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Re^3: Numification of strings

by chromatic (Archbishop)
on Aug 02, 2010 at 20:59 UTC ( #852543=note: print w/ replies, xml ) Need Help??


in reply to Re^2: Numification of strings
in thread Numification of strings

What's implicit about using numeric operators on strings? If you use string operators on strings, you get string behavior. If you use numeric operators (which you must do explicitly), you get numeric behavior. What isn't explicit about that?


Comment on Re^3: Numification of strings
Re^4: Numification of strings
by LanX (Abbot) on Aug 02, 2010 at 21:11 UTC
    well it's implicit if you use dynamically typed variables, i.e. you can't tell in advance if it's an integer or a number in the scalar.

    Cheers Rolf

Re^4: Numification of strings
by eyepopslikeamosquito (Canon) on Aug 03, 2010 at 04:54 UTC

    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.

      > 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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