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Re: Question about DayOfWeek

  • To: mathgroup at smc.vnet.net
  • Subject: [mg126199] Re: Question about DayOfWeek
  • From: Dana DeLouis <dana01 at me.com>
  • Date: Wed, 25 Apr 2012 00:31:03 -0400 (EDT)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com

> DayOfWeek[{1975,1,1}]==Thursday gives
> Wednesday == Thursday and not False, as it should? Why is it so?

Hi.  I believe the few examples given in Help on the function DayOfWeek using Equal ( == ) were incorrect.
The few examples given all just happen to return True.

Since Wednesday is a Symbol, and not a numeric value, I believe one should use SameQ.

Wednesday//Head
Symbol

What you are seeing is either this...
x==x
True

or this...
x==y
x==y

You want...

x===x
True
x===y
False

Using one of the Help examples, notice the following.

x==x==x
True

This expression reduced to x==y

x==x==y
x==y

Just note the following...

Needs["Calendar`"]

DayOfWeek[{2000, 1, 1}] 
Saturday

So, using a given example in Help,  they just happen to all be the same Symbol, so True...
DayOfWeek[{2000, 1, 1}] == DayOfWeek[{2000, 1, 1 }] == DayOfWeek[{2000, 1, 1}]
True

Here, the last is a different symbol, so reduces to ...

DayOfWeek[{2000, 1, 1}] == DayOfWeek[{2000, 1, 1 }] == DayOfWeek[{2000, 1, 2}]
Saturday==Sunday

So, it appears Equal doesn't work as expected, ...

Table[DayOfWeek[{2012,1,d}] == Monday,  {d,1,30}]  //TableForm

<omitted>

But SameQ appears to work comparing symbols...

Table[DayOfWeek[{2012,1,d}] === Monday, {d,1,30}]  //TableForm

False
True
False
False
...etc

= = = = = = = = = =
Good luck.   HTH  :>)
Dana DeLouis
Mac & Math 8
= = = = = = = = = =



On Apr 24, 5:37 am, Kent Holing <K... at statoil.com> wrote:
> DayOfWeek[{1975,1,1}]==Wednesday gives True (as it should), but DayOfWeek[{1975,1,1}]==Thursday gives
> Wednesday == Thursday and not False, as it should? Why is it so?





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