This is probably easy, but I have the following data:

In data frame 1:

index dat1
0 9
1 5

In data frame 2:

index dat2
0 7
1 6

I want a data frame with the following form:

index dat1 dat2
0 9 7
1 5 6

I've tried using the append method, but I get a cross join (i.e. cartesian product).

What's the right way to do this?

4

6 Answers

It seems in general you're just looking for a join:

> dat1 = pd.DataFrame({'dat1': [9,5]})
> dat2 = pd.DataFrame({'dat2': [7,6]})
> dat1.join(dat2) dat1 dat2
0 9 7
1 5 6

7

You can also use:

dat1 = pd.concat([dat1, dat2], axis=1)

1

Both join() and concat() way could solve the problem. However, there is one warning I have to mention: Reset the index before you join() or concat() if you trying to deal with some data frame by selecting some rows from another DataFrame.

One example below shows some interesting behavior of join and concat:

dat1 = pd.DataFrame({'dat1': range(4)})
dat2 = pd.DataFrame({'dat2': range(4,8)})
dat1.index = [1,3,5,7]
dat2.index = [2,4,6,8]
# way1 join 2 DataFrames
print(dat1.join(dat2))
# output dat1 dat2
1 0 NaN
3 1 NaN
5 2 NaN
7 3 NaN
# way2 concat 2 DataFrames
print(pd.concat([dat1,dat2],axis=1))
#output dat1 dat2
1 0.0 NaN
2 NaN 4.0
3 1.0 NaN
4 NaN 5.0
5 2.0 NaN
6 NaN 6.0
7 3.0 NaN
8 NaN 7.0
#reset index
dat1 = dat1.reset_index(drop=True)
dat2 = dat2.reset_index(drop=True)
#both 2 ways to get the same result
print(dat1.join(dat2)) dat1 dat2
0 0 4
1 1 5
2 2 6
3 3 7
print(pd.concat([dat1,dat2],axis=1)) dat1 dat2
0 0 4
1 1 5
2 2 6
3 3 7

5

You can assign a new column. Use indices to align correspoding rows:

df1 = pd.DataFrame({'A': [1, 2, 3], 'B': [10, 20, 30]}, index=[0, 1, 2])
df2 = pd.DataFrame({'C': [100, 200, 300]}, index=[1, 2, 3])
df1['C'] = df2['C']

Result:

 A B C
0 1 10 NaN
1 2 20 100.0
2 3 30 200.0

Ignore indices:

df1['C'] = df2['C'].reset_index(drop=True)

Result:

 A B C
0 1 10 100
1 2 20 200
2 3 30 300

Perhaps too simple by anyways...

dat1 = pd.DataFrame({'dat1': [9,5]})
dat2 = pd.DataFrame({'dat2': [7,6]})
dat1['dat2'] = dat2 # Uses indices from dat1

Result:

 dat1 dat2
0 9 7
1 5 6

Just a matter of the right google search:

data = dat_1.append(dat_2)
data = data.groupby(data.index).sum()

0

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