两者都返回每组第一行的DataFrame.在阅读API参考时,它首先说“计算第一组值”,但是当并排查看两个输出时,我看不到主要区别.
我错过了什么吗?
df = pd.DataFrame({'id' : [1,1,1,2,2,3,3,3,3,4,4,5,6,6,6,7,7],
'value' : ["first","second","second","first",
"second","first","third","fourth",
"fifth","second","fifth","first",
"first","second","third","fourth","fifth"]})
解决方法:
主要区别在于first()将跳转到第一个非null值,而head(1)则不会.
如果我把np.nan放到你的例子中:
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
(另外,如您所见,head()重置索引.)