Perl 101 (part 6) - The Perl Toolbox - Slice And Dice (
Page 6 of 8 )
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.
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