熊猫计算列中每个值的出现次数

Question

I have this dataframe:

I want to have a new column that counts only the first instances of the matchID in the column MatchID.

Specifically, it checks the matchID to see if it is unique. If it is unique it puts inputs a 1 in the new column row. If it is not unique but the FIRST instance of the matchID, it also inputs a 1 in the new column. If it is a duplicate and not the first instance it puts a zero in the new column.

Any help would be amazing. Switching from excel to pandas is much much harder then expected :).

Thanks in advance.

Answer 1

怎么样：

df['Count'] = (~df['MatchID'].duplicated()).astype(int)

Answer 2

Here's an approach based on a sample DataFrame:

# Some dummy data. The field ID is equivalent to MatchID
df = pd.DataFrame([("A",12),("B", 12), ("A",123)], columns=["id","val"])
# Create a temporary subset of the DF that matches the "first or unique" rule
first_or_unique = df.drop_duplicates(subset="id", keep="first")
# Populate the new match lookup series with 0 for all rows to begin with
df["match"] = 0
# Finally, use `.loc` along with the temporary DF's index to set the relevant
# rows to be 1
df.loc[first_or_unique.index.values, "match"] = 1