计算 pandas 中的物品数量 [英] Count number of items in pandas

问题描述

我知道这个问题相当琐碎，但这是我使用熊猫的第一天，所以请至少发布指向我应该阅读的文档的链接.

I know this question is quite trivial, but this is my first day of using pandas, so please at least post a link to the document which I should read.

基本上，我不知道如何计算每列的项目数.假设我有一个像这样的数据框:

Basically, I don't know how to count the number of items of each column. Suppose I have a data frame like this:

df = pandas.DataFrame({
    "Grade": [1, 2, 1, 1, 2, 1, 2, 2, 1],
    "Major": ["Science", "Art", "Engineering", "Science", "Science", "Science", "Science", "Engineering", "Engineering"]})

我希望得到以下结果:

Grade   Major           Number
1       Science         3
1       Engineering     2
1       Art             0
2       Science         2
2       Engineering     1
2       Art             1

推荐答案

我认为您需要

I think you need groupby with aggregating size:

print (df.groupby(['Grade','Major']).size().reset_index(name='Number'))
   Grade        Major  Number
0      1  Engineering       2
1      1      Science       3
2      2          Art       1
3      2  Engineering       1
4      2      Science       2

如果需要为每个组添加缺少的值，请添加 unstack 与 stack :

If need add missing values per group, add unstack with stack:

print (df.groupby(['Grade','Major'])
         .size()
         .unstack(fill_value=0)
         .stack()
         .reset_index(name='Number'))
   Grade        Major  Number
0      1          Art       0
1      1  Engineering       2
2      1      Science       3
3      2          Art       1
4      2  Engineering       1
5      2      Science       2

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