CODEWITHSUNDEEP
pythonastropy
usernumber
usernumber

Reputation: 2185

Astropy table add_row mismatch between number of vals and columns

I have an astropy table I'm reading from a file. For the sake of building a working example, let's make it

from astropy.table import Table, Column


t = Table(names=('a', 'b', 'c'))
t.add_row((1, 2.0, 4.2))
t.add_row((2, 5.0, 1.7))
t

I'm making another table with the same columns as t.

s = Table(names = t.colnames)

And I would like to add certain lines from t into s

row = t[t['a'] == 1]
s.add_row(row)

raises 

ValueError: Mismatch between number of vals and columns

Why am I getting this error, and how can I add row to s ?

Upvotes: 0

Views: 450

Answers (1)

Tom Aldcroft
Tom Aldcroft

Reputation: 2552

The easiest way to make a new table with the same columns as t is with

s = Table(t[0:0])

This is a slight trick that relies on making a zero-length version of t.

One problem with your code s = Table(names=t.colnames) is that without giving any information about the data types via the dtype parameter, this initializer gives all float64 columns.

Then to add certain rows to s you would do:

for row in t[t['a'] == 1]:  # Iterate over temporary table that is a subset of t  
    s.add_row(row)

In your original version, row is a Table not a Row and so using add_row() with a Table input does not work. The error message you got is definitely not helpful and I'll open an issue to make that more informative in this case.

It is important to note that in general making a table one row at a time will be slow, so if they tables are large then you should try to find another way. In this case you could do s = t[t['a'] == 1], or explore options using vstack (https://docs.astropy.org/en/stable/api/astropy.table.vstack.html).

Upvotes: 2

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