Re: Python has Traps; Does Euphoria have Programming Traps?
- Posted by DerekParnell (admin) Aug 20, 2013
- 1873 views
The following lines execute without any warning or error messages:
? 0 <= 5 <= 9 --1 ? 0 <= -5 <= 9 --1
Python (and a few other languages) allow this:
Python 2.7.3 (default, Sep 26 2012, 21:53:58) [GCC 4.7.2] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> 0 <= 5 <= 9 True >>> 0 <= -5 <= 9 False >>>
The Euphoria way of writing:
? 0 <= 5 and 5 <=9 -- 1 ? 0 <= -5 and 5 <= 9 -- 0
How do I explain what is happening?
The explanation is simply that Python supports chained conditionals whereas Euphoria doesn't. This would only be a trap for people coming from Python to Euphoria, as most languages do not support chained conditionals. C/C++ and Java, for example, do not.
The syntax ... ExprA CondA ExprB CondB ExprC is evaluated differently in the two languages.
In Python it is equivalent to ...
TempA = ExprA TempB = ExprB if TempA CondA TempB is TRUE then TempC = ExprC return TempB CondB TempC else return FALSE end if
And in Euphoria it is equivalent to ...
( ( ExprA CondA ExprB ) CondB ExprC )
Note that in both languages the expressions are only evaluated once, which is important if they have side effects. However, in Python, ExprC only gets evaluated if the first conditional test is true whereas in Euphoria it always gets evaluated.
For example ...
0 <= func(X) <= 9
To get the same effect in Euphoria as in Python, one cannot simply recode it as ...
( 0 <= func(X) ) and ( func(X) <= 9 )
because then the function is executed twice. Instead one needs to do ...
object tempA tempA = func(X) (0 <= tempA) and (tempA <= 9)
