主要区别在于first()
将跳到第一个非空值,而head(1)
won't.
如果我掉落np.nan
进入你的例子:
df = pd.DataFrame({'id' : [1,1,1,2,2,3,3,3,3,4,4,5,6,6,6,7,7],
'value' : [np.nan,"second","second","first",
"second","first","third","fourth",
"fifth","second","fifth","first",
"first","second","third","fourth","fifth"]})
然后我们有:
>>> df.groupby('id').head(1)
id value
0 1 NaN # NaN is included
3 2 first
5 3 first
9 4 second
11 5 first
12 6 first
15 7 fourth
>>> df.groupby('id').first()
value
id
1 second # NaN is skipped
2 first
3 first
4 second
5 first
6 first
7 fourth
(此外,如你所见,head()
重置索引。)