python - Pandas:获取组中出现次数最多的字符串值

我有以下数据帧:

item    response
1       A
1       A
1       B
2       A
2       A

我想为一个项目添加一个包含最多给定响应的列。这应该导致:

item    response  mostGivenResponse
1       A          A
1       A          A
1       B          A
2       C          C
2       C          C

我试过这样的事情:

df["responseCount"] = df.groupby(["ItemCode", "Response"])["Response"].transform("count")

df["mostGivenResponse"] = df.groupby(['ItemCode'])['responseCount'].transform(max)

但是 mostGivenResponse 现在是响应的计数而不是响应本身。

最佳答案

使用 value_counts 并返回第一个索引值:

df["responseCount"] = (df.groupby("item")["response"]
                        .transform(lambda x: x.value_counts().index[0]))

print (df)
   item response responseCount
0     1        A             A
1     1        A             A
2     1        B             A
3     2        C             C
4     2        C             C

或 collections.Counter.most_common :

from collections import Counter

df["responseCount"] = (df.groupby("item")["response"]
                         .transform(lambda x: Counter(x).most_common(1)[0][0]))

print (df)
   item response responseCount
0     1        A             A
1     1        A             A
2     1        B             A
3     2        C             C
4     2        C             C

编辑:

问题在于一个或多个 NaN s only 组，解决方案是使用 if-else 过滤:

print (df)
   item response
0     1        A
1     1        A
2     2      NaN
3     2      NaN
4     3      NaN

def f(x):
    s = x.value_counts()
    print (s)

    A    2
    Name: 1, dtype: int64
    Series([], Name: 2, dtype: int64)
    Series([], Name: 3, dtype: int64)

    #return np.nan if s.empty else s.index[0]
    return np.nan if len(s) == 0 else s.index[0]

df["responseCount"] = df.groupby("item")["response"].transform(f)
print (df)
   item response responseCount
0     1        A             A
1     1        A             A
2     2      NaN           NaN
3     2      NaN           NaN
4     3      NaN           NaN

关于python - Pandas:获取组中出现次数最多的字符串值，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/51288635/