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Modulus calculation to find decreasing Year-Quarter

  1. Nov 14, 2012 #1

    jjc

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    I am feeling silly because I can't seem to crack this, but maybe I have been starring at it too long. I am trying to find a math function that will yield a single unit decrement of the currently defined Year/Quarter. This is a software UI thing: I am trying to create a script to decrement, in single steps, the current Year/Quarter shown. The issue is that it needs to wrap around back to the start. Hence I was trying to use a Modulus operator.

    Here's what I need, decrementing the current quarter:
    Q1 -> Q4 (of the prior year)
    Q2 -> Q1
    Q3 -> Q2
    Q4 -> Q3

    I can get three of the four conditions via mod(), but I can't seem to work out one calculation that works for all. Adding another number somewhere is fine; I am really just trying to do this without having to also include an IF() statement to look for the Q1->Q4 condition.

    It doesn't have to deal with the "Q" string part; I already am splitting the string up and just handling the numeric portion separately. I then rebuild the string. Same with the year; that is handled in a separate step as well. I just need to calculate the quarter number, and I am trying to save 1 line of code. :)

    And if anyone cares, this is being done in FileMaker 12 scripting language. Has most of the modern conveniences, but not quite as flexible as most real full-blown languages. This is what I have currently (yields 3 correct results; slightly modified from actual code to make it more readable):

    ...
    res = Mod ( currQnum - 1 ; 4 );
    return res;

    Here is my working version for incrementing (modified again; Year-wrap around IS done with an IF statement, but not the Qrtr number):


    ...
    newQ = Mod ( currQNum ; 4 ) + 1 ;
    ...
    return newYr & " Q" & newQ

    Thanks
    Justin
     
  2. jcsd
  3. Nov 14, 2012 #2

    haruspex

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    The usual trick is to add the base first to make sure it doesn't go negative: Mod(val+4-1; 4)
     
  4. Nov 15, 2012 #3

    jjc

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    It seems like it wouldn't quite work in this situation; I could still end up with a zero result of the mod(); I want a result from 1 through 4. Val = 1 ; Mod (1 + 4 - 1 ; 4 ) = Mod (4) = 0. And if I add 4 to that, it works for that one, but not the others. I have thought of doing a subtraction ( 4 - Mod(...) ) but that just inverts the order of results, yielding f(3) = 1. But it does work for f(1).
     
  5. Nov 15, 2012 #4

    haruspex

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    OK, you didn't say the qtr numbers are in the range 1-4. I assumed 0-3. That's easy: subtract 1 at the start and add it back at the end. So to step back n quarters:
    1+Mod(4*n-1+x-n; 4)
     
  6. Nov 15, 2012 #5

    jjc

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    Yeah, I didn't explicitly mention it but my sample data showed it.

    Thanks for this function. I will assume that 'x' in your formula is the starting quarter number. n = 1 all the time, in my case.

    So that essentially reduces the formula to:
    1+Mod(x+2 ; 4 )

    When I work through it in my head it works! Now to test it... :)

    Thanks!
     
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