Pandas DataFrame to Numpy Array ValueError

Question

I am trying to convert a single column of a dataframe to a numpy array. Converting the entire dataframe has no issues.

df

  viz  a1_count  a1_mean     a1_std
0   0         3        2   0.816497
1   1         0      NaN        NaN 
2   0         2       51  50.000000

Both of these functions work fine:

X = df.as_matrix()
X = df.as_matrix(columns=df.columns[1:])

However, when I try:

y = df.as_matrix(columns=df.columns[0])

I get:

TypeError: Index(...) must be called with a collection of some kind, 'viz' was passed

EdChum · Accepted Answer · 2015-08-03 15:45:26Z

3

The problem here is that you're passing just a single element which in this case is just the string title of that column, if you convert this to a list with a single element then it works:

In [97]:
y = df.as_matrix(columns=[df.columns[0]])
y

Out[97]:
array([[0],
       [1],
       [0]], dtype=int64)

Here is what you're passing:

In [101]:
df.columns[0]

Out[101]:
'viz'

So it's equivalent to this:

y = df.as_matrix(columns='viz')

which results in the same error

The docs show the expected params:

DataFrame.as_matrix(columns=None) Convert the frame to its Numpy-array representation.

Parameters: columns: list, optional, default:None If None, return all columns, otherwise, returns specified columns

answered Aug 3, 2015 at 15:45

EdChum

397k204 gold badges836 silver badges583 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Adam_G Over a year ago

Thanks for the explanation. Why did it work for columns=df.columns[1:] without encapsulating that in a list?

EdChum Over a year ago

Because that returned an array but when it's a single element selection it returned the single object only

DeepSpace Over a year ago

Because df.columns[1:] is already a list. Consider: li = [1,2,3,4] li[1:] -> [2,3,4]

DeepSpace · Accepted Answer · 2015-08-03 15:45:55Z

3

as_matrix expects a list for the columns keyword and df.columns[0] isn't a list. Try df.as_matrix(columns=[df.columns[0]]) instead.

answered Aug 3, 2015 at 15:45

DeepSpace

82.1k12 gold badges119 silver badges166 bronze badges

Comments

Reed Richards · Accepted Answer · 2017-11-22 16:45:28Z

-1

Using the index tolist function works as well

df.as_matrix(columns=df.columns[0].tolist())

When giving multiple columns, for example, the ten first, then the command

df.as_matrix(columns=[df.columns[0:10]])

does not work as it returns an index. However, using

df.as_matrix(columns=df.columns[0:10].tolist())

works well.

answered Nov 22, 2017 at 16:45

Reed Richards

4,3988 gold badges46 silver badges55 bronze badges

Collectives™ on Stack Overflow

Pandas DataFrame to Numpy Array ValueError

3 Answers 3

3 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

3 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related