Trying to write numpy array (generated through a loop) into csv format

Question

I have a code that generates a 1-D numpy array in each iteration. I want the arrays to get appended to the end of a CSV file so that I can read all data from Excel. I am currently trying the following method:

for loop in range(0,10):
    # The following part generates the array 
    Array1 = numpy.array([4.3])  
    Array2 = numpy.array([10.2])
    Array3 = numpy.concatenate((Array1,Array2),axis=0)

    # The following part tries to generate a CSV writable array. But it fails :S
    if (loop == 0):
        ArrayMain = Array3
    else:
        # Trying to append the new array with the previous array
        ArrayMain = numpy.asarray(ArrayMain,Array3)  

# Trying to write the array into a .txt. file in .csv format
numpy.savetxt("ArrayMain.csv", ArrayMain, delimiter=",",fmt='%.3f')

This code is giving errors. Any idea how I can rectify it?

Which errors is it giving you?

Colin Dunklau
– Colin Dunklau

2012-08-21 16:42:59 +00:00
Commented Aug 21, 2012 at 16:42 — Colin Dunklau
– Colin Dunklau, Commented Aug 21, 2012 at 16:42

behnam · Accepted Answer · 2012-08-21 16:57:23Z

2

I think you need to use append function to append new array with previous array, asarray function converts input to array.

    else:
        # Trying to append the new array with the previous array
        ArrayMain = numpy.append(ArrayMain,Array3)

edited Aug 21, 2012 at 16:57

behnam

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answered Aug 21, 2012 at 16:45

MostafaR

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1 Comment

MostafaR Over a year ago

Yes, error was on ArrayMain = numpy.asarray(ArrayMain,Array3) line.

Pierre GM · Accepted Answer · 2012-08-21 17:44:35Z

1

You should really keep in mind that ndarrays are not really designed to be appended like that.

If you write your CSV file at once outside of the loop, you should consider using a list of arrays instead.

base = []
for i in range(10):
    base.append(...)
np.savetxt("ArrayMain.csv", base, ...)

That'll work provided the arrays you append to base have all the same size (np.savext will transform the list of arrays into an array itself).

Alternatively, you could open the file you want to write first, then use its handle in np.savetxt to update it at each iteration. In that case, don't forget to close it at the end...

answered Aug 21, 2012 at 17:44

Pierre GM

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Comments

acid_crucifix · Accepted Answer · 2012-08-21 16:57:23Z

0

Mostafar is right. numpy.asarray does not append lists

numpy.asarray([1,2]) -> array([1,2])

i.e. what it actually does is that it converts a list to an array. You can use the append function as he has defined. Or my personal favourite

numpy.r_

Usage:

numpy.r_[array([1,2]), array([3,4])] -> array([1,2,3,4])

also:

numpy.r_[array([1,2]), 3, [4,5], 6, array([7,8])] -> array([1, 2, 3, 4, 5, 6, 7, 8])

as you can see it can be used to concatenate various lists, arrays and single elements at one go.

answered Aug 21, 2012 at 16:57

acid_crucifix

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