Saving with numpy savetxt. Array elements as columns - numpy

Question

I am pretty new to Python and trying to kick my Matlab addiction. I am converting a lot of my lab's machine vision code over to Python but I am just stuck on one aspect of the saving. At each line of the code we save 6 variables in an array. I'd like these to be entered in as one of 6 columns in a txt file with bumpy.savetxt. Each iteration of the tracking loop would then add similar variables for that given frame as the next row in the txt file.
But I keep getting either a single column that just grows with every loop. I've attached a simple code to show my problem. As it loops through, there will be a variable generated that is called output. I would like this to be the three columns of the txt file and each iteration of the loop to be a new row. Is there any easy way to do this?
import numpy as np
dataFile_Path = "dataFile.txt"
dataFile_id = open(dataFile_Path, 'w+')
for x in range(0, 9):
variable = np.array([2,3,4])
output = x*variable+1
output.astype(float)
print(output)
np.savetxt(dataFile_id, output, fmt="%d")
dataFile_id.close()

Answer 1

In [160]: for x in range(0, 9):
...: variable = np.array([2,3,4])
...: output = x*variable+1
...: output.astype(float)
...: print(output)
...:
[1 1 1]
[3 4 5]
[5 7 9]
[ 7 10 13]
[ 9 13 17]
[11 16 21]
[13 19 25]
[15 22 29]
[17 25 33]
So you are writing one row at a time. savetxt normally is used to write a 2d array.
Notice that the print is still integers - astype returns a new array, it does not change things inplace.
But because you are giving it 1d arrays it writes those as columns:
In [177]: f = open('txt','bw+')
In [178]: for x in range(0, 9):
...: variable = np.array([2,3,4])
...: output = x*variable+1
...: np.savetxt(f, output, fmt='%d')
...:
In [179]: f.close()
In [180]: cat txt
1
1
1
3
4
5
5
7
9
if instead I give savetxt a 2d array ((1,3) shape), it writes
In [181]: f = open('txt','bw+')
In [182]: for x in range(0, 9):
...: variable = np.array([2,3,4])
...: output = x*variable+1
...: np.savetxt(f, [output], fmt='%d')
...:
...:
In [183]: f.close()
In [184]: cat txt
1 1 1
3 4 5
5 7 9
7 10 13
9 13 17
11 16 21
13 19 25
15 22 29
17 25 33
But a better approach is to construct the 2d array, and write that with one savetxt call:
In [185]: output = np.array([2,3,4])*np.arange(9)[:,None]+1
In [186]: output
Out[186]:
array([[ 1, 1, 1],
[ 3, 4, 5],
[ 5, 7, 9],
[ 7, 10, 13],
[ 9, 13, 17],
[11, 16, 21],
[13, 19, 25],
[15, 22, 29],
[17, 25, 33]])
In [187]: np.savetxt('txt', output, fmt='%10d')
In [188]: cat txt
1 1 1
3 4 5
5 7 9
7 10 13
9 13 17
11 16 21
13 19 25
15 22 29
17 25 33