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Re^4: Numification of strings

by eyepopslikeamosquito (Canon)
on Aug 03, 2010 at 04:54 UTC ( #852595=note: print w/ replies, xml ) Need Help??


in reply to Re^3: Numification of strings
in thread Numification of strings

Yes, I guess it is explicit in Perl. In terms of language design, the interesting question is whether the Python/Ruby demand to explicitly cast/coerce by calling int is "stronger/safer typing" or "ugly explicit casting" (frowned upon in C++, for example). Let's clarify with a little test program.

In Perl, running:

use strict; use warnings; my $x = "2abc"; my $y = $x + 42; print "x=", $x, " y=", $y, "\n";
produces:
Argument "2abc" isn't numeric in addition (+) at f.pl line 4. x=2abc y=44
That seems reasonable to me.

In Python, running:

x = "2abc" y = x + 42 print "x=", x, "y=", y, "\n"
produces:
Traceback (most recent call last): File "f.py", line 2, in <module> y = x + 42 TypeError: cannot concatenate 'str' and 'int' objects

while running:

x = "2abc" y = int(x) + 42 print "x=", x, "y=", y, "\n"
also fails with:
Traceback (most recent call last): File "f2.py", line 2, in <module> y = int(x) + 42 ValueError: invalid literal for int() with base 10: '2abc'
Python's int function is stricter in what it accepts than Perl and Ruby. Finally, running:
x = "2abc" y = int(x[0:1]) + 42 print "x=", x, "y=", y, "\n"
produces the desired result:
x= 2abc y= 44

In Ruby, running:

x = "2abc" y = x + 42 print "x=", x, "y=", y, "\n"
produces:
f.rb:2:in `+': can't convert Fixnum into String (TypeError) from f.rb:2
Just like Python. While running:
x = "2abc" y = x.to_i() + 42 print "x=", x, " y=", y, "\n"
produces:
x=2abc y=44
Ruby's to_i method being less strict than Python's int function.

I guess Ruby and Python need to do it that way because the + operator is overloaded for both numeric addition and string concatenation, while Perl has a separate string concatenation operator, and so there is no ambiguity.

Update: Python's stricter typing can produce other subtle differences. For example, in Python both 5 * "X" and "X" * 5 produce five Xs. Not so in Perl or Ruby. That is, the string multiply operator is commutative in Python, but not in Perl or Ruby. Not sure if string multiply commutativity is a bug or a feature. Curiously, the new Perl 5.10 smart match operator ~~ was commutative in Perl 5.10.0, then that was deemed a mistake and it was made non-commutative in Perl 5.10.1.


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Re^5: Numification of strings
by LanX (Canon) on Aug 03, 2010 at 13:03 UTC
    > I guess Ruby and Python need to do it that way because the + operator is overloaded for both numeric addition and string concatenation, while Perl has a separate string concatenation operator, and so there is no ambiguity.

    exactly, look at JS to see the mess happening, when you have implicit convertions combined with only one operator for addition AND concatenation.

    BTW: Ruby's .to_i and .to_f are closer to Perl than JS parseInt() and parseFloat(). Trying to convert a non-numeric string like "abc" in Perl and Ruby produces 0 but in JS it's NaN (not-a-number) a value which doesn't produce a warning!

    perl -we' print 4+"abc"' Argument "abc" isn't numeric in addition (+) at -e line 1. 4

    Cheers Rolf

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