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.

basic.pl

#!/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.

numbers.pl

#!/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.

strings.pl

#!/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.

bases.pl

#!/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.

scientific.pl

#!/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 *.

dynamic.pl

#!/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.

positional.pl

#!/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

Perl printf Documentation

This tutorial covered Perl’s printf function with practical examples demonstrating its powerful formatting capabilities.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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