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Re: List manipulation?


 > I got a problem with List manipulation. I have the following list item:
 >    a = -Subscript[50, 5]
 > and I would like to change the value of 50 to 360-50 and also change the
 > sign to +. First I tried to change the sign and then the value and I got
 > an outcome I didn't understand. Doing the other way round I get the
 > outcome I expected. Does anybody know what the difference is?
 > Here is the Example file:
 > a = -Subscript[50, 5]
 > a[[2, 1]] = 360 - a[[2, 1]]
 > a[[1]] = -1*a[[1]]
 > a
 > b = -Subscript[50, 5]
 > b[[1]] = -1*b[[1]]
 > b[[1]] = 360 - b[[1]]
 > b

what happens is that you change the OwnValue of b in place from




and finally to:


which you can check from looking at:

OwnValues[b] // FullForm

after every step. The subtlety is that in the second step the b[[1]] on 
the right hand side is evaluated. As usual the b itself is evaluated 
first, which will get rid of Times, and only after that Part ([[]]) 
actually is applied, which you can see from e.g.:

b = -Subscript[50, 5]
b[[1]] = -1*b[[1]]



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