Swapping columns in a numpy array?

by

There are two issues here. The first is that the data you pass to your function apparently isn’t a two-dimensional NumPy array — at least this is what the error message says.

The second issue is that the code does not do what you expect:

my_array = numpy.arange(9).reshape(3, 3)
# array([[0, 1, 2],
#        [3, 4, 5],
#        [6, 7, 8]])
temp = my_array[:, 0]
my_array[:, 0] = my_array[:, 1]
my_array[:, 1] = temp
# array([[1, 1, 2],
#        [4, 4, 5],
#        [7, 7, 8]])

The problem is that Numpy basic slicing does not create copies of the actual data, but rather a view to the same data. To make this work, you either have to copy explicitly

temp = numpy.copy(my_array[:, 0])
my_array[:, 0] = my_array[:, 1]
my_array[:, 1] = temp

or use advanced slicing

my_array[:, [0, 1]] = my_array[:, [1, 0]]

More Related Contents:

  1. Difference between numpy.array shape (R, 1) and (R,)
  2. Rolling window for 1D arrays in Numpy?
  3. How to pretty-print a numpy.array without scientific notation and with given precision?
  4. Understanding tensordot
  5. replace() method not working on Pandas DataFrame
  6. using an numpy array as indices of the 2nd dim of another array? [duplicate]
  7. How to evaluate the sum of values within array blocks
  8. Adding a background image to a plot
  9. numpy.unique with order preserved
  10. Numpy integer nan [duplicate]
  11. Get the position of the largest value in a multi-dimensional NumPy array
  12. Take multiple lists into dataframe
  13. Why does indexing numpy arrays with brackets and commas differ in behavior?
  14. Populate a Pandas SparseDataFrame from a SciPy Sparse Matrix
  15. Extract upper or lower triangular part of a numpy matrix
  16. Sum array by number in numpy
  17. Convert numpy.datetime64 to string object in python
  18. fitting exponential decay with no initial guessing
  19. How to plot line (polygonal chain) with numpy/scipy/matplotlib with minimal smoothing
  20. Matplotlib: how to make imshow read x,y coordinates from other numpy arrays?
  21. How to make numpy.argmax return all occurrences of the maximum?
  22. Why using an array as an index changes the shape of a multidimensional ndarray?
  23. TypeError: only length-1 arrays can be converted to Python scalars while plot showing
  24. Numpy isnan() fails on an array of floats (from pandas dataframe apply)
  25. read matlab v7.3 file into python list of numpy arrays via h5py
  26. How to access a column in a list of lists in python
  27. Is there a standard solution for Gauss elimination in Python?
  28. How to find linearly independent rows from a matrix
  29. Segmentation Fault using Python Shared Memory
  30. Convert Pandas dataframe to Sparse Numpy Matrix directly

Leave a Comment