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Show HN: I made a spreadsheet where formulas also update backwards

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Re: Show HN: I made a spreadsheet where formulas also update backwards

#71

Post by [deleted] »

fouronnes3 wrote: post #63
ximm wrote: post #58

Earlier quoted context omitted.

100% this. When I reached the end of that page I felt pranked because the obvious question was never answered. How are these cases resolved? Is it possible to fix some inputs and only update others? What if I sometimes want to change input A, and other times I want to update input B? All this should be explained as early as possible.

You can do it and it is explained, actually. Use # as a prefix to indicate a constant, e.g.: #50 will be a constant and not a variable. In the future I'd like to support more user input constraints, in particular domain constraints for variables. So you could tell the solver that this cell must remain in some interval, and it would respect that interval instead of assigning any real value.

[deleted]

Re: Show HN: I made a spreadsheet where formulas also update backwards

#72

Post by dizhn »

nico wrote: post #43
Suppafly wrote: post #40

Earlier quoted context omitted.

Lots of people working in different fields end up reinventing things that have been known to math for centuries, often in clunky roundabout ways. I imagine some of them figure out things not known to math, but it's far more likely to go the other way.

> Lots of people working in different fields end up reinventing things that have been known to math for centuries I remember reading, about a year or two ago, about a medical doctor that published a paper rediscovering calculus (I just looked it up, it happened in 1994, there’s been many articles and videos about it)

It's not clear from the Wikipedia article linked below whether she was rediscovering part of calculus or knowingly rebranding it. Do you know more details?

Re: Show HN: I made a spreadsheet where formulas also update backwards

#73

Post by dizhn »

zoky wrote: post #67
fouronnes3 wrote: post #62

Earlier quoted context omitted.

Jokes aside, let's say someone does figure out how to break RSA over a weekend project. The evil options are easy to come up with, but what is the actually responsible, ethical, thing to do? Never tell anyone?

Contact a known and trusted security researcher who can verify to the world that you did what you said you did, so everyone else can have as much time as possible to figure out exactly how fucked they are. Doing nothing isn’t an option; once someone figures something like that out, it signifies that conditions were ripe for the discovery to be made, and it’s only a matter of time before it’s discovered again independ…

Also fairly reasonable to assume it has already been done by someone who had a motive to break it and is keeping quiet.

Re: Show HN: I made a spreadsheet where formulas also update backwards

#75

Post by dmurray »

esafak wrote: post #29
throw310822 wrote: post #7

Hm? I don't get it. What's the point of calculating backwards non-invertible operations such as addition? Isn't the result just arbitrary?

It is for fun. Commercial products do not support this because functions are generally not invertible.

I would expect the opposite.

Commercial products are run by product managers: they do whatever the business needs that day, and if it doesn't work for most inputs, "that's fine, our users will only ever need addition". Fun open source projects, run by the same programmer who does the implementation, obsess over finding the generic solution to inverting a function and end up with a version that isn't useful for anyone's specific case.

Re: Show HN: I made a spreadsheet where formulas also update backwards

#76

Post by geon »

rahimnathwani wrote: post #21
willrshansen wrote: post #10

The first example on the main page has a formula with two variables being updated from changing one value. The immediate question I have is if I change the output, where does the extra degree of freedom come from on the inputs? Does one stay locked in place? Unclear. I am a huge fan of the concept though. It's been bugging me for years that my spreadsheet doesn't allow editing text fields after filtering and sorting…

Does one stay locked in place? Unclear. If you set C1=A1+B1 then, when you set a value for C1, A1 and B1 are each half of that value, even if they started off unbalanced.

It would make more sense to preserve the ratio if possible.

Re: Show HN: I made a spreadsheet where formulas also update backwards

#77

Post by graemep »

b-karl wrote: post #15

In Excel you have goal seek for this functionality. I believe it does some form of numerical solving of the equation system. Good for every situation when you need to solve equations! In the context of using spreadsheets I think about solving simple financial or maybe construction/mechanical design problems where you don’t want to solve it manually or program it and a spreadsheet is a quick and useful interface.

This is very different in practice, because it is pervasive rather than something you have to set up for particular cases.

If this was usual it would help a lot with people's tendency to hard code the correct answer rather than fix formulae. Just that aspect of it would be a huge improvement. People do this all the time with simple financial problem, for example.

A lot of what people use spreadsheets for is not all that simple. Again, especially with financial applications. People manage huge amounts of money using horribly complex models implemented in Excel.

Re: Show HN: I made a spreadsheet where formulas also update backwards

#78

Post by geon »

InsideOutSanta wrote: post #64
jamestimmins wrote: post #38

Earlier quoted context omitted.

My brother once suggested that there are probably bits of code/algorithms that would be world changing if they were released in academic journals, but instead were written by some unknowing programmer in an afternoon for their job coding embedded systems for refrigerators. This particular example may be unlikely, but it's a very fun idea.

Iirc, Heisenberg reinvented Matrix calculations to solve a problem in quantum physics. Not being a mathematician, he wasn't aware of the concept. Born recognized what Heisenberg had done and introduced him to his own reinvention.

PBS Space Time released a video with that story yesterday: https://www.youtube.com/watch?v=c-Q5r3THR3M

Re: Show HN: I made a spreadsheet where formulas also update backwards

#79

Post by geon »

Suppafly wrote: post #69
Towaway69 wrote: post #50

Earlier quoted context omitted.

Folks shouldn’t be afraid to “rediscover” stuff. Primarily because the learnings you make are the same as the original “discoverer”. Without those learnings, you might not be able to arrive at your true destination.

>Folks shouldn’t be afraid to “rediscover” stuff. Luckily no one is suggesting that.

A lot of people suggest that. So many that it has become an idiom. "Don't reinvent the wheel."

Re: Show HN: I made a spreadsheet where formulas also update backwards

#80

Post by big-chungus4 »

adonovan wrote: post #39

“Formulas that update backwards” is the main idea behind neural networks such as LLMs: the computation network produces a value, the error in this value is computed, and then the error quantity is pushed backward through the network; this relies on the differentiability of the function computed at each node in the network.

"Formulas that update backwards" isn't really the main idea behind neural networks. It's an efficient way of computing gradients, but there are other ways. For example forward propagation would compute a jacobian-matrix product of input wrt output with an identity matrix. Backpropagation is similar to bidi-calc to the same extent as it is similar to many other algorithms which traverse some graph backward.

I think you should be able to use bidi-calc to train a neural net, altough I haven't tried. You'd define a neural net, and then change it's random output to what you want it to output. However as I understand it, it won't find a good solution. It might find a least squares solution to the last layer, then you'd want previous layer to output something that reduces error of the last layer, but bidi-calc will no longer consider last layer at all.

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