For Loop clarification

Question

I have the following dataset:

plgs = {{{1522., 388.5}, {1543.18, 414.971}, {1522., 406.5}}, {{1508.5, 
421.5}, {1508.5, 400.528}, {1515.69, 404.923}, {1511.83, 
416.5}}, {{565.455, 408.545}, {576.778, 397.222}, {594., 
426.5}, {614.375, 467.25}}, {{559.605, 386.053}, {548.065, 
416.826}, {213.929, 667.429}, {538.434, 363.636}}, {{1503.8, 
392.6}, {1502.82, 395.128}, {1369.24, 389.057}, {1491.38, 
347.071}}, {{1560.31, 482.308}, {1603.53, 445.263}, {1621.91, 
433.98}, {1715.11, 637.109}}, {{1097.69, 735.5}, {1091.99, 
283.009}, {1145.27, 324.965}, {1284.22, 735.5}}, {{246.529, 
735.5}, {648.257, 525.978}, {814.646, 461.982}, {821.031, 
735.5}}, {{821.031, 735.5}, {814.646, 461.982}, {889.082, 
353.262}, {928.152, 300.524}, {971.65, 735.5}}, {{1502.82, 
395.128}, {1503.35, 396.805}, {1434.43, 560.485}, {1281.53, 
410.475}, {1369.24, 389.057}}, {{1503.35, 396.805}, {1508.5, 
400.528}, {1508.5, 421.5}, {1437.37, 563.76}, {1434.43, 
560.485}}, {{1543.18, 414.971}, {1546., 416.5}, {1511.83, 
416.5}, {1515.69, 404.923}, {1522., 406.5}}, {{577.11, 
395.394}, {591., 393.5}, {594., 393.5}, {594., 426.5}, {576.778, 
397.222}}, {{576.682, 394.591}, {559.605, 386.053}, {538.434, 
363.636}, {544.174, 351.773}, {587.882, 338.591}}, {{577.11, 
395.394}, {576.682, 394.591}, {587.882, 338.591}, {591., 
336.776}, {591., 393.5}}, {{1484.16, 735.5}, {1446.59, 
574.589}, {1560.31, 482.308}, {1715.11, 637.109}, {1762.07, 
735.5}}, {{758.94, 231.5}, {591., 336.776}, {587.882, 
338.591}, {544.174, 351.773}, {451.872, 231.5}}, {{1646., 
231.5}, {1520., 384.5}, {1503.8, 392.6}, {1491.38, 
347.071}, {1643.73, 231.5}}, {{1762.07, 735.5}, {1715.11, 
637.109}, {1621.91, 433.98}, {1745.26, 231.5}, {2001.25, 
231.5}, {2001.25, 735.5}}, {{594., 426.5}, {594., 393.5}, {889.082,
353.262}, {814.646, 461.982}, {648.257, 525.978}, {614.375, 
467.25}}, {{971.65, 735.5}, {928.152, 300.524}, {974.766, 
231.5}, {1058.03, 231.5}, {1091.99, 283.009}, {1097.69, 
735.5}}, {{576.682, 394.591}, {577.11, 395.394}, {576.778, 
397.222}, {565.455, 408.545}, {548.065, 416.826}, {559.605, 
386.053}}, {{451.872, 231.5}, {544.174, 351.773}, {538.434, 
363.636}, {213.929, 667.429}, {208.75, 671.951}, {208.75, 
231.5}}, {{1284.22, 735.5}, {1145.27, 324.965}, {1281.53, 
410.475}, {1434.43, 560.485}, {1437.37, 563.76}, {1446.59, 
574.589}, {1484.16, 735.5}}, {{974.766, 231.5}, {928.152, 
300.524}, {889.082, 353.262}, {594., 393.5}, {591., 393.5}, {591., 
336.776}, {758.94, 231.5}}, {{1603.53, 445.263}, {1560.31, 
482.308}, {1446.59, 574.589}, {1437.37, 563.76}, {1508.5, 
421.5}, {1511.83, 416.5}, {1546., 416.5}}, {{1643.73, 
231.5}, {1491.38, 347.071}, {1369.24, 389.057}, {1281.53, 
410.475}, {1145.27, 324.965}, {1091.99, 283.009}, {1058.03, 
231.5}}, {{208.75, 735.5}, {208.75, 671.951}, {213.929, 
667.429}, {548.065, 416.826}, {565.455, 408.545}, {614.375, 
467.25}, {648.257, 525.978}, {246.529, 735.5}}, {{1520., 
384.5}, {1522., 388.5}, {1522., 406.5}, {1515.69, 
404.923}, {1508.5, 400.528}, {1503.35, 396.805}, {1502.82, 
395.128}, {1503.8, 392.6}}, {{1745.26, 231.5}, {1621.91, 
433.98}, {1603.53, 445.263}, {1546., 416.5}, {1543.18, 
414.971}, {1522., 388.5}, {1520., 384.5}, {1646., 231.5}}}

Now, I want to make a for loop that can find the adjacent neighbor for last three polygon values. Here is the following code I tried for finding neighbor of the last three polygon. But, it gives error. How can I run the for loop?

For[i = 1, i < sv + 1, i++, 
  iLength = Length[plgs[[-i]]]
  neigh[[i]] = 
    Select[
      plgs[[;; -i - 1]], 
      Length[Union[Join[#, plgs[[-i]]]]] =!= Length[#] + iLength &]]
  aa = neigh[[1]]

You have some basic syntax issues. Read this post What are the most common pitfalls awaiting new users?. In particular the body of he For is taking the results of each line and multiplying them together. You need to use the terminating semicolon. — Edmund, Feb 01 '16 at 03:01
I can not comprehend your question. What do you mean by "adjacent neighbor" and "last three polygons"? Also, why are using a for-loop at all, when only want to compute neigh[[1]]? — m_goldberg, Feb 01 '16 at 03:04
Please fix the basic syntax issues that Edmund describes, then see if you have further problems and update the question if necessary. — Mr.Wizard, Feb 01 '16 at 06:27

score 1 · Answer 1 · edited Apr 13 '17 at 12:55

You have several post leading up to this one and it appears that you have been working up to this for a couple of days. Please read What are the most common pitfalls awaiting new users?. This will really help you out as Wolfram Language is different most traditional programming languages.

You don't need a loop to code this the Wolfram Language way.

Taking adjacent to mean that a polygon shares a vertex with the central polygon then an example of how to code this and display this is.

Graphics[{White, EdgeForm[Black], Polygon@plgs,
  LightGray, Polygon@ 
   Flatten[Through[(Select /@ Thread@ContainsAny[plgs[[-3 ;;]]])[plgs[[;; -4]]]], 1],
  Blue, Polygon@plgs[[-3 ;;]]}]

Have a read of how each of the functions work in the documentation. Map (\@) and Span (;;) are particularly useful. These and a few others pretty much negate the need for loops in most cases.

Hope this helps.

For Loop clarification

1 Answers1