STACK: How to write F_{1x} in input?

STACK: How to write F_{1x} in input?

by Martin Kraska -
Number of replies: 3

If I have vectors F_1 and F_2, then how would I adress their x and y components in STACK input?

In Latex, I'd write F_1_x or F_{1x}. 

Whatever I try, in STACK input, the validation kills it:

  • F_1 and F1 are allowed but that is just part of what I want
  • F1x is not allowed, Validation suggests F1*x
  • F_1x is not allowed. Validation suggests  F_1*x
  • F_x1 is allowed but wrong, because the coordinate index should be right to the count (with or without deeper subscript level)
  • F_1_x is not allowed. Validation suggests F_1_*x.

BTW, this issue came up, because students tried to write F_1x instead of F_1*cos(phi), which would have been the correct solution. 

Even adding F_1x or similar to the allowed words doesn't help.

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In reply to Martin Kraska

Re: STACK: How to write F_{1x} in input?

by Christopher Sangwin -
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Martin,
Sorry about this. Whenever we have an insert *s option we have to make some kind of decisions about what we will, and also will not, accept. Have you checked all the other insert*s options? (I haven't had time, sorry!) I'm happy to discuss specific proposals if you have them.
Chris
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In reply to Christopher Sangwin

Re: STACK: How to write F_{1x} in input?

by Martin Kraska -
Chris,
after some erratic inconsistency in my beginning STACK activities I settled to use "Insert stars for space" in order not to force students to write * everywhere. It is related to the fact that I have to fight hard to prevent them from using stars as times operator in ordinary formulas in their reports. It is like fighting against windmills. I use to ask whether they really mean a convolution if they write F*a. So I thought it might be a good idea to not insist on stars.

If the naming restrictions are related to the selected insert *s option I have to reconsider that decision, because the names mentioned in the TO post are quite common in mechanics. There are even worse names like R_{m,N} which one could write as R_m_N with the convention that there is just a single subscript level.

Yet I think that none of the reported naming restrictions are really needed to enable insert *s for whitespace, because spaces will never be a valid part of a name.
Another story is about what subscript convention should be used in valitation output and display in CAS text.

After initially carelessly using implicit subscripting for F1 M0 and similar names I found that students tried to also write Fx and Av, or Ieta, where implicit indexing doesn't work.
So I started to insist on _ as subscript indicator/operator. This significantly reduced complaints and blindly poking around while answering the questions,

Given that strict convention, formatting should become easier. Whatever follows the first underscore goes to the subscript no matter how many characters or numbers. Repeated underscores could be written verbatim or replaced by some other delimiter, R_m_N would display as R_{m\_N}.

Martin
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In reply to Martin Kraska

Re: STACK: How to write F_{1x} in input?

by Martin Kraska -
Actually, R_m_N works, problems arise if numbers are part of the subscript, just like F_1x
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