demonstrates basic string (%s) and decimal (%d) formatting. The placeholders are replaced by the corresponding variables in order.
$ ./basic.pl Name: Alice, Age: 28
Number Formatting
printf provides extensive number formatting options.
#!/usr/bin/perl
use strict;
use warnings;
use v5.34.0;
my $price = 19.99;
my $quantity = 5;
my $total = $price * $quantity;
printf("Price: \$%.2f\n", $price);
printf("Quantity: %03d\n", $quantity);
printf("Total: \$%8.2f\n", $total);
We format floating-point numbers with 2 decimal places, pad integers with leading zeros, and control field width for alignment.
$ ./numbers.pl Price: $19.99 Quantity: 005 Total: $ 99.95
String Formatting
String formatting controls field width, padding, and alignment.
#!/usr/bin/perl
use strict;
use warnings;
use v5.34.0;
my $item1 = "Notebook";
my $item2 = "Pen";
my $price1 = 2.99;
my $price2 = 1.49;
printf("%-15s \$%5.2f\n", $item1, $price1);
printf("%-15s \$%5.2f\n", $item2, $price2);
The - flag left-aligns strings within a 15-character field. Numbers are right-aligned in a 5-character field with 2 decimal places.
$ ./strings.pl Notebook $ 2.99 Pen $ 1.49
Hexadecimal and Octal Output
printf can display numbers in different bases.
#!/usr/bin/perl
use strict;
use warnings;
use v5.34.0;
my $num = 255;
printf("Decimal: %d\n", $num);
printf("Hexadecimal: %x\n", $num);
printf("Hexadecimal (uppercase): %X\n", $num);
printf("Octal: %o\n", $num);
printf("With prefixes: %#x %#o\n", $num, $num);
The %x and %X formats output hexadecimal, %o outputs octal. The # flag adds base prefixes (0x for hex, 0 for octal).
$ ./bases.pl Decimal: 255 Hexadecimal: ff Hexadecimal (uppercase): FF Octal: 377 With prefixes: 0xff 0377
Scientific Notation
Large or small numbers can be displayed in scientific notation.
#!/usr/bin/perl
use strict;
use warnings;
use v5.34.0;
my $avogadro = 6.02214076e23;
my $electron_mass = 9.10938356e-31;
printf("Avogadro's number: %.4e\n", $avogadro);
printf("Electron mass: %.4e kg\n", $electron_mass);
printf("Alternative format: %.4g\n", $avogadro);
%e uses scientific notation, while %g chooses between %f and %e based on the number's magnitude. Precision controls significant digits.
$ ./scientific.pl Avogadro's number: 6.0221e+23 Electron mass: 9.1094e-31 kg Alternative format: 6.022e+23
Variable Width and Precision
Width and precision can be specified dynamically using *.
#!/usr/bin/perl
use strict;
use warnings;
use v5.34.0;
my $pi = 3.141592653589793;
my $width = 10;
my $precision = 4;
printf("Fixed: %.*f\n", $precision, $pi);
printf("Dynamic width: %*.*f\n", $width, $precision, $pi);
The * in the format string means "take the next argument as this value". This allows runtime control over formatting parameters.
$ ./dynamic.pl Fixed: 3.1416 Dynamic width: 3.1416
Positional Parameters
Parameters can be referenced by position in the format string.
#!/usr/bin/perl
use strict;
use warnings;
use v5.34.0;
my $name = "Bob";
my $age = 42;
my $score = 95.5;
printf("%2\$s is %1\$d years old and scored %3\$.1f%%\n",
$age, $name, $score);
The n$ syntax specifies which argument to use (1-based index). This allows reordering or reusing arguments in the format string.
$ ./positional.pl Bob is 42 years old and scored 95.5%
Best Practices
- Validate inputs: Ensure values match format specifiers.
- Use constants: For frequently used format strings.
- Consider locale: Number formatting varies by region.
- Watch precision: Excessive precision can mislead.
- Document complex formats: Explain non-obvious formats.
Source
This tutorial covered Perl's printf function with practical
examples demonstrating its powerful formatting capabilities.
Author
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