pandas
1. Import pandas under the alias pd.
import pandas as pd
2. Print the version of pandas that has been imported.
pd.__version__
3. Print out all the version information of the libraries that are required by the pandas library.
pd.show_versions()
DataFrame basics¶
A few of the fundamental routines for selecting, sorting, adding and aggregating data in DataFrames¶
Difficulty: easy
Note: remember to import numpy using:
import numpy as np
Consider the following Python dictionary data and Python list labels:
data = {'animal': ['cat', 'cat', 'snake', 'dog', 'dog', 'cat', 'snake', 'cat', 'dog', 'dog'],
'age': [2.5, 3, 0.5, np.nan, 5, 2, 4.5, np.nan, 7, 3],
'visits': [1, 3, 2, 3, 2, 3, 1, 1, 2, 1],
'priority': ['yes', 'yes', 'no', 'yes', 'no', 'no', 'no', 'yes', 'no', 'no']}
labels = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j']
(This is just some meaningless data I made up with the theme of animals and trips to a vet.)
4. Create a DataFrame df from this dictionary data which has the index labels.
import numpy as np
data = {'animal': ['cat', 'cat', 'snake', 'dog', 'dog', 'cat', 'snake', 'cat', 'dog', 'dog'],
'age': [2.5, 3, 0.5, np.nan, 5, 2, 4.5, np.nan, 7, 3],
'visits': [1, 3, 2, 3, 2, 3, 1, 1, 2, 1],
'priority': ['yes', 'yes', 'no', 'yes', 'no', 'no', 'no', 'yes', 'no', 'no']}
labels = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j']
df = pd.DataFrame(data, index=labels)
5 Display a summary of the basic information about this DataFrame and its data (hint: there is a single method that can be called on the DataFrame).
df.info()
# ...or...
df.describe()
6 Return the first 3 rows of the DataFrame df.
df.iloc[:3]
# or equivalently
df.head(3)
7 Select just the 'animal' and 'age' columns from the DataFrame df.
df.loc[:, ['animal', 'age']]
# or
df[['animal', 'age']]
8 Select the data in rows [3, 4, 8] and in columns ['animal', 'age'].
df.loc[df.index[[3, 4, 8]], ['animal', 'age']]
9 Select only the rows where the number of visits is greater than 3.
df[df['visits'] > 3]
10. Select the rows where the age is missing, i.e. it is NaN.
df[df['age'].isnull()]
11. Select the rows where the animal is a cat and the age is less than 3.
df[(df['animal'] == 'cat') & (df['age'] < 3)]
12. Select the rows the age is between 2 and 4 (inclusive).
df[df['age'].between(2, 4)]
13. Change the age in row 'f' to 1.5.
df.loc['f', 'age'] = 1.5
14. Calculate the sum of all visits in df (i.e. the total number of visits).
df['visits'].sum()
15. Calculate the mean age for each different animal in df.
df.groupby('animal')['age'].mean()
16. Append a new row 'k' to df with your choice of values for each column. Then delete that row to return the original DataFrame.
df.loc['k'] = [5.5, 'dog', 'no', 2]
# and then deleting the new row...
df = df.drop('k')
17. Count the number of each type of animal in df.
df['animal'].value_counts()
18. Sort df first by the values in the 'age' in decending order, then by the value in the 'visit' column in ascending order (so row i should be first, and row d should be last).
df.sort_values(by=['age', 'visits'], ascending=[False, True])
19. The 'priority' column contains the values 'yes' and 'no'. Replace this column with a column of boolean values: 'yes' should be True and 'no' should be False.(★★★)
df['priority'] = df['priority'].map({'yes': True, 'no': False})
20. In the 'animal' column, change the 'snake' entries to 'python'.
df['animal'] = df['animal'].replace('snake', 'python')
21. For each animal type and each number of visits, find the mean age. In other words, each row is an animal, each column is a number of visits and the values are the mean ages (hint: use a pivot table).
df.pivot_table(index='animal', columns='visits', values='age', aggfunc='mean')
DataFrames: beyond the basics¶
Slightly trickier: you may need to combine two or more methods to get the right answer¶
Difficulty: medium
The previous section was tour through some basic but essential DataFrame operations. Below are some ways that you might need to cut your data, but for which there is no single "out of the box" method.
22 You have a DataFrame df with a column 'A' of integers. For example:
df = pd.DataFrame({'A': [1, 2, 2, 3, 4, 5, 5, 5, 6, 7, 7]})
How do you filter out rows which contain the same integer as the row immediately above?
You should be left with a column containing the following values:
1, 2, 3, 4, 5, 6, 7
df = pd.DataFrame({'A': [1, 2, 2, 3, 4, 5, 5, 5, 6, 7, 7]})
df.loc[df['A'].shift() != df['A']]
# Alternatively, we could use drop_duplicates() here. Note
# that this removes *all* duplicates though, so it won't
# work as desired if A is [1, 1, 2, 2, 1, 1] for example.
df.drop_duplicates(subset='A')
23. Given a DataFrame of random numeric values:
df = pd.DataFrame(np.random.random(size=(5, 3)))
# this is a 5x3 DataFrame of float values
how do you subtract the row mean from each element in the row?
df = pd.DataFrame(np.random.random(size=(5, 3)))
df.sub(df.mean(axis=1), axis=0)
24. Suppose you have DataFrame with 10 columns of real numbers, for example:
df = pd.DataFrame(np.random.random(size=(5, 10)), columns=list('abcdefghij'))
Which column of numbers has the smallest sum? Return that column's label.
df = pd.DataFrame(np.random.random(size=(5, 10)), columns=list('abcdefghij'))
df.sum().idxmin()
25. How do you count how many unique rows a DataFrame has (i.e. ignore all rows that are duplicates)?
df = pd.DataFrame(np.random.randint(0, 2, size=(10, 3)))
len(df) - df.duplicated(keep=False).sum()
# or perhaps more simply...
len(df.drop_duplicates(keep=False))
The next three puzzles are slightly harder.
26. In the cell below, you have a DataFrame df that consists of 10 columns of floating-point numbers. Exactly 5 entries in each row are NaN values.
For each row of the DataFrame, find the column which contains the third NaN value.
You should return a Series of column labels: e, c, d, h, d
nan = np.nan
data = [[0.04, nan, nan, 0.25, nan, 0.43, 0.71, 0.51, nan, nan],
[ nan, nan, nan, 0.04, 0.76, nan, nan, 0.67, 0.76, 0.16],
[ nan, nan, 0.5 , nan, 0.31, 0.4 , nan, nan, 0.24, 0.01],
[0.49, nan, nan, 0.62, 0.73, 0.26, 0.85, nan, nan, nan],
[ nan, nan, 0.41, nan, 0.05, nan, 0.61, nan, 0.48, 0.68]]
columns = list('abcdefghij')
df = pd.DataFrame(data, columns=columns)
(df.isnull().cumsum(axis=1) == 3).idxmax(axis=1)
27. A DataFrame has a column of groups 'grps' and and column of integer values 'vals':
df = pd.DataFrame({'grps': list('aaabbcaabcccbbc'),
'vals': [12,345,3,1,45,14,4,52,54,23,235,21,57,3,87]})
For each group, find the sum of the three greatest values. You should end up with the answer as follows:
grps
a 409
b 156
c 345
df = pd.DataFrame({'grps': list('aaabbcaabcccbbc'),
'vals': [12,345,3,1,45,14,4,52,54,23,235,21,57,3,87]})
df.groupby('grps')['vals'].nlargest(3).sum(level=0)
28. The DataFrame df constructed below has two integer columns 'A' and 'B'. The values in 'A' are between 1 and 100 (inclusive).
For each group of 10 consecutive integers in 'A' (i.e. (0, 10], (10, 20], ...), calculate the sum of the corresponding values in column 'B'.
The answer should be a Series as follows:
A
(0, 10] 635
(10, 20] 360
(20, 30] 315
(30, 40] 306
(40, 50] 750
(50, 60] 284
(60, 70] 424
(70, 80] 526
(80, 90] 835
(90, 100] 852
df = pd.DataFrame(np.random.RandomState(8765).randint(1, 101, size=(100, 2)), columns = ["A", "B"])
df.groupby(pd.cut(df['A'], np.arange(0, 101, 10)))['B'].sum()
DataFrames: harder problems¶
These might require a bit of thinking outside the box...¶
...but all are solvable using just the usual pandas/NumPy methods (and so avoid using explicit for loops).
Difficulty: hard
29 Consider a DataFrame df where there is an integer column 'X':
df = pd.DataFrame({'X': [7, 2, 0, 3, 4, 2, 5, 0, 3, 4]})
For each value, count the difference back to the previous zero (or the start of the Series, whichever is closer). These values should therefore be
[1, 2, 0, 1, 2, 3, 4, 0, 1, 2]
Make this a new column 'Y'.
df = pd.DataFrame({'X': [7, 2, 0, 3, 4, 2, 5, 0, 3, 4]})
izero = np.r_[-1, (df == 0).values.nonzero()[0]] # indices of zeros
idx = np.arange(len(df))
y = df['X'] != 0
df['Y'] = idx - izero[np.searchsorted(izero - 1, idx) - 1]
# http://stackoverflow.com/questions/30730981/how-to-count-distance-to-the-previous-zero-in-pandas-series/
# credit: Behzad Nouri
# Here's an alternative approach based on a [cookbook recipe](http://pandas.pydata.org/pandas-docs/stable/cookbook.html#grouping):
df = pd.DataFrame({'X': [7, 2, 0, 3, 4, 2, 5, 0, 3, 4]})
x = (df['X'] != 0).cumsum()
y = x != x.shift()
df['Y'] = y.groupby((y != y.shift()).cumsum()).cumsum()
And another approach using a groupby operation:
df = pd.DataFrame({'X': [7, 2, 0, 3, 4, 2, 5, 0, 3, 4]})
df['Y'] = df.groupby((df['X'] == 0).cumsum()).cumcount()
# We're off by one before we reach the first zero.
first_zero_idx = (df['X'] == 0).idxmax()
df['Y'].iloc[0:first_zero_idx] += 1
30 Consider the DataFrame constructed below which contains rows and columns of numerical data.
Create a list of the column-row index locations of the 3 largest values in this DataFrame. In this case, the answer should be:
[(5, 7), (6, 4), (2, 5)]
df = pd.DataFrame(np.random.RandomState(30).randint(1, 101, size=(8, 8)))
df.unstack().sort_values()[-3:].index.tolist()
# http://stackoverflow.com/questions/14941261/index-and-column-for-the-max-value-in-pandas-dataframe/
# credit: DSM
31 You are given the DataFrame below with a column of group IDs, 'grps', and a column of corresponding integer values, 'vals'.
df = pd.DataFrame({"vals": np.random.RandomState(31).randint(-30, 30, size=15),
"grps": np.random.RandomState(31).choice(["A", "B"], 15)})
Create a new column 'patched_values' which contains the same values as the 'vals' any negative values in 'vals' with the group mean:
vals grps patched_vals
0 -12 A 13.6
1 -7 B 28.0
2 -14 A 13.6
3 4 A 4.0
4 -7 A 13.6
5 28 B 28.0
6 -2 A 13.6
7 -1 A 13.6
8 8 A 8.0
9 -2 B 28.0
10 28 A 28.0
11 12 A 12.0
12 16 A 16.0
13 -24 A 13.6
14 -12 A 13.6
df = pd.DataFrame({"vals": np.random.RandomState(31).randint(-30, 30, size=15),
"grps": np.random.RandomState(31).choice(["A", "B"], 15)})
def replace(group):
mask = group<0
group[mask] = group[~mask].mean()
return group
df.groupby(['grps'])['vals'].transform(replace)
# http://stackoverflow.com/questions/14760757/replacing-values-with-groupby-means/
# credit: unutbu
32 Implement a rolling mean over groups with window size 3, which ignores NaN value. For example consider the following DataFrame:
df = pd.DataFrame({'group': list('aabbabbbabab'),
'value': [1, 2, 3, np.nan, 2, 3, np.nan, 1, 7, 3, np.nan, 8]})
df
group value
0 a 1.0
1 a 2.0
2 b 3.0
3 b NaN
4 a 2.0
5 b 3.0
6 b NaN
7 b 1.0
8 a 7.0
9 b 3.0
10 a NaN
11 b 8.0
The goal is to compute the Series:
0 1.000000
1 1.500000
2 3.000000
3 3.000000
4 1.666667
5 3.000000
6 3.000000
7 2.000000
8 3.666667
9 2.000000
10 4.500000
11 4.000000
E.g. the first window of size three for group 'b' has values 3.0, NaN and 3.0 and occurs at row index 5. Instead of being NaN the value in the new column at this row index should be 3.0 (just the two non-NaN values are used to compute the mean (3+3)/2)
df = pd.DataFrame({'group': list('aabbabbbabab'),
'value': [1, 2, 3, np.nan, 2, 3, np.nan, 1, 7, 3, np.nan, 8]})
g1 = df.groupby(['group'])['value'] # group values
g2 = df.fillna(0).groupby(['group'])['value'] # fillna, then group values
s = g2.rolling(3, min_periods=1).sum() / g1.rolling(3, min_periods=1).count() # compute means
s.reset_index(level=0, drop=True).sort_index() # drop/sort index
# http://stackoverflow.com/questions/36988123/pandas-groupby-and-rolling-apply-ignoring-nans/</pre>
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