Translating a string into a number (Homework Help)

[mcduff]

This seems made for Maps. A lot less elegant and performant than Altenbach's solution, but will easily allow you to change the key to different letters, symbols, or even non-printing characters if needed.

[wiebe@CARYA]

Maps are certainly faster than a linear search (like log(O) vs (O), although IIRC theoretically log(log(O)) is possible).

A sorted array is also a valid option. Still log(O), but searching a sorted array is faster than searching a map (as a map search has to traverse the pointers of the binary tree, a sorted array doesn't). Inserting into a sorted array is slower, but there's none of that in this exercise.

But a (simple array index) LUT is much faster than both (something like (1) vs log(O)).

As the character is treated (for now) as a U8, a LUT would still be clear and the fastest.

I'd certainly make sure the code lives in something contained, like a vilib or lvclass  By hiding the implementation and making low level functions private, you make sure you can change (and test) the implementation without problems.

[altenbach]

For a map with 10 elements, the difference between a plain search O(N) and a map O(logN) does not justify the map overhead. That only starts to matter for larger collections. A LUT is O(1) and the max size here is tiny (256 elements) that can be constant folded.

[Paddle_to_the_sea21]

Awesome thank you! So I ended up trying a different approach to this (attached). Now my problem is I can't seem to get the right output but it may be because of my case structure. As well, for the message I switched it so that it isnt a,b,c etc but can be other characters. I have attached a screenshot as well as the VI that I am currently working on for this problem. 

[mcduff]

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@altenbach wrote:

For a map with 10 elements, the difference between a plain search O(N) and a map O(logN) does not justify the map overhead. That only starts to matter for larger collections. A LUT is O(1) and the max size here is tiny (256 elements) that can be constant folded.

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I get it that a LUT is faster, but don't completely understand "does not justify the map overhead". Are you referring to setting up the map, populating the map, reading elements from map... 

[altenbach]

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@mcduff wrote:

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@altenbach wrote:

For a map with 10 elements, the difference between a plain search O(N) and a map O(logN) does not justify the map overhead. That only starts to matter for larger collections. A LUT is O(1) and the max size here is tiny (256 elements) that can be constant folded.

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I get it that a LUT is faster, but don't completely understand "does not justify the map overhead". Are you referring to setting up the map, populating the map, reading elements from map... 

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A map is an advanced data structure and while only about log2(N) comparisons need to be made for a lookup, it is a more expensive operation per comparison. (In this case, the map is fixed and insertion and deletion operations are irrelevant).

(For more details, have a look at my 2019 NI-Week presentation)

Big O complexity Says nothing about speed, just about the limiting behavior as a function of N.

[altenbach]

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@Paddle_to_the_sea21 wrote:

Awesome thank you! So I ended up trying a different approach to this (attached). Now my problem is I can't seem to get the right output but it may be because of my case structure. As well, for the message I switched it so that it isnt a,b,c etc but can be other characters. I have attached a screenshot as well as the VI that I am currently working on for this problem. 

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If you want more people to look at your VI, do a "save for previous" (2020 or below) before attaching. I currently cannot open it.

Form the picture, there is a lot of "code smell".

• Apparently the problem has changed and now you have two characters per symbol.
• The result must be an integer, not a floating point number. There should be nothing orange!
• No matter what numeric output data type you pick, you'll quickly run out of bits for longer inputs. You cannot have a scalar numeric output for a general solution

[Paddle_to_the_sea21]

I have saved it for the 2016 version hopefully this works!

[NIquist]

You are on the right track.  One good debug trick is to turn on the little light bulb so you can watch your code run on the block diagram.  

You can also right-click a wire and use a probe.  Try probing the string output of you loop before the String to Number VI to see why it can't decode a number.  (you should also be using the DECIMAL string function, not the Fractional 😉).

Hint for your problem:  Using two quotes that look like an empty string is NOT an empty string.  Erase them or use the empty string constant in the string palette.

[NIquist]

Also don't forget to verify your attempt to ignore undefined characters by concatenating a -1 into your string.  How is the string to number conversion going to like a dash in the string???  And what happens when there are many undefined characters?  Your loop iterations are not taking them into account are they?  Try some long strings with a mix of good and bad inputs.  Use probes and the light bulb to see what's happening in your code...