What are Decision Trees in C Programming ? See Examples

In programming, especially when working with C, we often deal with complex decision-making situations. These decisions involve choosing an execution path based on multiple conditions. To make this process simpler and error-free, programmers use Decision Trees and Decision Tables before writing the actual code.

These tools help visualize logic, handle multiple conditions clearly, and reduce mistakes in conditional branching.

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1. What Are Decision Trees?

A Decision Tree is a graphical representation of decisions and their possible outcomes. It uses a tree-like structure where each node represents a condition or decision, and each branch represents the result of that decision.

Why Use Decision Trees?

• To handle complex nested conditions
• To visualize multi-level decision-making
• To avoid confusion in if-else chains
• To check all possible paths easily

Decision trees are especially useful when many conditions depend on each other.

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2. Structure of a Decision Tree

A decision tree contains:

• Root Node: The first main decision
• Branches: Yes/No or True/False paths
• Internal Nodes: Sub-decisions
• Leaf Nodes: Final outcome or action

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3. Example of a Decision Tree in C Programming

Problem:

Write a program that decides if a person is eligible to vote and if they need a voter ID.

Logic:

• Must be 18 or older to vote.
• If age ≥ 18, check if they have a voter ID.

Decision Tree Diagram (Text Form)

                     [Start]
                        |
                 Is age >= 18?
                 /            \
              Yes              No
              |                |
   Do you have voter ID?     [Not Eligible]
         /       \
      Yes         No
      |           |
 [Eligible]   [Apply for ID]

Equivalent C Code

#include <stdio.h>

int main() {
    int age, id;

    printf("Enter age: ");
    scanf("%d", &age);

    if (age >= 18) {
        printf("Do you have a voter ID? (1 for Yes, 0 for No): ");
        scanf("%d", &id);

        if (id == 1)
            printf("Eligible to vote.");
        else
            printf("Eligible but must apply for Voter ID.");
    } else {
        printf("Not eligible to vote.");
    }
    return 0;
}

Decision trees help plan these nested decisions clearly.

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4. What Are Decision Tables?

A Decision Table is a structured way to represent complex logic using rows and columns.
It lists all possible conditions and the resulting actions in a tabular format.

Why Use Decision Tables?

• Ensures no combination of conditions is missed
• Best for business logic or rule-based programs
• Avoids logical duplication
• Easy to read and maintain

Decision tables are especially useful when multiple conditions can occur together.

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5. Structure of a Decision Table

A decision table consists of:

• Condition Stub: List of conditions
• Condition Entries: True/False or values
• Action Stub: List of possible actions
• Action Entries: What action happens when conditions are met

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6. Example of a Decision Table in C

Problem:

Determine electricity bill discount based on two conditions:

1. Customer Type (Domestic / Commercial)
2. Units Consumed (>500 or ≤500)

Decision Table

Condition / Rule	Rule 1	Rule 2	Rule 3	Rule 4
Customer Type = Domestic	T	T	F	F
Units > 500	T	F	T	F
Action
Discount = 20%	Yes	No	No	No
Discount = 10%	No	Yes	Yes	No
No Discount	No	No	No	Yes

Interpretation of Rules

• Rule 1: Domestic + Units > 500 → 20% Discount
• Rule 2: Domestic + Units ≤ 500 → 10% Discount
• Rule 3: Commercial + Units > 500 → 10% Discount
• Rule 4: Commercial + Units ≤ 500 → No Discount

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C Code Implementation

#include <stdio.h>

int main() {
    char type;
    int units;
    float discount = 0;

    printf("Enter Customer Type (D/C): ");
    scanf(" %c", &type);

    printf("Enter Units Consumed: ");
    scanf("%d", &units);

    if (type == 'D' || type == 'd') {            // Domestic
        if (units > 500)
            discount = 20;
        else
            discount = 10;
    }
    else if (type == 'C' || type == 'c') {       // Commercial
        if (units > 500)
            discount = 10;
        else
            discount = 0;
    }

    printf("Applicable Discount: %.2f%%", discount);
    return 0;
}

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7. Decision Trees vs Decision Tables

Feature	Decision Tree	Decision Table
Visual Form	Diagram (tree structure)	Table (rows & columns)
Best For	Step-by-step decisions	Multiple combined conditions
Detection Of	Flow errors	Missing condition combinations
Easy For	Beginners	System analysts

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8. When to Use What?

Use a Decision Tree When:

• Decisions occur one after another
• There are hierarchical “if-else” conditions
• You want a clear visual of the flow

Use a Decision Table When:

• Multiple conditions interact simultaneously
• You need to test all combinations
• You want to avoid missing edge cases

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Conclusion

Decision Trees and Decision Tables are powerful tools that make programming logic clear, structured, and error-free. They help visualize conditions, organize logic, and avoid mistakes before writing code in C. Whether you’re a student or a professional, using these methods improves problem-solving and builds better programs.

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