Merging 2 dataframes

Question

I am trying to merge the following 2 dataframes.

df.head():

 x y z w 0 0.0056 11 824 51 1 0.0056 138 546 16 2 0.0056 328 1264 40 3 0.0056 1212 553 91 4 0.0056 1839 388 48

df1.head():

 x y z 0 5539.0567 12243 27 1 5873.2923 14474 1540 2 3975.9776 11353 699 3 1508.5975 8250 628 4 66.7913 11812 538

using the following command:

df1 = df1.merge(df, how='left',left_on=(['x','y','z']),right_index=True)

and the following error crops up:

ValueError: len(left_on) must equal the number of levels in the index of "right"

In total df has 11458060 rows and df1 has 2528243 rows

I don't really know what this means. Can anyone tell me what I might be doing wrong?

If you indicate left_on = something in your merge, you also have to indicate right_on. This is only useful when the columns in your two dataframes don't have the same names. Otherwise just use on = something. — ysearka
– ysearka, Commented May 4, 2016 at 8:16

jezrael · Accepted Answer · 2016-05-04 08:04:37Z

I think you need merge on columns x, y and z:

print df1.merge(df, how='left', on=['x','y','z'])

Docs.

Parameter on in merge:

on: Columns (names) to join on. Must be found in both the left and right DataFrame objects. If not passed and left_index and right_index are False, the intersection of the columns in the DataFrames will be inferred to be the join keys

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