Valueerror: per-column arrays must each be 1-dimensional

Error: ValueError: per-column arrays must each be 1-dimensional

This error occurs when trying to pass a multidimensional array as input to a function or operation that expects one-dimensional arrays.

Explanation

In Python, a one-dimensional array is typically represented as a list or a NumPy array with a single dimension. It is structured as a sequence of elements accessed by indices. On the other hand, a multidimensional array is an array with more than one dimension, comprised of nested lists or arrays.

Many functions and operations in Python expect one-dimensional arrays as input. When you encounter this error, it means that you have provided an array with multiple dimensions where only one dimension is expected.

Examples

1. Example 1:

   Consider the following code snippet that tries to sum the elements of two two-dimensional arrays using NumPy’s sum() function:

   import numpy as np
   
   array1 = np.array([[1, 2], [3, 4]])
   array2 = np.array([[5, 6], [7, 8]])
   
   result = np.sum([array1, array2])
   print(result)
         

   In this example, we have two two-dimensional arrays, array1 and array2. We try to sum them by passing both arrays as an argument to np.sum(). However, since np.sum() expects one-dimensional arrays, it raises the ValueError: per-column arrays must each be 1-dimensional error.

   To fix this, we can either flatten the arrays before passing them to np.sum() or use the axis parameter to specify the axis along which the sum should be performed:

   result = np.sum([array1.flatten(), array2.flatten()])
   print(result)  # Output: 36
   # or
   result = np.sum([array1, array2], axis=0)
   print(result)  # Output: [[ 6  8] [10 12]]
         

2. Example 2:

   Another example is attempting to pass a two-dimensional array to functions that expect one-dimensional arrays, such as the .sort() method of a list:

   my_list = [[3, 1, 2], [6, 5, 4]]
   my_list.sort()
         

   In this case, my_list.sort() will raise the same ValueError because sort() expects a one-dimensional list to sort the elements. To fix this, we need to flatten the list before sorting:

   my_list = [[3, 1, 2], [6, 5, 4]]
   flattened_list = [item for sublist in my_list for item in sublist]
   flattened_list.sort()
   print(flattened_list)  # Output: [1, 2, 3, 4, 5, 6]