How to write a string to numpy array?

Question

I have two attributes age (float) and name (string) that belongs to a person. I want to write them to csv file, hence, I used numpy to store data, then write it to csv.

import numpy as np
import random
age_name = np.empty((2, 10))
print (age_name.shape)
for i in range (10):
    age = random.randint(0,100)
    name =  'ABC'
    age_name[0,i]=age
    age_name[1,i]=name
print (age_name)

I got the error

Traceback (most recent call last): File "python", line 9, in ValueError: could not convert string to float: 'ABC'

It may be a not a good option because the data has both string and float number, could you suggest to me a good way that can easy to store to csv file?

you can pass dtype = object to np.empty or use a custom data-type if efficiency matters. — hilberts_drinking_problem
– hilberts_drinking_problem, Commented Dec 6, 2018 at 20:40
age_name = np.empty((2, 10), dtype=str) and print (age_name) will be [['8' '2' '6' '4' '9' '9' '7' '6' '2' '4'] ['A' 'A' 'A' 'A' 'A' 'A' 'A' 'A' 'A' 'A']] — John
– John, Commented Dec 6, 2018 at 20:46
@trailing_whitespace It worked. Thanks. But the solution cannot write to csv file. Let try it np.savetxt("age_name.csv", age_name, delimiter=",") — John
– John, Commented Dec 6, 2018 at 21:19

user10597469 · Accepted Answer · 2018-12-06 20:51:23Z

5

Older versions of numpy had chararrays and numarrays, but now you just pass a dtype to tell numpy that the array needs to contain more than just numbers. Using dtype=str will only take one string per element, so you will get "a" "a" "a"..., instead of the whole string "abc". To get that, you pass object as the dtype.

import numpy as np
import random
age_name = np.zeros((2, 10), dtype=object)
print (age_name)
for i in range (10):
    age = random.randint(0,100)
    name =  'ABC'
    age_name[0,i]=age
    age_name[1,i]=name
print (age_name)

>>>[[0 0 0 0 0 0 0 0 0 0]
>>> [0 0 0 0 0 0 0 0 0 0]]
>>>[[24 67 72 59 44 4 71 16 17 82]
>>> ['ABC' 'ABC' 'ABC' 'ABC' 'ABC' 'ABC' 'ABC' 'ABC' 'ABC' 'ABC']]

answered Dec 6, 2018 at 20:51

user10597469

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2 Comments

John Over a year ago

It worked. Thanks. But the solution cannot write to csv file. Let try it np.savetxt("age_name.csv", age_name, delimiter=",")

user10597469 Over a year ago

@John You have to include the fact that it is a string in the format to use numpy.savetxt. Try: np.savetxt("age_name.csv", age_name, delimiter=",", fmt="%s")

mirekphd · Accepted Answer · 2023-02-27 09:55:50Z

0

For heterogenous data (with differently typed columns, some of which contain str/object data) use pandas.DataFrames.

For data of "mixed types" where (legitimate) strings can be found within each/some column(s) interspersed with numeric values, use python dicts.

Only for uniform data (typically numeric) use numpy.ndarrays.

answered Feb 27, 2023 at 9:55

mirekphd

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