Merging DataFrames in Pandas: Join, Merge, and Concat
Merge, join, and concatenate are three essential operations for handling DataFrames in pandas. In this article, we will look at how to use these functions to combine DataFrames.
We’ll cover how to use join, merge, and concat to merge DataFrames and their different applications.
Joining DataFrames
Joining two DataFrames involves combining them into one DataFrame based on a shared index or column in both DataFrames. Joining is similar to SQL join statements, but pandas has four types of join statements: left, right, outer, and inner join.
The join() function is used to merge two dataframes horizontally (based on index or column) or vertically (based on rows). It merges two dataframes into one if they have the same index or column.
Let’s look at an example:
import pandas as pd
df1 = pd.DataFrame({
"key": ["A", "B", "C", "D"],
"value1": [1, 2, 3, 4]
})
df2 = pd.DataFrame({
"key": ["B", "D", "E", "F"],
"value2": [5, 6, 7, 8]
})
df = df1.join(df2.set_index('key'), on='key', how='inner')
print(df)
The output should be:
key value1 value2
1 B 2 5
3 D 4 6
In this example, we’re joining two DataFrames, df1 and df2, with columns key and value1, and key and value2, respectively. We set the index for df2 as key, and then use the join method.
We combined both DataFrames based on their key column using inner join, resulting in a DataFrame with values at indices where the key value exists in both DataFrames, which are B and D.
Merging DataFrames
Merging DataFrames is another approach to combine DataFrames. Unlike the join method, the merge method combines DataFrames based on their columns values, instead of their indices.
The merge function is used to merge two pandas DataFrame objects into one. It works similarly to SQL join statements.
Let’s consider one more example:
df3 = pd.DataFrame({
"key": ["A", "B", "C", "D"],
"value1": [1, 2, 3, 4]
})
df4 = pd.DataFrame({
"key": ["B", "D", "E", "F"],
"value2": [5, 6, 7, 8]
})
df = pd.merge(df3, df4, on='key', how='inner')
print(df)
The output should be:
key value1 value2
0 B 2 5
1 D 4 6
In this example, we merged df3 and df4 by key. The result is a DataFrame with values at indices where the key value exists in both DataFrames, which are B and D.
Concatenating DataFrames
Concatenating DataFrames is a way to stack DataFrames on top of each other or side-by-side. It involves combining multiple DataFrames into one using either their index or column values.
The concat() function can be used to combine multiple dataframes to create a single one. We can use this for column binding or row binding operations.
Let’s see how concatenation works:
df5 = pd.DataFrame({
"key": ["A", "B", "C", "D"],
"value3": [9, 10, 11, 12],
})
frames = [df3, df5]
concatenated_df = pd.concat(frames)
print(concatenated_df)
The output should be:
key value1 value3
0 A 1 NaN
1 B 2 NaN
2 C 3 NaN
3 D 4 NaN
0 A NaN 9
1 B NaN 10
2 C NaN 11
3 D NaN 12
Here, we created a new DataFrame df5 and combined both df3 and df5 DataFrames by concatenation. It should be noted that columns that don’t match (e.g., value3 and value1) will be represented as NaN.
Conclusion
In conclusion, joining, merging, and concatenating dataframes are powerful techniques for data cleaning, processing, and analysis. Knowing how to use these functions can help you manipulate DataFrames effectively, saving time and improving workflow.
Joining using join() is useful for horizontal merging. Merging using merge() is useful for merging data frames based on their columns.
Concatenating using concat() is an effective strategy for combining DataFrames side-by-side or vertically. With these Pandas functions, you have more control over your dataframes, presenting them in a format suitable for your analysis or visualization needs.
Example 2: Merge DataFrames Using Merge
The merge() function is a powerful tool to combine two DataFrames based on their defined columns. It offers flexibility in merging dataframes based on common columns, non-shared values, and the direction of the merge (left, right, outer, or inner).
The syntax for merge() function is:
merged_df = pd.merge(df1, df2, on='key', how='inner')
Here, df1 and df2 are the two DataFrames to be merged, ‘key‘ is the common column for joining and inner is the type of merge operation. Lets consider an example to understand the merge() function.
# Import pandas library
import pandas as pd
# Create a dictionary of data
data1 = {
'id': [1, 2, 3, 4, 5],
'name': ['Steve', 'Jack', 'Tom', 'Robert', 'Alex'],
'age': [29, 31, 50, 18, 42],
'salary': [10.5, 9.2, 7.8, 8.5, 12.9]
}
# Create a dictionary of data
data2 = {
'id': [5, 6, 7, 8],
'name': ['Lucas', 'David', 'Natasha', 'Oscar'],
'age': [21, 38, 29, 44],
'salary': [5.1, 9.7, 7.6, 11.3]
}
# Create two dataframes from data
df1 = pd.DataFrame(data1)
df2 = pd.DataFrame(data2)
# Merge two dataframes
merged_df = pd.merge(df1, df2, on='id', how='inner')
# Print merged dataframe
print(merged_df)
In this example, two DataFrames (df1 and df2) are created using the dictionaries containing the data, with a common column ‘id.’ A new DataFrame is then created by merging the two dataframes based on this common column id using inner join. The output is:
id name_x age_x salary_x name_y age_y salary_y
0 5 Alex 42 12.9 Lucas 21 5.1
As we can see, the output DataFrame is merged based on the common id column, which is five in this case, and an inner join operation has been applied.
Since there is only one shared value, we get a resultant DataFrame with only one row.
Example 3: Merge DataFrames Using Concat
Concatenating DataFrames in pandas involves stacking DataFrames on top of each other or side-by-side based on their index or columns. It’s useful for combining data from different sources or processing data that’s already in different data frames.
The syntax for the concat() function is:
concatenated_df = pd.concat([df1, df2])
Here, df1 and df2 are two dataframes to be concatenated. Let’s consider an example to understand the concat() function.
# Import pandas module
import pandas as pd
# Create a dictionary of data
data1 = {
'id': [1, 2, 3, 4, 5],
'name': ['Steve', 'Jack', 'Tom', 'Robert', 'Alex'],
'age': [29, 31, 50, 18, 42],
'salary': [10.5, 9.2, 7.8, 8.5, 12.9]
}
# Create a dictionary of data
data2 = {
'id': [6, 7, 8],
'name': ['David', 'Natasha', 'Oscar'],
'age': [38, 29, 44],
'salary': [9.7, 7.6, 11.3]
}
# Create two dataframes from data
df1 = pd.DataFrame(data1)
df2 = pd.DataFrame(data2)
# Concatenate two dataframes
concatenated_df = pd.concat([df1, df2])
# Print concatenated dataframe
print(concatenated_df)
In this example, two data frames have been created using dictionaries of data. A new DataFrame is created using the ‘concat‘ function, which concatenates both DataFrames.
The output is:
id name age salary
0 1 Steve 29 10.5
1 2 Jack 31 9.2
2 3 Tom 50 7.8
3 4 Robert 18 8.5
4 5 Alex 42 12.9
0 6 David 38 9.7
1 7 Natasha 29 7.6
2 8 Oscar 44 11.3
As we can see, the concatenated DataFrame has 8 rows and all data from both the DataFrames has been stacked together.
Conclusion
Merging and concatenating data frames are essential operations in data analysis. Pandas provides a powerful set of functions (join(), merge(), and concat()) to effectively combine data frames.
We hope this article helped you understand how to use these functions and their applications with examples. With these tools, you can combine and analyze data across multiple data frames quickly and easily.
In addition to the merge and concatenation operations we’ve discussed earlier, there are many other common operations in Pandas for data manipulation, aggregation, cleaning, and exploratory data analysis (EDA). Here, we’ll discuss some of the most commonly used operations.
Selecting Data
The ability to appropriately select data from a DataFrame is essential in data analysis. Pandas offers several methods for selecting data, including .loc, .iloc and the .at and .iat properties.
The .loc property selects data based on matching labels, while .iloc selects based on integer position. The .at and .iat properties allow us to retrieve a single value from a DataFrames location based on label or integer position.
# Import pandas module
import pandas as pd
# Create a dictionary of data
data = {
'id': [1, 2, 3, 4, 5],
'name': ['Steve', 'Jack', 'Tom', 'Robert', 'Alex'],
'age': [29, 31, 50, 18, 42],
'salary': [10.5, 9.2, 7.8, 8.5, 12.9]
}
# Create a DataFrame from data
df = pd.DataFrame(data)
# Select data based on label
print(df.loc[2:4, 'name'])
# Select data based on integer position
print(df.iloc[2:4, 2])
# Select data based on label or integer position
print(df.at[3, 'name'])
print(df.iat[3, 2])
Handling Missing Data
Missing data is a common issue in data analysis. Pandas provides several methods for handling missing data, including dropna(), fillna(), and isna().
The dropna() function drops rows or columns with missing values, while fillna() fills the missing values based on various methods.
# Import pandas module
import pandas as pd
import numpy as np
# Create a dictionary of data
data = {
'id': [1, 2, np.nan, 4, 5],
'name': ['Steve', 'Jack', 'Tom', None, 'Alex'],
'age': [29, np.nan, 50, 18, 42],
'salary': [10.5, 9.2, np.nan, 8.5, 12.9]
}
# Create a DataFrame from data
df = pd.DataFrame(data)
# Find rows with missing data
print(df.isna())
# Remove rows or columns with missing values
print(df.dropna())
# Fill missing values with mean of column
print(df.fillna(df.mean()))
# Fill missing values with data from previous or next row
print(df.fillna(method='ffill'))
print(df.fillna(method='bfill'))
Aggregating and Analyzing Data
Aggregating and analyzing data is crucial in data analysis. Pandas provides several functions for aggregating and analyzing data, including groupby(), mean(), sum(), max(), min(), and count().
# Import pandas module
import pandas as pd
# Create a dictionary of data
data = {
'name': ['Steve', 'Jack', 'Tom', 'Robert', 'Alex', 'Steve', 'Jack', 'Alex'],
'age': [29, 31, 50, 18, 42, 35, 32, 43],
'salary': [10.5, 9.2, 7.8, 8.5, 12.9, 14.5, 11.4, 18.2]
}
# Create a DataFrame from data
df = pd.DataFrame(data)
# Group data by name and calculate mean salary and age
print(df.groupby('name').agg({'salary': 'mean', 'age': 'mean'}))
# Find the highest salary and age for each name
print(df.groupby('name').agg({'salary': 'max', 'age': 'max'}))
# Count the number of occurrences of each name
print(df.groupby('name').count())
Filtering Data
Filtering data is a useful operation in data analysis. Pandas provides several methods for filtering data based on specific conditions, including the .loc and .iloc methods, as well as the .query() method and Boolean indexing.
# Import pandas module
import pandas as pd
# Create a dictionary of data
data = {
'name': ['Steve', 'Jack', 'Tom', 'Robert', 'Alex', 'Steve', 'Jack', 'Alex'],
'age': [29, 31, 50, 18, 42, 35, 32, 43],
'salary': [10.5, 9.2, 7.8, 8.5, 12.9, 14.5, 11.4, 18.2]
}
# Create a DataFrame from data
df = pd.DataFrame(data)
# Filter data based on a condition
print(df.loc[df.age > 35])
# Filter data based on multiple conditions
print(df.query('age > 30 and salary > 10'))
# Filter data based on a list of names
names = ['Tom', 'Alex']
print(df[df['name'].isin(names)])
Conclusion
Pandas offers a range of powerful built-in functions for handling data, including selecting, cleaning, aggregating, filtering, and analyzing data. By using these operations, data analysts can efficiently manipulate and process data and gain valuable insights into the nature of the data.
In summary, Pandas provides powerful functions for data manipulation, aggregation, cleaning, and EDA, including selecting data, handling missing data, aggregating and analyzing data, and filtering data. By utilizing these functions, data analysts can efficiently analyze data and gain valuable insights.
The importance of knowing these operations is critical for data analysis and should be considered essential for anyone looking to work with data using Pandas. Overall, mastering these functions is essential to effectively processing large and complex datasets, and can provide a significant competitive advantage in the field of data analysis.