Python: "IndexError: invalid index into a 0-size array"

Question

Obviously, there are loads of threads concerning index errors. But I couldn't find one that helped me out.

I use numpy.loadtxt to read in a function f(a,b).

a, b, f = np.loadtxt(filename, delimiter=' ', usecols=(0,1,2), unpack=True)

To get a 2d plot I found a solution in another thread, where you raster the (a,b)-plane and associate an integer tuple (ida, idb) with each point.

ncols, nrows = np.round((a.max() - a.min()) / da).astype(np.int) , np.round((b.max() - b.min()) / db).astype(np.int)
ida = np.round((a - a.min()) / da - 1).astype(np.int)
idb = np.round((b - b.min()) / db - 1).astype(np.int)

Then put f into the a grid.

grid = np.empty((nrows, ncols), dtype=np.float)
grid[idb, ida] = f

This worked out well for several data files. But now, suddenly it doesn't. Instead it reports the error in the title when reaching the grid[ida,idb]-line. So I looked for a mistake in the data file, but I couldn't find any. I tried

print ida.shape
print idb.shape
print f.shape

which returns

(3107,)
(3107,)
(3107,)

so all arrays match well in size and especially none of them is an empty array. What am I missing here?

Thanks, Alice

Thanks to seberg's advice, I found the mistake: In loadtxt I imported the wrong columns from the file. One happened to be constant. So I got nrows=0. And grid was the 0-size-array. — Alice Schwarze
– Alice Schwarze, Commented Dec 7, 2013 at 13:54
I can't close the thread thoguh without an answer it seems. And I can't write an answer myself until 8h have passed. — Alice Schwarze
– Alice Schwarze, Commented Dec 7, 2013 at 13:55

alice · Accepted Answer · 2013-12-09 12:49:35Z

1

Thanks to seberg's advice, I found the mistake (and I also found old stackoverflow account):

a, b, f = np.loadtxt(filename, delimiter=' ', usecols=(0,1,2), unpack=True)

I imported the wrong columns from the file. One happened to be constant. So I got nrows=0. And grid was the 0-size-array.

Sorry...

answered Dec 9, 2013 at 12:49

alice

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