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Pandas Add Column

You are never stuck with just the data you are given. Instead, you can add new columns to a DataFrame. This has many names, such as transforming, mutating, and feature engineering.

You can create new columns from scratch, but it is also common to derive them from other columns, for example, by adding columns together or by changing their units.

Deriving a Column

Using a dog dataset, let's say you want to add a new column to your DataFrame that has each dog's height in meters instead of centimeters.

On the left-hand side of the equals, you use square brackets with the name of the new column you want to create, in this case, height_m. On the right-hand side, you have the calculation.

dogs["height_m"] = dogs["height_cm"] / 100
print(dogs)
        name        breed   color   height_cm   weight_kg   date_of_birth   height_m
0      Bella     Labrador   Brown          56          24      2013-07-01       0.56
1    Charlie       Poodle   Black          43          24      2016-09-16       0.43
2       Lucy    Chow Chow   Brown          46          24      2014-08-25       0.46
3     Cooper    Schnauzer    Gray          49          17      2016-09-16       0.49
4        Max     Labrador   Black          59          29      2016-09-16       0.59
5     Stella    Chihuahua     Tan          18           2      2016-09-16       0.18
6     Bernie  St. Bernard   White          77          74      2016-09-16       0.77

Notice that both the existing and the derived column are in the dataframe you modified.

Adding a Column

In this example, you will calculate doggy mass index and add it as a column to your dataframe. BMI stands for body mass index, which is calculated by weight in kilograms divided by their height in meters, squared.

dogs["height_m"] = dogs["weight_kg"] / dogs["height_m"] ** 2
print(dogs.head())
        name        breed   color   height_cm   weight_kg   date_of_birth   height_m          bmi
0      Bella     Labrador   Brown          56          24      2013-07-01       0.56    76.530612
1    Charlie       Poodle   Black          43          24      2016-09-16       0.43   129.799892
2       Lucy    Chow Chow   Brown          46          24      2014-08-25       0.46   113.421550
3     Cooper    Schnauzer    Gray          49          17      2016-09-16       0.49    70.803832
4        Max     Labrador   Black          59          29      2016-09-16       0.59    83.309394

Again, the new column is on the left-hand side of the equals, but this time, our calculation involves two columns.

Adding a Column With Multiple Manipulations

The real power of pandas comes in when you combine all the skills that you have learned so far. Let's figure out the names of skinny, tall dogs.

First, to define the skinny dogs, you take the subset of dogs that have a BMI of less than 100. Next, you sort the height in descending order of height to get the tallest skinny dogs at the top.

Finally, this time you will only keep the columns you are interested in.

bmi_lt_100 = dogs[dogs["bmi"] < 100]
bmi_lt_100_height = bmi_lt_100.short_values("height_cm", ascending=False)
bmi_lt_100_height[["name", "height_cm", "bmi"]]
        name      height_cm           bmi
4        Max             59     83.309394
0      Bella             56     76.530612
3     Cooper             49     70.803832
5     Stella             18     61.728395

Here, you can see that Max is the tallest dog with a BMI of under 100.

Interactive Example

In the below example, you add a new column to DataFrame homelessness, named total, containing the sum of the individuals and family_members columns. Then, add another column to homelessness, named p_individuals, containing the proportion of homeless people in each state who are individuals. Finally, print the homelessness dataframe.

# Add total col as sum of individuals and family_members
homelessness["total"] = homelessness["individuals"] + homelessness["family_members"]

# Add p_individuals col as proportion of individuals
homelessness["p_individuals"] = homelessness["individuals"] / homelessness["total"]

# See the result
print(homelessness)

When we run the above code, it produces the following result:

                region                 state  individuals  family_members  state_pop     total  p_individuals
0   East South Central               Alabama       2570.0           864.0    4887681    3434.0          0.748
1              Pacific                Alaska       1434.0           582.0     735139    2016.0          0.711
2             Mountain               Arizona       7259.0          2606.0    7158024    9865.0          0.736
3   West South Central              Arkansas       2280.0           432.0    3009733    2712.0          0.841
4              Pacific            California     109008.0         20964.0   39461588  129972.0          0.8
...
48      South Atlantic         West Virginia       1021.0           222.0    1804291    1243.0          0.821
49  East North Central             Wisconsin       2740.0          2167.0    5807406    4907.0          0.558
50            Mountain               Wyoming        434.0           205.0     577601     639.0          0.679

Try it for yourself.

To learn more about adding new columns in pandas, please see this video from our course Data Manipulation with pandas .

This content is taken from DataCamp’s Data Manipulation with pandas course by Maggie Matsui and Richie Cotton.