Stops are one good approach, echo statements another good approach, temporary =: another, and an identity function which updates a variable is another, and the dissect and trace facilities are also good approaches.
Basically we’re talking about choice of tools, and our understanding of the problems we’re trying to find. Sometimes it’s best to stick with just one approach, but they all have some value. — Raul On Wednesday, April 18, 2018, Devon McCormick <[email protected]> wrote: > On "=." in scripts: I never thought of using =: because it's easy to set a > stop in a script if I want to examine those local values and this also > allows access to everything else in the function context without changing > any code. For example: > > 13!:3 'J2StdCvtNums 0 4 6 9 10 11 12 17 21 22 24 26 28' *NB. Set stop > lines* > 13!:0]1 *NB. Halt on interrupt* > 'S' J2StdCvtNums _3.14 6.02e_23 *NB. Convert J numbers to std rep* > -3.1400000000000001 0.0000000000000000 *NB. Didn't stop - why?* > 13!:3 'J2StdCvtNums *:* 0 4 6 9 10 11 12 17 21 22 24 26 28' *NB. Dyadic > stops* > 'S' J2StdCvtNums _3.14 6.02e_23 > |stop: J2StdCvtNums > | 'S'=x > |J2StdCvtNums[:0] > 13!:4'' *NB. Resume execution from current line.* > |stop > | ' '={.,y > |J2StdCvtNums[:0] NB. Same line because multiple statements. > 13!:4'' > |stop > | toStd=.-.'J'-:''$'2'-.~,x > |J2StdCvtNums[:4] > 13!:0]0 *NB. Exit debug* > 13!:0]1 NB. Halt on interrupt > > > On Wed, Apr 18, 2018 at 6:21 PM, Henry Rich <[email protected]> wrote: > > > On =: in scripts: When I replace =. with =: for debugging purposes, I > > always precede it with 2 spaces: > > > > name =: value > > > > Then after testing a global replace of ' =:' with '=.' removes the > global > > assignments. > > > > Henry Rich > > > > > > On 4/18/2018 6:16 PM, Marshall Lochbaum wrote: > > > >> This is a perfectly good use of the forums in my book. At the very > >> least, we've had similar posts before and no one's complained. > >> > >> Overall your program looks pretty good to me. It's not leveraging the > >> Full Power of J™, but it's a great start. You know what I. does! > >> > >> There are some ways to use arrays to do more of your work for you. You > >> write > >> > >> 'minCut maxCut' =. x > >> ... > >> minWanted =: (<.minCut*#srtData){srtData > >> maxWanted =: (<.maxCut*#srtData){srtData > >> > >> but this is the same as > >> > >> 'minWanted maxWanted' =: (<.x*#srtData){srtData > >> > >> since each of * <. and { has rank zero, that is, maps over arrays, on > >> the side of x. (Unrelatedly, these should probably use =. rather than =: > >> to avoid global assignment. It looks like they might have been =: for > >> debugging, but if so, it's good practice to look over the code again and > >> change them back when you're done.) > >> > >> A more sophisticated example is the plane-scaling code in displayFits. > >> You use different code for each different number of scalings to do: > >> > >> r =. minmax scaleT2D 0{imarray > >> select. 3<.{.$imarray NB. use up to 3 planes > >> case. 3 do. > >> g =. minmax scaleT2D 1{imarray > >> b =. minmax scaleT2D 2{imarray > >> case. 2 do. > >> b =. minmax scaleT2D 1{imarray > >> g =. -: r+b NB. average of red and blue > >> case. 1 do. > >> 'g b'=. r;r > >> end. > >> > >> Instead, you could do all the scalings in one go: > >> ((#imarray) is like ({.$imarray), but it is a scalar and not a vector.) > >> > >> numScales =. 3<.#imarray NB. use up to 3 planes > >> scaled =. minmax scaleT2D"_ _1 numScales {. imarray > >> if. numScales < 3 do. > >> avg =. (+/ % #) scaled > >> scaled =. scaled , (3-numScales)#,:avg > >> end. > >> 'r g b' =. scaled > >> > >> This example first uses scaleT2D with an explicit rank to operate on > >> each of the first numScales planes. If you're not familiar with it, my > >> favorite explanation of the rank conjunction (") is > >> http://www.jsoftware.com/help/jforc/loopless_code_i_verbs_have_r.htm. > >> A rank of (_ _1), that is, infinity on the left and minus one on the > >> right, means to apply the function to the entire left argument and each > >> subarray of the right argument, effectively mapping over only the right > >> argument. I could have used a rank of 2 rather than _1 on the right, but > >> rank _1 is more flexible--it would work even if the planes were instead > >> three-dimensional. > >> > >> After this compuation, scaled is a three-dimensional array of the first > >> numScales planes of imarray. The average of its planes can be found with > >> the standard averaging function (+/ % #), which happens to be the > >> canonical example of a fork. If you're interested, these are described > >> at http://code.jsoftware.com/wiki/Vocabulary/fork. > >> > >> An exponent like (2^8) unfortunately always evaluates to a > >> floating-point number. I habitually use (<.@^) when I know the arguments > >> are integers to avoid accidentally promoting to floats. > >> > >> > >> I actually gave a talk on a task similar to the parsing you do here. My > >> task was to parse the header of a simple wave file. A script version > >> (there's no recording that I'm aware of) is at > >> http://code.jsoftware.com/wiki/Community/Conference2014/Talk > >> s/UsingDataAsCode > >> and the current script that I use based on this is > >> https://github.com/mlochbaum/JSound/blob/master/wav.ijs. > >> This is certainly difficult material for a beginning J programmer, but > >> the techniques discussed in the talk might be useful for you as you gain > >> more skill. > >> > >> One neat trick similar to the one used there: rather than write > >> > >> bitpix =. hdata getHdrVal 'BITPIX' > >> naxis =. hdata getHdrVal 'NAXIS' > >> naxis1 =. hdata getHdrVal 'NAXIS1' > >> naxis2 =. hdata getHdrVal 'NAXIS2' > >> naxis3 =. hdata getHdrVal 'NAXIS3' > >> > >> you might use > >> > >> fields =. ;: 'BITPIX NAXIS NAXIS1 NAXIS2 NAXIS3' > >> (fields) =. hdata&getHdrVal&.> fields > >> > >> and then the fields will be defined in variables BITPIX, etc. > >> > >> Another very cool trick that I found after the talk would be useful > >> here: > >> > >> select. bitpix > >> case. _32 do. adata =. (naxis3,naxis2,naxis1)$ |. _1 fc |. > >> rdata > >> case. _64 do. adata =. (naxis3,naxis2,naxis1)$ |. _2 fc |. > >> rdata > >> case. 16 do. adata =. (naxis3,naxis2,naxis1)$ |. _1 ic |. > >> rdata > >> case. 32 do. adata =. (naxis3,naxis2,naxis1)$ |. _2 ic |. > >> rdata > >> end. > >> > >> The idea is to make an adverb (mine is audioconvert in wav.ijs) which > >> takes a format specification like bitpix and returns a conversion > >> function. Something like: > >> > >> rawconvert =: 1 : 0 > >> select. u > >> case. _32 do. _1&fc > >> case. _64 do. _2&fc > >> case. 16 do. _1&ic > >> case. 32 do. _2&ic > >> case. do. 'Invalid format' assert 0 > >> end. > >> ) > >> > >> which can then be called with > >> > >> cvt =. bitpix rawconvert NB. A conversion function > >> adata =. (naxis3,naxis2,naxis1)$ |. cvt |. rdata > >> > >> Now the really cool thing is that J knows how to invert each of these > >> functions. So when you want to store the data again, you just use > >> (cvt^:_1)! This is a J-only technique: I don't know of any other > >> language that has the functional capabilities and the automatic > >> inversion required to do it. > >> > >> There's always more to say but I think that's enough for one email. Good > >> luck with your future J projects! > >> > >> Marshall > >> > >> On Wed, Apr 18, 2018 at 02:47:02PM -0400, Thomas Hickey wrote: > >> > >>> As one of my first J programs, I've written a minimal viewer for FITS > >>> files, the standard way of sharing astronomical data, especially > images. > >>> > >>> I'd appreciate it if someone could take a look at it and let me know > how > >>> the code (80 lines of J) looks. I know I've replaced some really > awkward > >>> code in it, so I imagine there are better ways of doing lots of things. > >>> > >>> The code is on GitHub at https://github.com/ThomasBHickey/JFits, > along > >>> with a sample FITS file in the fitsSample folder. Pointers on > >>> organizing J > >>> code on GitHub would be appreciated as well. > >>> > >>> Or is there a better way to accomplish this than the programming forum? > >>> I > >>> know this is longer than most of the questions that are posted here, > but > >>> I > >>> think it is an interesting application of J. > >>> > >>> --Th > >>> ---------------------------------------------------------------------- > >>> For information about J forums see http://www.jsoftware.com/forums.htm > >>> > >> ---------------------------------------------------------------------- > >> For information about J forums see http://www.jsoftware.com/forums.htm > >> > > > > > > --- > > This email has been checked for viruses by AVG. > > http://www.avg.com > > > > > > ---------------------------------------------------------------------- > > For information about J forums see http://www.jsoftware.com/forums.htm > > > > > > -- > > Devon McCormick, CFA > > Quantitative Consultant > ---------------------------------------------------------------------- > For information about J forums see http://www.jsoftware.com/forums.htm ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
