Styjun

So, I have to do these two functions, one that saves a binary matrix on a .bin file and one that reads that same file and returns the numpy.array.

My problem is that when I try to .vstack both line and final image (I basically want to save a Black and White image) I get this message error:

'ValueError: all the input array dimensions except for the concatenation axis must match exactly'

which makes sense because after the I read the second line binaryLine and final image have different length, for some reason I can't understand.

def save_binary_matrix(img, fileName):
 file = open(fileName, "w+")
 heigth, width = img.shape
 image = convert_BW_to_0_1(img) # converts from 0 and 255 to 0 and 1
 for y in range(heigth):
 for x in range(0, width, 8):
 bits = image[y][x:x+8]# gets every 8 bits
 s = ''
 # converts bits to a string
 for i in range(len(bits)):
 s = s + str(bits[i])
 file.write(str(int(s,2)))# saves the string as a integer
 file.write("n")# line change
 file.close()

def read_binary_matrix(fileName):
 file = open(fileName, "r")
 #saves first line of the file
 finalImage = np.array([])
 line = file.readline()
 for l in range(len(line)):
 if line[l] != 'n':
 finalImage = np.append(finalImage, np.array([int(x) for x in list('0:08b'.format(int(line[l])))]))
 #reads and saves other lines
 for line in file:
 binaryLine = np.array([])
 for l in range(len(line)):
 if line[l] != 'n':
 #read and saves line as binary value
 binaryLine = np.append(binaryLine, np.array([int(x) for x in list('0:08b'.format(int(line[l])))]))
 finalImage = np.vstack((finalImage, binaryLine))
 return finalImage

Twice you create a np.array([]). Pay attention to its shape:

In [140]: x = np.array([]) 
In [141]: x.shape 
Out[141]: (0,)

It does work in np.append - that's because without axis parameter, append is just concatenate((x, y), axis=0), e.g. adding a (0,) shape and a (3,) shape to create a (3,) shape:

In [142]: np.append(x, np.arange(3)) 
Out[142]: array([0., 1., 2.])

But vstack doesn't work. It makes its inputs into 2d arrays, and joins them on the first axis:

In [143]: np.vstack((x, np.arange(3))) 
ValueError: all the input array dimensions except for the concatenation axis must match exactly

So we are joining (0,) and (3,) on a new first axis, e.g. (1,0) and (1,3) on first axis. The 0 and 3 don't match, hence the error.

vstack works when joining a (3,) with a (3,) and a (1,3), and a (4,3). Note the common 'last' dimension.

The underlying problem is that you are trying to emulate the list append, without fully understanding dimensions, or what concatenate does. It makes a whole new array each time. np.append is not a clone of list.append!.

What you should be doing is starting with a [] list (or two), append new values to that, making a list of lists. Then np.array(alist) to turn that into an array (provided, of course that all sublists match in size).

I haven't paid attention to your write, or how you read the lines, so can't say whether that makes sense or not.