pandas map() Function – Examples

pandas map() function from Series is used to substitute each value in a Series with another value, that may be derived from a function, a dict or a Series. Since DataFrame columns are series, you can use map() to update the column and assign it back to the DataFrame.

pandas Series is a one-dimensional array-like object containing a sequence of values. Each of these values is associated with a label called index. We can create a Series by using an array-like object (e.g., list) or a dictionary.

pandas map() Key Points –

• This method defined only in Series and not present in DataFrame.
• map() accepts dict, Series, or callable
• You can use this to perform operations on a specific column of a DataFrame as each column in a DataFrame is Series.
• map() when passed a dictionary/Series will map elements based on the keys in that dictionary/Series. Missing values will be recorded as NaN in the output.
• Series.map() operate on one element at time

1. Syntax of pandas map()

The following is the syntax of the pandas map() function. This accepts arg and na_action as parameters and returns a Series.

# Syntax of Series.map()
Series.map(arg, na_action=None)

Following are the parameters

• arg – Accepts function, dict, or Series
• na_action – Accepts ignore, None. Default set to None.

Let’s create a DataFrame and use it with map() function to update the DataFrame column.

# Create a pandas DataFrame.
import pandas as pd
import numpy as np
technologies= {
    'Fee' :[22000,25000,23000,np.NaN,26000],
    'Duration':['30days','50days','30days','35days','40days']
          }
df = pd.DataFrame(technologies)
print(df)

Yields below output.

       Fee Duration
0  22000.0   30days
1  25000.0   50days
2  23000.0   30days
3      NaN   35days
4  26000.0   40days

2. Series.map() Example

You can only use the Series.map() function with the particular column of a pandas DataFrame. If you are not aware, every column in DataFrame is a Series. For example, df[‘Fee’] returns a Series object. Let’s see how to apply the map function on one of the DataFrame column and assign it back to the DataFrame.

# Using Lambda Function
df['Fee'] = df['Fee'].map(lambda x: x - (x*10/100))
print(df)

Yields below output. This example substitutes 10% from the Fee column value.

       Fee Duration
0  19800.0   30days
1  22500.0   50days
2  20700.0   30days
3      NaN   35days
4  23400.0   40days

You can also apply a function with the lambda as below. This yields the same output as above.

# Using custom function
def fun1(x):
    return x/100
df['Fee'] = df['Fee'].map(lambda x:fun1(x))

3. Handling NaN by using na_action param

The na_action param is used to handle NaN values. The default option for this argument is None, using which the NaN values are passed to the mapping function and may result in incorrect. You can also use ‘ignore’, where no action is performed.

# Let's add the currently to the Fee
df['Fee'] = df['Fee'].map('{} RS'.format)
print(df)

Yields below output. Notice that the Value for Fee column for index 3 is ‘nan RS’ which doesn’t make sense.

       Fee Duration
0  198.0 RS   30days
1  225.0 RS   50days
2  207.0 RS   30days
3    nan RS   35days
4  234.0 RS   40days

Now let’s use the na_action=’ignore’. This ignores the updating when it sees the NaN value.

# Use na_action param
df['Fee'] = df['Fee'].map('{} RS'.format, na_action='ignore')
print(df)

Yields below output

        Fee Duration
0  198.0 RS   30days
1  225.0 RS   50days
2  207.0 RS   30days
3       NaN   35days
4  234.0 RS   40days

4. Using map() with Dictionary

Alternatively, you can also use the dictionary as the mapping function.

# Using Dictionary for mapping
dict_map = {'30days':'35 Days','50days':'55 Days',
            '40days':'45 Days'}
updateSer = df['Duration'].map(dict_map)
df['Duration'] = updateSer
print(df)

Yields below output.

        Fee Duration
0  198.0 RS  35 Days
1  225.0 RS  55 Days
2  207.0 RS  35 Days
3       NaN      NaN
4  234.0 RS  45 Days

5. Complete Example of pandas map() Function

# Create a pandas DataFrame.
import pandas as pd
import numpy as np
technologies= {
    'Fee' :[22000,25000,23000,np.NaN,26000],
    'Duration':['30days','50days','30days','35days','40days']
          }
df = pd.DataFrame(technologies)
print(df)

# Using Lambda Function
df['Fee'] = df['Fee'].map(lambda x: x - (x*10/100))
print(df)

# Using custom function
def fun1(x):
    return x/100
ser = df['Fee'].map(lambda x:fun1(x))
print(ser)

# Let's add the currently to the Fee
df['Fee'] = df['Fee'].map('{} RS'.format)
print(df)

df['Fee'] = df['Fee'].map('{} RS'.format, na_action='ignore')
print(df)

# Using Dictionary for mapping
dict_map = {'30days':'35 Days','50days':'55 Days',
            '40days':'45 Days'}
updateSer = df['Duration'].map(dict_map)
df['Duration'] = updateSer
print(df)

Conclusion

In this article, I have explained map() function is from the Series which is used to substitute each value in a Series with another value and returns a Series object, since DataFrame is a collection of Series, you can use the map() function to update the DataFrame.

Related Articles

• Pandas – Create DataFrame From Multiple Series
• Pandas Difference Between map, applymap and apply Methods
• Pandas apply map (applymap()) Explained
• Pandas Groupby Transform
• Pandas Read Multiple CSV Files into DataFrame
• Pandas Apply Function to Every Row
• Pandas apply() with Lambda Examples
• Pandas Concatenate Two Columns

References

• https://pandas.pydata.org/docs/reference/api/pandas.Series.map.html