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Re: Logical operators [message #93871 is a reply to message #93867]

Sun, 13 November 2016 19:05

Phillip Bitzer
Messages: 223
Registered: June 2006

Senior Member

On Sunday, November 13, 2016 at 8:30:50 AM UTC-6, Glan wrote:
> Hi
> I am trying to do
> if( ( (a gt 50) and ( b lt 50) ) or ((a lt 50) and ( b gt 50)) then begin
> c=! NANQ
>
> ...but the multiplie logical operator not working.
> Can you help
> GlanPlon

First, and just to clarify for others, "and" and "or" or not, strictly speaking, logical operators (&&, ||). They are bitwise. Here, they essentially work the same as the logical ops, since the result of the relational operators (lt,gt, etc.) lead to ones or zeros. But, the logical operators have the added benefit of "short-circuiting," so that 1 || _some_expression is true, without evaluating _some_expression. Similarly, 0 && _some_expression is false, without evaluating _some_expression.

With that said, a simple test shows the code does not always evaluate to true:

IDL> a = 0 & b=0
IDL> ( (a gt 50) and ( b lt 50) ) or ((a lt 50) and ( b gt 50))
0
IDL> a = 0 & b=51
IDL> ( (a gt 50) and ( b lt 50) ) or ((a lt 50) and ( b gt 50))
1
IDL> a = 51 & b=0
IDL> ( (a gt 50) and ( b lt 50) ) or ((a lt 50) and ( b gt 50))
1
IDL> a = 51 & b=51
IDL> ( (a gt 50) and ( b lt 50) ) or ((a lt 50) and ( b gt 50))
0

(Your code seems to be missing a parentheses, BTW.)

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[Message index]

		Logical operators By: Gompie on Sun, 13 November 2016 06:30
		Re: Logical operators By: Jim Pendleton on Sun, 13 November 2016 07:40
		Re: Logical operators By: Gompie on Sun, 13 November 2016 08:18
		Re: Logical operators By: Jim Pendleton on Sun, 13 November 2016 08:39
		Re: Logical operators By: Phillip Bitzer on Sun, 13 November 2016 19:05

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