Nested structures - Step-by-Step Execution

Concept Flow - Nested structures

Define inner struct

↓

Define outer struct with inner struct inside

↓

Create outer struct variable

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Assign values to inner struct fields

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Assign values to outer struct fields

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Access and use nested fields

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Program output or further use

This flow shows how an inner structure is defined and used inside an outer structure, then how values are assigned and accessed step-by-step.

Execution Sample

C

struct Point {
    int x;
    int y;
};

struct Rectangle {
    struct Point topLeft;
    struct Point bottomRight;
};

Defines two structures: Point and Rectangle, where Rectangle contains two Points as nested structures.

Execution Table

Step	Action	Variable/Field	Value	Notes
1	Define struct Point	Point	Defined	Structure with fields x and y
2	Define struct Rectangle	Rectangle	Defined	Contains two Point structs: topLeft and bottomRight
3	Create variable rect of type Rectangle	rect	Uninitialized	Memory allocated for rect
4	Assign rect.topLeft.x = 1	rect.topLeft.x	1	Set x of topLeft point
5	Assign rect.topLeft.y = 2	rect.topLeft.y	2	Set y of topLeft point
6	Assign rect.bottomRight.x = 3	rect.bottomRight.x	3	Set x of bottomRight point
7	Assign rect.bottomRight.y = 4	rect.bottomRight.y	4	Set y of bottomRight point
8	Access rect.topLeft.x	rect.topLeft.x	1	Read value 1
9	Access rect.bottomRight.y	rect.bottomRight.y	4	Read value 4
10	End of example			All nested fields assigned and accessed

💡 All nested structure fields assigned and accessed successfully.

Variable Tracker

Variable	Start	After Step 4	After Step 5	After Step 6	After Step 7	Final
rect.topLeft.x	undefined	1	1	1	1	1
rect.topLeft.y	undefined	undefined	2	2	2	2
rect.bottomRight.x	undefined	undefined	undefined	3	3	3
rect.bottomRight.y	undefined	undefined	undefined	undefined	4	4

Key Moments - 2 Insights

Why do we write rect.topLeft.x instead of just rect.x?

Because x is a field inside the nested struct Point, which is inside rect as topLeft. The execution_table rows 4 and 8 show accessing nested fields requires specifying each level.

Can we assign values to rect.topLeft and rect.bottomRight directly?

Yes, but only by assigning a whole Point struct. Here, we assign individual fields step-by-step as shown in rows 4 to 7 for clarity.

Visual Quiz - 3 Questions

Look at the execution_table at step 6, what is the value of rect.bottomRight.x?

A 3

B 4

C 2

D undefined

💡 Hint

Check the 'Value' column at step 6 in the execution_table.

At which step is rect.topLeft.y assigned a value?

A Step 4

B Step 5

C Step 6

D Step 7

💡 Hint

Look for assignment actions to rect.topLeft.y in the execution_table.

If we tried to access rect.x directly, what would happen?

A It would give the value of rect.topLeft.x

B It would give the value of rect.bottomRight.x

C It would cause a compile error

D It would return zero

💡 Hint

Recall that x is inside nested structs, not directly inside rect; see key_moments explanation.

Concept Snapshot

Nested structures in C allow one struct to contain another.
Use dot notation to access inner fields: outer.inner.field.
Assign values step-by-step to nested fields.
Access nested fields by specifying each level.
Useful for grouping related data hierarchically.

Full Transcript

This example shows how nested structures work in C. First, we define an inner struct Point with fields x and y. Then, we define an outer struct Rectangle that contains two Points named topLeft and bottomRight. We create a variable rect of type Rectangle. We assign values to each nested field step-by-step: rect.topLeft.x, rect.topLeft.y, rect.bottomRight.x, and rect.bottomRight.y. Finally, we access these nested fields to use their values. This demonstrates how to organize data inside nested structures and how to access and assign their fields correctly.