Question

‘lo all. I am a self-described admitted noob in iPhone programming (having a much longer perl & web background — 30 years)…but took the plunge last week and bought a couple of good books. After cramming and reading well over 1000 pages — and understanding it pretty well, I am well on my way to a good first Native iPhone app. My problem is this: I do not know how to do a simple Geographic (lat/long) point-in-polygon routine in Objective-C. I have 2 ways of doing this. One in C (the first code example) and one in JavaScript (the second code example):

// this is the poly.h file

int pnpoly(int nvert, float *vertx, float *verty, float testx, float testy);


// this is the poly.c file

#include "poly.h"
int pnpoly(int nvert, float *vertx, float *verty, float testx, float testy){
 int i, j, c = 0;
 for (i = 0, j = nvert-1; i < nvert; j = i++) {
 if ( ((verty[i]>testy) != (verty[j]>testy)) &&
  (testx < (vertx[j]-vertx[i]) * (testy-verty[i]) / (verty[j]-verty[i]) + vertx[i]) )
    c = !c;
 }
 return c;
}

or this (in Javascript):

function _isPointInPoly(poly, pt){
 for(var c = false, i = -1, l = poly.length, j = l - 1; ++i < l; j = i)
  ((poly[i].y <= pt.y && pt.y < poly[j].y) || (poly[j].y <= pt.y && pt.y < poly[i].y))
  && (pt.x < (poly[j].x - poly[i].x) * (pt.y - poly[i].y) / (poly[j].y - poly[i].y) + poly[i].x)
  && (c = !c);
 return c;
}

(either one will work if i could get them converted)

So, to try this out…I put the .h file and .c file into xcode with my iPhone project. The only question now is how to call this from Objective-C and get the result.. 🙂

BTW: I searched the Great God Google all last night to get the answer to this but just TRY to search for “including C in an Objective-C iPhone app”, etc.. you get so many entries and none have to do with this! 🙂 Just letting you know I tried google before posting here.

Okay, my issues:

1. How do I call the pnpoly from Objective-C?
2. What types do i call it using? (int is fine, but the float
   *vertx is obviously an array of floats..which NSArray does not have
   — that I can find)

(EDIT: HERE IS MORE INFO. I AM ASKING FOR HELP CONTRUCTING THE ARRAYS THAT WOULD BE PASSED AS WELL)

The question was not asked fully.

The routine (in objective-c) would be like this: (assuming this is coded right)

NSMutableArray *latitudeArray = [[NSMutableArray alloc] init];
NSMutableArray *longitudeArray = [[NSMutableArray alloc] init];

// coordinates surrounding 1 inifite loop.

[latitudeArray addObject:@"37.32812557141369"];
[longitudeArray addObject:@"-122.0320253896318"];
[latitudeArray addObject:@"37.32821852349916"];
[longitudeArray addObject:@"-122.0289014325174"];
[latitudeArray addObject:@"37.33021046381746"];
[longitudeArray addObject:@"-122.0289300638158"];
[latitudeArray addObject:@"37.33042111092124"];
[longitudeArray addObject:@"-122.0279574092159"];
[latitudeArray addObject:@"37.33395972491337"];
[longitudeArray addObject:@"-122.0279263955651"];
[latitudeArray addObject:@"37.33363270879559"];
[longitudeArray addObject:@"-122.0320527775551"];
[latitudeArray addObject:@"37.32812557141369"];
[longitudeArray addObject:@"-122.0320253896318"];


int nvert = [[latitudeArray count] intvalue];

// 37.33189399206268 x -122.0296274412866 should return true

float testx =37.33189399206268;
float testy =-122.0296274412866;

int y_or_n = pnpoly(int nvert, float *vertx, float *verty, float testx, float testy);

I should’ve made it clear that I am learning Objective-c but FOUND that C routine–so was not sure how to construct either the C variables to call it with or the routine to call it with.

I know this is asking a lot…but it is really puzzling to me. Can anyone help me?
Thanks so much.

Jann

Answer 1

Here’s en example of how it would be used:

int nCoords = 4;
float vertexXCoords[n] = {0.0, 0.0, 20.0, 20.0};
float vertexYCoords[n] = {0.0, 20.0, 20.0, 0.0};
NSPoint testPoint = NSMakePoint(5, 10);

BOOL testPointIsInPoly = pnpoly(nCoords, xCoords, yCoords, testPoint.x, testPoint.y);

Note that there’s nothing specific to Objective-C in here. This is C code (though it does use the Cocoa BOOL and NSPoint C types). Since Objective-C is a strict superset of C, any valid C code is also valid Objective-C. This is also a case in which Objective-C’s unique features would not be particularly useful. (Numerical calculations in general are less complex and more readable in plain C.)