If the same element occurs two times in a dataframe?

Question

The first column in the dataframe is a random list of studentIDs. I would like to find out if there is any studentID that occur two times. If this is the case, I would like to print out the two lines where it happens.

StudentID   Name
s123456     Michael
s123789     Peter
s123789     Thomas 
s123579     Marie

I would like to print out:

"Two students have the same student id in line {} and {}"

And what have you tried so far ? [This question may be downvoted because it shows neither research effort nor attempt](http://idownvotedbecau.se/noattempt/) — vincrichaud, Jun 15 '18 at 13:41
Possible duplicate of [How do I get a list of all the duplicate items using pandas in python?](https://stackoverflow.com/questions/14657241/how-do-i-get-a-list-of-all-the-duplicate-items-using-pandas-in-python) — Derek Brown, Jun 15 '18 at 14:23

Dillon · Answer 1 · 2018-06-15T15:09:11.310

1

df = df.reset_index()  # So a row value is visible after the groupby

# Check how the df looks
print(df)
   index StudentID     Name
0      0   s123456  Michael
1      1   s123789    Peter
2      2   s123789   Thomas
3      3   s123579    Marie

def my_func(x):
    count = len(x)
    rows = " and ".join(x.astype(str))
    return "{} students have the same student ID in line {}".format(count, rows)

df = df[df.StudentID.duplicated(False)].groupby('StudentID')['index'].unique().map(my_func)

# Print results
for i in df:
    print(i)

2 students have the same student ID in line 1 and 2

edited Jun 15 '18 at 15:09

answered Jun 15 '18 at 13:20

Dillon

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But I want to print out in which line in the original dataframe, the two studentIDs are. – Laura Røtzler Holm Jun 15 '18 at 13:41
@LauraRøtzlerHolm do you mean you also want to print `s123789` as part of the sentence? – Dillon Jun 15 '18 at 13:43
No i want to print out the rownumbers of the two studentIDs. like, row number 1 and 2. – Laura Røtzler Holm Jun 15 '18 at 13:48
@LauraRøtzlerHolm I have updated the answer accordingly – Dillon Jun 15 '18 at 14:19

score 0 · Answer 2 · answered Jun 15 '18 at 15:23

Here's one way using f-strings, available in Python 3.6+:

# example data
StudentID   Name
s123456     Michael
s123789     Peter
s123789     Thomas 
s123577     Joe
s123456     Mark
s123458     Andrew

# get duplicates StudentIDs
dups = df.loc[df['StudentID'].duplicated(keep=False), 'StudentID'].unique()

# iterate duplicates
for stid in dups:
    dup_idx = df[df['StudentID'] == stid].index.tolist()
    print(f'{len(dup_idx)} Students have the same student id in lines: {dup_idx}')

2 Students have the same student id in lines: [0, 4]
2 Students have the same student id in lines: [1, 2]

If the same element occurs two times in a dataframe?

2 Answers2