Slice And Dice - Perl

Next up, the substr() function. As the name implies, this is the function that allows you to slice and dice strings into smaller strings. Here's what it looks like:

---

substr(string, start, length)

---

where "string" is a string or a scalar variable containing a string, "start" is the position to begin slicing at, and "length" is the number of characters to return from "start".

Here's a Perl script that demonstrates the substr() operator:

---

#!/usr/bin/perl
# get a string
print "Gimme a line!\n";
$line = ;
chomp ($line);
# get a chunk size
print "How many characters per slice?\n";
$num_slices = ;
chomp ($num_slices);
$length = length($line);
$count = 0;
print "Slicing...\n";
# slice the string into sections
while (($num_slices*$count) < $length)
{
$temp = substr($line, ($num_slices*$count), $num_slices);
$count++;
print $temp . "\n";
}

---

Here, after getting a string and a block size, we've used a "while" loop and a counter to keep slicing off pieces of the string and displaying them on separate lines.

And here's what it looks like:

---

Gimme a line!
The cow jumped over the moon, giggling madly as a purple pumpkin with fat
ears exploded into confetti
How many characters per slice?
11
Slicing...
The cow jum
ped over th
e moon, gig
gling madly
as a purpl
e pumpkin w
ith fat ear
s exploded
into confet
ti

---

You've already used the print() function extensively to sent output to the console. However, the print() function doesn't allow you to format output in any significant manner - for example, you can't write 1000 as 1,000 or 1 as 00001. And so clever Perl programmers came up with the printf() function, which allows you to define the format in which data is printed to the console.

Consider a simple example - printing decimals:

---

#!/usr/bin/perl
print (5/3);

---

And here's the output:

---

1.66666666666667

---

As you might imagine, that's not very friendly. Ideally, you'd like to display just the "significant digits" of the result. And so, you'd use the printf() function:

---

#!/usr/bin/perl
printf "%1.2f", (5/3);

---

which returns

---

1.67

---

A quick word of explanation here: the Perl printf() function is very similar to the printf() function that C programmers are used to. In order to format the output, you need to use "field templates", templates which represent the format you'd like to display.

Some common field templates are:

---

%s string
%c character
%d decimal number
%x hexadecimal number
%o octal number
%f float number

---

You can also combine these field templates with numbers which indicate the number of digits to display - for example, %1.2f implies that Perl should only display two digits after the decimal point.

Here are a few more examples of printf() in action:

---

printf("%05d", 3); # returns 00003
printf("$%2.2f", 25.99); # returns $25.99
printf("%2d%", 56); # returns 56%

---

And here's a calculator which uses the printf() function to display numbers in various numerical bases like hexadecimal and octal.

---

#!/usr/bin/perl
print "Enter a number: ";
chomp($number = );
printf("In decimal format: %d\n",$number);
printf("In hexadecimal format: %x\n",$number);
printf("In octal format: %o\n",$number);

---

Perl also comes with a sprintf() function, which is used to send the formatted output to a variable instead of standard output.

This article copyright Melonfire 2000. All rights reserved.

>>> More Perl Programming Articles          >>> More By Vikram Vaswani and Harish Kamath, (c) Melonfire

blog comments powered by Disqus