Candidate telemetry diagnostic, error autopsy, and step-by-step query construction walkthrough.
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33 solved
First attempt fail
Evaluated submissions
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Unlocked answer
Extract all email addresses from a block of mixed text. Return them as a sorted list of unique lowercased emails.
An email matches the pattern: local-part@domain.tld where local-part allows letters, digits, dots, underscores, plus, and hyphens; and domain allows letters, digits, dots, and hyphens.
Operating on a DataFrame slice without `.copy()`, producing `SettingWithCopyWarning`, or using `.apply(axis=1)` with custom Python functions when vectorized NumPy / pandas column operations could run 50x faster with 0 memory overhead.
Interviewers assess whether you write idiomatic, vectorized pandas code instead of slow row-by-row procedural Python loops.
Construct the solution logically from first principles to avoid typical edge case pitfalls.
Use boolean masks with `.loc[row_mask, col_list]` to avoid chained assignment warnings.
subset = df.loc[df['status'] == 'completed'].copy()
Use numpy/pandas native operations like `.groupby()`, `.transform()`, or column arithmetic.
df['rev_share'] = df['revenue'] / df.groupby('country')['revenue'].transform('sum')Sort values and reset index to match the required evaluation output contract.
result = df.sort_values('rev_share', ascending=False).reset_index(drop=True)import re
text = '''
For inquiries, contact Sales@Topfolio.in or info@topfolio.in.
Support tickets go to help@databuoy.io or backup-help@databuoy.io.
DO NOT email random.guy.92@gmail.com — that's not us.
Marketing: marketing+promo@topfolio.in (note the +promo segment).
Repeat: SALES@topfolio.in should dedupe with sales@topfolio.in.
'''
emails = re.findall(r"[\w.+\-]+@[\w.\-]+\.\w+", text)
result = sorted({e.lower() for e in emails})
Real code patterns candidates submit that fail the grading suite.
results = []
for i, row in df.iterrows():
results.append(row['val'] * 2)Three recurring syntax and semantic traps relevant to this problem domain.
Modifying a filtered slice like df[df.age > 30]['salary'] = 5000 modifies an ephemeral copy instead of the underlying dataframe.
df[df['active'] == True]['status'] = 'verified' # ❌ SettingWithCopyWarning
df.loc[df['active'] == True, 'status'] = 'verified' # ✅ Idiomatic inplace assignment
Calling df.groupby('category').agg(...) places 'category' into the MultiIndex/Index, making subsequent column references fail.
res = df.groupby('dept')['salary'].mean(); print(res['dept']) # ❌ KeyErrorres = df.groupby('dept', as_index=False)['salary'].mean() # ✅ Keeps column intactIterating rows with for index, row in df.iterrows() is an anti-pattern in data science interviews.
for i, r in df.iterrows(): df.at[i, 'total'] = r['price'] * r['qty'] # ❌ Slow
df['total'] = df['price'] * df['qty'] # ✅ C-speed vectorization
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