P2-1.2 Translating Formulas into Code to Calculate with Data¶

Section ID: P2-1.2 Version: v2026.09.08

In P2-1.1, we treated mathematics as the language for reading AI computation. Now we look at its role in actual learning. When studying AI, formulas, code, and data are not separate things. They show the same computation in three different forms.

1. A formula compresses the computational structure.
2. Code executes the computational procedure.
3. Data provides the values to calculate with and the results to inspect.

This section shows how formulas, code, and data relate to the same calculation.

Later, even when you read formulas for means, vectors, loss, and gradients, you should be able to read together to what data this expression is applied and where it is executed in code.

Here, we organize together formula, code, data, output, and shape. If 1.1 first fixed why mathematics should be read as a computational language, here we organize how that computational language points to the same calculation inside documents, code cells, and real values.

A Shared Example to Start With¶

We bring over the same small table from the previous section.

This table can be used as the smallest scene in Chapter 1 where formulas, code, and data meet.

• As data, it is a small table with two columns, x and y.
• As a formula, it is material for writing a mean, a function, and an error.
• As code, it is an input that can be translated into lists, arrays, and variables.

What the reader easily misses here is that table, formula, and code can look like different study topics. But in practice, they are only different surfaces for reading the same scene. The purpose of this section is to fix that correspondence.

Core Criteria: Where Formulas, Code, and Data Meet¶

• You can read a formula as a compressed expression of computation.
• You can view code as the translation of that formula into an executable procedure.
• You can view data as both the values fed into the computation and the results that must be checked afterward.
• You can explain one small mean computation from the viewpoints of formula, code, and data.
• You prepare to move back and forth between formula notation and Python practice in later sections.

Three Criteria¶

This section shows the place where mathematics and programming meet. The three viewpoints that act as standards while reading are the following.

One Calculation Seen in Three Ways¶

Take the mean as the simplest example. A mean is the calculation that adds several values and divides by the count.

If you pull out only the score column from the table above, the data becomes 55, 65, 80, 90.

Written as a formula, it becomes the following.

Written as code, it becomes the following.

Problem situation:

• even for the same mean calculation, once you move from a formula to code, you may not immediately connect what execution order it unfolds into

Input:

• the list of values scores = [55, 65, 80, 90]

Expected output:

• the mean value 72.5

Concept to check:

• sum(scores) / len(scores) in code is the mean formula written out as an execution order
• even if formula, code, and data look different, they can point to the same calculation

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# scores is the quiz-score list for one student group.
scores = [55, 65, 80, 90]

# mean summarizes the score list as one representative value.
mean = sum(scores) / len(scores)

print(mean)

Example execution result:

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72.5

The three expressions look different, but they do the same thing.

• data: the score bundle 55, 65, 80, 90
• formula: the compressed structure that adds the values and divides by the count
• code: the actual execution procedure the computer follows
• output: the value checked after the computation ends

This viewpoint matters. In AI documents, a formula alone can look abstract. Code alone can look like only library usage. Data alone can look like only a list of numbers. Only when the three are connected does it become visible what is being computed.

This example is useful because the same table can later be reread through different questions. Mean score is one summary value, the relationship between x and y is a problem of function and prediction, and the gap between predicted and real values is a problem of loss. In other words, one data scene becomes the reference point that ties together many explanations in Chapter 1.

Formulas Give Names and Compress Relationships¶

The first things to look for in a formula are the names and relationships, rather than the exact shapes of the symbols.

This expression does not look complicated, but it is one of the most important basic structures in AI documents.

• x: the input
• f: the function or model that turns input into output
• y: the output

In a machine-learning context, you can read this as input data x passes through the model f and becomes output y.

If you attach it directly to the table above, x is study time and y is score. Even before there is a complex model, the thought experiment "a function that predicts a score from study time" is enough to fix the input-transformation-output structure.

In code, it may appear as the following.

Problem situation:

• even if you can read the formula y = f(x), you may not immediately feel how it appears in real code

Input:

• the input value x
• the model or function model that turns input into output

Output:

• the execution result y

Concept to check:

• the input-transformation-output relationship of the formula is expressed with the same structure in code
• the goal of this example is reading relationship expression rather than carrying out a complex calculation

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# x is the input passed into the model, and y is the output returned by it.
y = model(x)

The formula and the code do not use exactly the same grammar. But both show the same relationship: input becomes output through some transformation. Once you can read that relationship, it becomes easier to place the later ideas of model, prediction, loss, and learning.

Data Enters in a Computable Shape¶

Real-world data is usually not computed on immediately as it is. Sentences, images, tables, and logs first become computable values.

For example, the sentence "I am studying AI again" can become tokens, numeric IDs, and vectors. An image can continue into pixels, a numeric array, and a tensor. Table data can enter in the form of an array, matrix, or DataFrame based on rows and columns.

At that point, the shape of the data becomes important. Whether there is one value, a list of values, a table with rows and columns, or an array with multiple axes changes the formula and the code that will be used.

Problem situation:

• even with the same numbers, one value, a vector, and a matrix are treated as different shapes in computation

Input:

• the scalar-like array one_value
• the one-dimensional array vector
• the two-dimensional array matrix

Expected output:

• the shape of each array

Concept to check:

• data shape is the basic information that distinguishes computable structure
• NumPy shows that even the same numbers are separated by axis and dimension

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# This example uses a NumPy array shape to check how many examples and variables the input data contains.
import numpy as np

# one_value, vector, and matrix show that the same numbers can be stored in different shapes.
one_value = np.array(3)
vector = np.array([1, 2, 3])
matrix = np.array([[1, 2, 3], [4, 5, 6]])

# shape is the observation point for checking each array's axes and size.
print(one_value.shape)
print(vector.shape)
print(matrix.shape)

Example execution result:

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()
(3,)
(2, 3)

This example is not intended to teach advanced NumPy usage. It is for checking that, even with the same numbers, the meaning of the computation changes depending on the shape in which they are stored.

The study-time and score table above is the same. If you look only at the score column, it can be treated like a one-dimensional vector. But if you look at x and y together, it becomes a table structure with two columns. The later reasons for separating X and y or checking shape also ultimately come from turning what kind of question this data is for into a computable form.

Code Turns a Formula into an Executable Procedure¶

A formula shows the computational structure briefly, but real execution needs code. For example, a mean is simple as a formula.

But in code, you must decide where the data is, how many items there are, by what function they will be added, and where the result will be stored.

Problem situation:

• a mean formula with sigma is short, but in code the computation steps may need to be written in a finer-grained way

Input:

• the score list scores = [55, 65, 80, 90]

Output:

• the count n
• the total total
• the mean mean

Concept to check:

• in code, one line of a formula can unfold into several intermediate variables and execution stages
• not only the final result but also intermediate variables become observation points that help interpret the formula

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# scores is the data that the mean formula will calculate over.
scores = [55, 65, 80, 90]

# n and total are intermediate observation values before the mean is made.
n = len(scores)
total = sum(scores)
mean = total / n

print(n)
print(total)
print(mean)

Example execution result:

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4
290
72.5

Code can become longer than a formula. But code reveals the execution order. It prepares the values, counts them, adds them, divides by the count, and finally stores the result. If you look at 4, 290, and 72.5 in order, you can immediately check into what intermediate stages a one-line sigma formula unfolds in code.

Push the same scene a bit further, and later you start storing it as study_hours = [1, 2, 3, 4] and quiz_scores = [55, 65, 80, 90], separating input and output. From there, you naturally move from mean computation alone into model computation that describes a relationship.

The same thing happens in AI learning. In a paper or textbook, a loss function may appear as a short formula, but in real code the sequence unfolds into many lines: data loading, preprocessing, model execution, loss computation, and parameter update.

Results Must Be Interpreted Again¶

Understanding does not end just because a calculation is over. The output still has to be interpreted again in the context of the problem.

If the mean is 72.5, that means the center of the four students' scores lies roughly around 72.5 points.

The same is true in machine learning.

If loss is 0.2, it means that when the gap between model output and reference was computed in some specific way, the result was 0.2.

If accuracy is 90%, it means that 90% of the evaluation data were predicted correctly. But which cases were wrong must still be checked separately.

A numeric result does not explain its own meaning. You must check together what data it came from, by what formula it was computed, and what code was executed.

Connecting It to a Small Learning Flow¶

If you shrink AI learning as much as possible, you can read it as the flow of preparing data, feeding it into a model, obtaining an output, comparing it with a reference to compute loss, adjusting parameters, and repeating this process.

This is exactly the place where formula, code, and data all enter together.

• data provides the input and target
• formula expresses the model output, loss, and update direction
• code executes this calculation in real order
• results are checked as metrics, loss, and prediction

This is why Part 2 handles mathematics and Python together. If you see only formulas, execution does not appear. If you see only code, the intention of the computation can become blurry. If you see only data, it is hard to see what the model is learning.

Data Count and the Denominator of the Mean¶

The four students' scores, [55, 65, 80, 90], have a sum of 290 and a count of 4, so the mean is 72.5 points. For this data, sum(scores) / 4 and sum(scores) / len(scores) give the same result.

What happens if we add a fifth student's score of 100? The sum becomes 390, and the number of students becomes 5.

The mean for the five students is 78 points. The value 97.5 comes from adding five scores but dividing by four students. The code has performed the division, but the result is not the mean of this data.

The denominator n in the mean formula is the number of values being added. In code, len(scores) obtains that count from the current score list. When data is added, both the sum and the count must change to preserve the meaning of the formula in code.

Checklist¶

• Can you explain that formula, code, and data can express the same computation in different ways?
• Can you read y = f(x) as the relationship among input, function, and output?
• Can you explain that the shape of data affects the way a computation is carried out?
• Can you explain that code turns a formula into an executable procedure?
• Can you explain that output must be interpreted together with data, formula, code, and problem context?
• Can you explain the flow that mathematics is checked through code, code is verified with data, and the result is interpreted again by returning to the formula and problem definition?
• When formula, code, and data start looking separated, can you tie them back together as different expressions of the same computation?

Sources and References¶

• Marc Peter Deisenroth, A. Aldo Faisal, Cheng Soon Ong, Mathematics for Machine Learning, Cambridge University Press, 2020, checked on 2026-07-19.
• Ian Goodfellow, Yoshua Bengio, Aaron Courville, Deep Learning, MIT Press, 2016, checked on 2026-07-19.
• Charles R. Harris et al., Array Programming with NumPy, Nature, 2020, checked on 2026-07-19.
• NumPy Developers, numpy.ndarray.shape, NumPy User Guide, checked on 2026-07-19. This is the direct reference for explaining shape as array dimension and length information.
• scikit-learn developers, Glossary of Common Terms and API Elements, scikit-learn User Guide, checked on 2026-07-19. This is a reference for the convention of reading X as input data and y as the target.