'2 apples' + '2 oranges' == 4

[J Krugman]

I can't find the documentation for the behavior illustrated in the
Subject line (i.e. in Perl arithmetic operations are defined for
strings beginning with numbers[*]). Any pointers would be appreciated.
(Also, meta-pointers on how one would go about finding this
documentation if one didn't know already would also be very welcome.)

TIA!

jill

* Actually, the format of the strings Perl will perform arithmetic
on is more compilicated than I described above. Optional leading
whitespace is permitted, for example.

 

[Tassilo v. Parseval]

Also sprach J Krugman:

I can't find the documentation for the behavior illustrated in the
Subject line (i.e. in Perl arithmetic operations are defined for
strings beginning with numbers[*]). Any pointers would be appreciated.
(Also, meta-pointers on how one would go about finding this
documentation if one didn't know already would also be very welcome.)

A string like '2 apples' and '2 oranges' in numeric context will both
become 2. So if you add them, they are both evaluated in numeric
context, so that becomes 2 + 2, and hence 4.

The rules for string to number conversion are not so hard. One simple
rule is that perl scans everything up to the first non-digit character
and throws everything beginning with this character away. So the
number-prefix remains and becomes the number.

Tassilo

 

[xhoster]

I can't find the documentation for the behavior illustrated in the
Subject line (i.e. in Perl arithmetic operations are defined for
strings beginning with numbers[*]). Any pointers would be appreciated.

The warnings you get if you enable warnings would be a good start.

Or perldoc perlintro:

Scalars
A scalar represents a single value:

my $animal = "camel";
my $answer = 42;

Scalar values can be strings, integers or floating point
numbers, and Perl will automatically convert between them as
required.

Xho

 

[John Bokma]

The rules for string to number conversion are not so hard. One simple
rule is that perl scans everything up to the first non-digit character
and throws everything beginning with this character away. So the
number-prefix remains and becomes the number.

perl -e "print '3e4' * 4"

 

[J Krugman]

Also sprach J Krugman:

I can't find the documentation for the behavior illustrated in the
Subject line (i.e. in Perl arithmetic operations are defined for
strings beginning with numbers[*]). Any pointers would be appreciated.
(Also, meta-pointers on how one would go about finding this
documentation if one didn't know already would also be very welcome.)

A string like '2 apples' and '2 oranges' in numeric context will both
become 2. So if you add them, they are both evaluated in numeric
context, so that becomes 2 + 2, and hence 4.

The rules for string to number conversion are not so hard. One simple
rule is that perl scans everything up to the first non-digit character
and throws everything beginning with this character away. So the
number-prefix remains and becomes the number.

I'm sure the rules are straightforward, but I don't understand why
they aren't documented. It is not wise to use undocumented features
even if they are extremely straightforward.

jill

 

[Tassilo v. Parseval]

Also sprach John Bokma:

perl -e "print '3e4' * 4"

Which is why I wrote "One simple rule". It implies there are others.

Tassilo

 

[John Bokma]

Also sprach John Bokma:


Which is why I wrote "One simple rule". It implies there are others.

I think the rule is: scans everything that looks like a number, including
scientific notation, sign etc., and stops when it can't add anything more
to this number.

Correct me if I am wrong.