Why loops are frowned upon in data science

In computer science, looping is one of the most fundamental operations. It saves you from manually doing the same thing over and over.

You can tell your computer to do something ten, a hundred, or even a million times, and it will happily do it. It’s no wonder many people feel so proud when they successfully write a for loop in their first programming language.

Here’s an example of implementing a for loop in the Python programming language. Say we had a list of three-letter government agencies in lowercase, but we wanted them to be in uppercase. After all, that’s the only way they will be recognizable.

flowchart TD
    Start([Start])
    A[Initialize List of Lowercase Characters]
    B{More Characters in List?}
    C[Get Next Character]
    D[Convert Character to Uppercase]
    E[Add Uppercase Character to New List]
    F([End Loop])

    Start --> A --> B
    B -->|Yes| C --> D --> E --> B
    B -->|No| F

We can do the task show in the flowchart above manually, which won’t be much work because our list only has 8 agencies. But you can imagine how tedious this may be if we had a hundred or a thousand agencies. Fortunately, we can use a for loop to do this tedious work.

agencies = ['cia', 'fbi', 'nsa', 'epa', 'irs', 'dhs', 'faa', 'doe']

# Loop through the list and convert each agency to uppercase
for i in range(len(agencies)):
    agencies[i] = agencies[i].upper()

agencies

['CIA', 'FBI', 'NSA', 'EPA', 'IRS', 'DHS', 'FAA', 'DOE']

While looping may save you a lot of time in computer science, it can cost you a lot of time in data science. In fact, using for loops in data science operations is frowned upon. Why is this so?

When performing data science operations on a dataset, it is important to understand the difference between normal operations and vectorized operations. Normal operations happen in series (one by one), while vectorized operations happen in parallel (all at once). You can conclude that vectorized operations are faster than normal operations.

Let’s perform these operations on a real dataset. I’ll show 3 examples of pandas code that output the same dataframe.

Dataset

The dataset below shows the quantity of units of some sales. Our task is to create a new column Qty_plus_20_pct that increases quantity values by 20%.

import pandas as pd
from pathlib import Path

(pd.read_parquet(f"{Path('../../../')}/datasets/sales_total_2018.parquet")
         [['name','quantity']]
         )

	name	quantity
0	Barton LLC	39
1	Trantow-Barrows	-1
2	Kulas Inc	23
3	Kassulke, Ondricka and Metz	41
4	Jerde-Hilpert	6
...	...	...
1495	Fritsch, Russel and Anderson	12
1496	Frami, Hills and Schmidt	37
1497	Stokes LLC	14
1498	Pollich LLC	3
1499	Will LLC	38

1500 rows × 2 columns

Looping

The following piece of code loops through the dataframe. Writing code like this to do a mathematical operation would be considered normal in computer science, however, in data science, it would be a BIG no-no. Software engineers are used to writing loops in their code, which is why they find writing data science code less intuitive.

sales = (pd.read_parquet(f"{Path('../../../')}/datasets/sales_total_2018.parquet")
         [['name','quantity']]
         )

for i in range(len(sales)):
    sales.at[i, 'Qty_plus_20_pct'] = sales.at[i, 'quantity'] * 1.20

sales

	name	quantity	Qty_plus_20_pct
0	Barton LLC	39	46.8
1	Trantow-Barrows	-1	-1.2
2	Kulas Inc	23	27.6
3	Kassulke, Ondricka and Metz	41	49.2
4	Jerde-Hilpert	6	7.2
...	...	...	...
1495	Fritsch, Russel and Anderson	12	14.4
1496	Frami, Hills and Schmidt	37	44.4
1497	Stokes LLC	14	16.8
1498	Pollich LLC	3	3.6
1499	Will LLC	38	45.6

1500 rows × 3 columns

Apply

Apply also loops through the dataframe under the hood. However, it’s faster than the normal looping demonstrated above because the looping happens in the C programming language layer. Remember, Python is a slow programming language, but when you use it in conjunction with pandas, which is written in the C programming language, it becomes fast. That’s why Python is capable of manipulating even large quantities of data. Unsurprisingly, performing the mathematical operation with apply is faster than the previous method.

sales = (pd.read_parquet(f"{Path('../../../')}/datasets/sales_total_2018.parquet")
         [['name','quantity']]
         )

# Function to increase quantity by 20%
def add_20_percent(x):
    return x * 1.20

sales['Qty_plus_20_pct'] = sales['quantity'].apply(add_20_percent)

sales

	name	quantity	Qty_plus_20_pct
0	Barton LLC	39	46.8
1	Trantow-Barrows	-1	-1.2
2	Kulas Inc	23	27.6
3	Kassulke, Ondricka and Metz	41	49.2
4	Jerde-Hilpert	6	7.2
...	...	...	...
1495	Fritsch, Russel and Anderson	12	14.4
1496	Frami, Hills and Schmidt	37	44.4
1497	Stokes LLC	14	16.8
1498	Pollich LLC	3	3.6
1499	Will LLC	38	45.6

1500 rows × 3 columns

Vectorized

Dataframes are like matrices in that they have rows and columns. The only difference is that a matrix can only store values of the same data type, while a dataframe can store values of different data types. For this reason, you can perform vectorized operations on dataframes just like you can on matrices.

In the code below, we multiply 1.20 directly on the dataframe, targeting the quantity column. With this operation, multiplication happens at once for all the rows of the dataframe.

sales = (pd.read_parquet(f"{Path('../../../')}/datasets/sales_total_2018.parquet")
         [['name','quantity']]
         )

# Incease qty by 20 percent (vectorized)
(sales
 .assign(Qty_plus_20_pct=sales['quantity']*1.20)
 )

	name	quantity	Qty_plus_20_pct
0	Barton LLC	39	46.8
1	Trantow-Barrows	-1	-1.2
2	Kulas Inc	23	27.6
3	Kassulke, Ondricka and Metz	41	49.2
4	Jerde-Hilpert	6	7.2
...	...	...	...
1495	Fritsch, Russel and Anderson	12	14.4
1496	Frami, Hills and Schmidt	37	44.4
1497	Stokes LLC	14	16.8
1498	Pollich LLC	3	3.6
1499	Will LLC	38	45.6

1500 rows × 3 columns

Timing

Now that I’ve proved that all three versions of the code above produce the same dataframe, let me test the speed at which each piece of code runs the mathematical operation.

1. With normal looping

%%timeit
sales = (pd.read_parquet(f"{Path('../../../')}/datasets/sales_total_2018.parquet")
         [['name','quantity']]
         )

for i in range(len(sales)):
    sales.at[i, 'Qty_plus_20_pct'] = sales.at[i, 'quantity'] * 1.20

sales

15.2 ms ± 27.2 μs per loop (mean ± std. dev. of 7 runs, 100 loops each)

2. With apply

%%timeit
sales = (pd.read_parquet(f"{Path('../../../')}/datasets/sales_total_2018.parquet")
         [['name','quantity']]
         )

# Function to increase quantity by 20%
def add_20_percent(x):
    return x * 1.20

sales['Qty_plus_20_pct'] = sales['quantity'].apply(add_20_percent)

sales

736 μs ± 2.62 μs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)

3. With vectorizaiton

%%timeit
sales = (pd.read_parquet(f"{Path('../../../')}/datasets/sales_total_2018.parquet")
         [['name','quantity']]
         )

# Incease qty by 20 percent (vectorized)
(sales
 .assign(Qty_plus_20_pct=sales['quantity']*1.20)
 )

650 μs ± 1.06 μs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)

I’ve proved that the vectorized operation runs faster. You may think the difference in speed is negligible, but that’s only because this is a small dataset. On a massive dataset the difference becomes noticeable.

Conclusion

When working with dataframes or tabular data, it’s crucial to use vectorized operations for mathematical computations. This approach yields faster results and lowers computational costs.

For string (text) data, however, vectorized operations may not always be feasible. In such cases, opt for the apply function instead of looping through the dataframe, as looping can be inefficient and slow.

To learn how to efficiently manipulate data, check out my Polars course. Polars is a new dataframe library that’s faster than pandas.