Treating expressions as symbols

Asked May 11 '15 at 11:38

Active May 11 '15 at 15:10

Viewed 67 times

I'm trying to do something like this:

test = {x, y};
With[{test[[1]] = 3}, x/y]

However, the following error is printed:

With::lvset: "Local variable specification {test[[1]]=3} contains test[[1]]=3, which is an assignment to test[[1]]; only assignments to symbols are allowed"

I'm assuming this is because Mathematica treats the list elements as expressions and not as symbols correct? Setting x directly works, but I need the index for a loop functionality. Any hint on how to force Mathematica to interpret test[[1]] as a symbol?

edited May 11 '15 at 15:10

m_goldberg

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asked May 11 '15 at 11:38

F. E.

1

You could use a rule replacement x /. x -> 3 – image_doctor May 11 '15 at 11:45
3

With[{# = 3}, x/y] &[test[[1]]] if x has no value. – Kuba May 11 '15 at 11:49
related: 83126 – Kuba May 11 '15 at 11:50
thanks, that did the trick! any insight on why my method does not work? trying to avoid these kinds of mistakes in the future.. – F. E. May 11 '15 at 12:35
3

With has attribute HoldAll hence test[[1]] isn't evaluated to x prior to trying to make the assignment. – Bob Hanlon May 11 '15 at 13:12
1

@BobHanlon not only With is the problem. Set is HoldFirst too. – Kuba May 11 '15 at 13:46
1

Ok, this is complex issue, I think you should read this 28610 (probably duplicate) topic and linked/related ones. – Kuba May 11 '15 at 14:01

Treating expressions as symbols

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