Perl-Python-a-Day: one-liner loop Functional Style


Xah Lee

One-Liner Loop in Functional Style

Xah Lee, 200510

Today we show a example of a loop done as a one-liner of Functional
Programing style.

Suppose you have a list of file full paths of images:


For those ending with -s (indicate a smaller version), you want to
change it to the full version without the -s, if it exists. Here's the

# -*- coding: utf-8 -*-
# python

import re, os.path


# change the image path to the full sized image, if it exists
# that is, if image ends in -s.jpg, find one without the '-s'.
for myPath in imgPaths:
(dirName, fileName) = os.path.split(myPath)
(fileBaseName, fileExtension)=os.path.splitext(fileName)
if(fileBaseName[-2:] == '-s'):
p2=os.path.join(dirName,fileBaseName[0:-2]) + fileExtension
if os.path.exists(p2): p=p2

print imgPaths2

But how do you do it in a functional programing style? Namely,
something of the from imgPath2=f(imgPath), where the f is some
function. Normally, the f would be a pure function construct made up on
the spot, that is, lambda. But Python's lambda is limited to only a
expression as its body. Nevertheless, one can achieve a functional
style with some workaround, using map twice. Here's the code:

imgPaths3 = map( lambda x: os.path.exists(x[1]) and x[1] or x[0], \
map(lambda x: (x, re.sub( r"^(.+?)-s(\.[^.]+)$",r"\1\2", x)),

The first map:

newList = map(lambda x: (x, re.sub( r"^(.+?)-s(\.[^.]+)$",r"\1\2", x)),

generate a list of pairs (x, y), where x is the same element in
imgPaths, and y is one without the -s. Then, a second map:

map( lambda x: os.path.exists(x[1]) and x[1] or x[0], newList,

checks if the path y exists, if so, use that, else, use the x part. The
function body is essentially of a conditional EXPRESSION, of the from
(test, if true return result1, else return result2). This form of a
test that returns a expression is very important in functional
programing. Because, in functional programing a common pattern is to
sequence functions and passing values. One cannot stop in the middle
and use a block structure. Here's how this form's syntax in several

test? trueExpression: falseExpression (C, Perl)
If[test, trueExpression, falseExpression] (Mathematica)
(test trueExpression falseExpression) (LISP)

In Python, there's no such form for this, but a semantically equivalent
workaround is to sequence boolean expressions as used in our example.

test and trueExpression or falseExpression (Python)

This works because it exploits a particular behavior of how Python
treats boolean sequences. In Python, “x and y†returns y if x is
true. And, “x or y†returns x if x is true. This behavior is
compatible with the mathematical sense of booleans. For example,
mathematically “x and y†should be true if both are true, yet in
Python if x is true then y is returned, and if y is true then this is
compatible with the math sense, but if y is false then the whole result
is also false, so it also satisfies the math sense. Similar is the case
of “x or yâ€. The point here is that one of the element is returned,
instead of a real True or False value. Therefore, in a twisted way this
can be used for the If[test, trueExpression, falseExpression] form.

result1= True and 4 or 5 # returns 4
result2= False and 4 or 5 # returns 5

print result1
print result2

Such language behavior is a result of the laziness of the compiler
implementation, particular seen in unix shells (&& and ||). The problem
with this design is that codes relying on the the returned element of a
boolean sequence does not clearly indicate the programer's intention.
It works by language pecularities, instead of expressed program logic.

For the official doc on evaluating booleans, see:

Here's the Perl code of the same loop block:

# perl
use File::Basename;


# change the image path to the full sized image, if it exists
# that is, if image ends in -s.jpg, find one without the '-s'.
for $myPath (@imgPaths){
($fileBaseName, $dirName, $fileExtension) = fileparse($myPath,
('\.[^.]+$') );
if (substr($fileBaseName,-2,2) eq '-s') {
$p2= $dirName . '/' .
substr($fileBaseName,0,length($fileBaseName)-2) . $fileExtension;
print $p2, "\n";
if (-e $p2) { $p=$p2}

use Data::Dumper;
print Dumper(\@imgPaths2)

In Perl, this can be written in a functional style one-liner:

@imgPaths3= map{
$y = $_;
$y =~ s/^(.+?)-s(\.[^.]+)$/$1$2/;
-e $y ? $y : $_;
} @imgPaths;

For the official doc of Perl's map, type in command line: “perldoc -f
this article is archived at:

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