C Programming | CodeBhavya

🔀 8. Decision Making

🎓 Concept-First Explanation

This lesson is organized in the same order a faculty member would normally teach it: meaning → purpose → syntax/model → internal working → examples → common confusion → safe use. The guided examples from the earlier lesson are preserved afterward for reinforcement.

8.1 What Is Decision Making?

🎯 A Program Sometimes Has to Choose

So far, our programs have mostly followed statements from top to bottom. Real programs cannot always do that. They need to choose a different action depending on the data.

Input / current state
        ↓
     Condition
      /      \\
   true      false
    ↓          ↓
 Action A    Action B
        \\    /
         Continue

ifRun a block only when a condition is true

if-elseChoose between two alternatives

else-ifChoose among several conditions

switchChoose among discrete constant cases

?:Choose one of two values inside an expression

gotoTransfer control to a labeled statement; use carefully

💡 Main idea

Decision making is not about memorizing if syntax. It is about controlling which path the program follows.

8.2 Conditions in C — What Counts as True?

When C evaluates a controlling expression such as the expression inside if, zero means false and any nonzero value means true.

0false

1true

-5true

100true

int x = -5;

if (x)
{
    puts("x is nonzero");
}

⚠️ Important:

A condition does not have to equal exactly 1. Any nonzero value is treated as true. Relational and logical operators commonly produce 0 or 1, but if can test other scalar expressions too.

8.3 Simple `if` — Run Something Only When Needed

Use if when there is an action that should happen only if a condition is satisfied.

if (temperature > 40)
{
    puts("High temperature");
}

Evaluate temperature > 40
          ↓
       Is it true?
       /       \\
     YES        NO
      ↓          ↓
 print message  skip block
      \          /
       ↓        ↓
       continue after if

The condition is evaluated first.
If it is nonzero, the controlled statement/block executes.
If it is zero, the block is skipped.
Execution then continues with the statement after the if.

🎯 Good habit:

Use braces even when there is only one statement. They make the intended block obvious and reduce maintenance mistakes.

8.4 `if-else` — Choose Between Two Alternatives

if (n % 2 == 0)
{
    puts("Even");
}
else
{
    puts("Odd");
}

n % 2 == 0
     ↓
  ┌──┴──┐
 true  false
  ↓       ↓
Even     Odd
  └──┬────┘
     ↓
  continue

For one execution of this statement, one branch is selected. The else block is the fallback when the if condition is false.

💡 When to use it

Use if-else when the problem naturally says “if this happens, do A; otherwise, do B.”

8.5 `else-if` Ladder — Check Conditions in Order

if (marks >= 90)
    grade = 'A';
else if (marks >= 75)
    grade = 'B';
else if (marks >= 60)
    grade = 'C';
else
    grade = 'D';

marks >= 90 ?  → A
      ↓ no
marks >= 75 ?  → B
      ↓ no
marks >= 60 ?  → C
      ↓ no
     else     → D

Conditions are evaluated from top to bottom.
The first true condition selects its branch.
Once a branch is selected, the remaining conditions are skipped.
If no condition is true, the final else executes, if present.

⚠️ Condition order matters

If a broad condition is placed before a narrower condition, the later branch may never be reached. Write conditions from the most appropriate first test to the fallback cases.

8.6 Nested `if` — A Decision Inside Another Decision

if (usernameValid)
{
    if (passwordValid)
    {
        puts("Login allowed");
    }
    else
    {
        puts("Wrong password");
    }
}

Username valid?
      ↓
   ┌──┴──┐
  NO    YES
  ↓       ↓
stop   Password valid?
          ↓
       ┌──┴──┐
      NO    YES
      ↓       ↓
 wrong     allowed

Nested decisions are useful when the second question makes sense only after the first condition succeeds.

🎯 Readability rule

Deep nesting can make code difficult to understand. When the logic becomes complicated, consider clearer conditions, early returns, or separate functions.

8.7 The Dangling `else` — A Common Beginner Trap

if (a > 0)
    if (b > 0)
        puts("both positive");
    else
        puts("b is not positive");

⚠️ Which if owns the else?

In C, an else belongs to the nearest unmatched if. Therefore the else above belongs to if (b > 0).

Braces remove the ambiguity and make the intended structure visible:

if (a > 0)
{
    if (b > 0)
        puts("both positive");
}
else
{
    puts("a is not positive");
}

💡 CodeBhavya rule:

For beginner code and production code, braces are usually the safer and clearer choice.

8.8 `switch` — Multi-Way Selection

switch is useful when one expression is compared against a set of discrete constant values.

switch (choice)
{
    case 1:
        puts("Add");
        break;

    case 2:
        puts("Delete");
        break;

    default:
        puts("Invalid choice");
}

switchExpression being tested

caseConstant value to match

breakLeave the switch

defaultRuns when no case matches

⚠️ Not for every condition

switch is not a general replacement for conditions such as marks >= 90 or age > 18 && age < 60. It is designed around matching an expression with case values.

8.9 `break` and Fall-Through in `switch`

After a matching case is entered, execution continues through following statements until a control-flow transfer such as break occurs or the switch ends.

switch (grade)
{
    case 'A':
    case 'B':
        puts("Pass with strong grade");
        break;

    default:
        puts("Other grade");
}

grade == 'A' ─┐
              ├→ same block → break → exit switch
grade == 'B' ─┘

no match ───────────────→ default

When multiple labels intentionally share one block, fall-through is useful. When it happens accidentally because a break was forgotten, it becomes a bug.

🎯 Interview point:

break exits the nearest enclosing switch or loop. It does not mean “go to the next case.”

8.10 `if-else` vs `switch` — Choose by the Problem

Use `if/else` when	Use `switch` when
Ranges or inequalities matter	One expression is matched against discrete constant values
Several variables participate in the condition	A menu, command or state maps naturally to cases
Complex logical expressions are required	Many exact-value alternatives would be clearer as cases

💡 Example

marks >= 90 is naturally an if condition. A menu choice such as 1 = Add, 2 = Delete, 3 = Exit is naturally expressed with switch.

8.11 Conditional Operator `?:` — Choose a Value

int max = (a > b) ? a : b;

condition: a > b
       ↓
   ┌───┴───┐
 true     false
  ↓          ↓
  a          b
   \        /
    → value ←

The conditional operator is an expression. It evaluates the condition and produces one of two expression results.

conditionTest to perform

?Choose the true expression

true expressionResult when condition is true

:Separates the two choices

false expressionResult when condition is false

⚠️ Don't overuse it

A short value selection can be clear with ?:. Large nested conditional expressions can become harder to read than a normal if-else.

8.12 `goto` and Labels — Unconditional Transfer

goto transfers control directly to a labeled statement in the same function.

if (resource2_failed)
    goto cleanup_resource1;

/* normal work */

cleanup_resource1:
    release_resource1();

💡 Where it can be useful

In low-level C, a carefully controlled goto is sometimes used for cleanup when several resources may need to be released on different failure paths.

⚠️ Why beginners are told to avoid it

Unstructured jumps can make control flow difficult to follow. Do not use goto merely to replace normal if, loops or functions. Understand structured control flow first.

8.13 Boundary Conditions Are Part of the Algorithm

A decision is not complete until you know what happens at the boundary values.

if (marks >= 40)
    puts("Pass");
else
    puts("Fail");

39Fail

40Pass

41Pass

The difference between > 40 and >= 40 changes the result for exactly one important boundary value.

🎯 Placement habit

Whenever you write a condition, test at least one value below, one value at, and one value above the boundary.

8.14 Decision-Design Method — Think Before Writing `if`

1. Understand the requirement
          ↓
2. Identify the possible outcomes
          ↓
3. Identify the condition for each outcome
          ↓
4. Check boundary values
          ↓
5. Choose if / if-else / else-if / switch
          ↓
6. Write the simplest clear structure
          ↓
7. Test normal + boundary + invalid cases

Step 1What decision does the problem require?

Step 2What are all possible outcomes?

Step 3What exact condition selects each outcome?

Step 4What happens at equality and boundaries?

Step 5Which C construct communicates the rule best?

Step 6Can another programmer understand it quickly?

💡 Example: student result

Requirement: marks below 40 → Fail; 40–59 → Pass; 60–74 → First Class; 75–89 → Distinction; 90+ → Outstanding.

Before coding, write the ranges clearly. Then order the conditions so every value is covered exactly once.

⚠️ Avoid “condition guessing”

Do not start writing if statements before deciding the ranges. Most decision bugs come from missing boundaries, overlapping conditions or incorrect condition order—not from the if syntax itself.

8.15 Quick Revision

📌 if → One condition

📌 if-else → Two alternatives

📌 else-if → Multiple conditions

📌 Nested if → Decision inside another decision

📌 switch → Multiple discrete choices

📌 break → Exit switch

📌 default → No case matched

📌 Always test boundary cases.

INTERACTIVE LEARNING

🎬 Decision Making — else-if Path

Follow how marks = 82 moves through an else-if ladder and selects exactly one grade.

PROGRAM TRACING

🔎 Program Tracing — Decision Making

Trace an else-if ladder and observe how C stops checking conditions after the first true branch.

8.16 Quick MCQs

Select an answer first, then click Check Answer. A correct choice becomes green. If the answer is wrong, your choice becomes red and the correct option becomes green. The explanation appears below.

PRACTICE

8.17 🎯 Practice Problems

Decision making becomes easy when you practice conditions and boundary cases. Use 💻 Solve It Yourself first, open Hint only when needed, and use Show Program after attempting the problem.

📈 Decision Making Practice Progress

Solved0 / 12

Completed with Solution0

Total Score0 / 1200

Completion0%

A problem counts as Solved when all test cases pass without opening the full solution. Problems completed after viewing the official solution are tracked separately.

1. Positive, Negative or Zero

Problem 1: Check whether a number is positive, negative or zero.

Input: One integer n.

Output: Print Positive, Negative, or Zero.

Official C Program

#include <stdio.h>

int main()
{
    int n;
    scanf("%d", &n);

    if (n > 0)
        printf("Positive");
    else if (n < 0)
        printf("Negative");
    else
        printf("Zero");

    return 0;
}

2. Even or Odd

Problem 2: Check whether a number is even or odd.

Input: One integer n.

Output: Print Even or Odd.

Official C Program

#include <stdio.h>

int main()
{
    int n;
    scanf("%d", &n);

    if (n % 2 == 0)
        printf("Even");
    else
        printf("Odd");

    return 0;
}

3. Largest of Two Numbers

Problem 3: Find the largest of two numbers.

Input: Two integers a and b.

Output: Print the larger value.

Official C Program

#include <stdio.h>

int main()
{
    int a, b;
    scanf("%d %d", &a, &b);

    if (a > b)
        printf("%d", a);
    else
        printf("%d", b);

    return 0;
}

4. Largest of Three Numbers

Problem 4: Find the largest of three numbers.

Input: Three integers a, b and c.

Output: Print the largest value.

Official C Program

#include <stdio.h>

int main()
{
    int a, b, c;
    scanf("%d %d %d", &a, &b, &c);

    if (a >= b && a >= c)
        printf("%d", a);
    else if (b >= a && b >= c)
        printf("%d", b);
    else
        printf("%d", c);

    return 0;
}

5. Pass or Fail

Problem 5: Check whether a student has passed or failed.

Input: One integer marks. Assume 40 is the pass mark.

Output: Print Pass or Fail.

Official C Program

#include <stdio.h>

int main()
{
    int marks;
    scanf("%d", &marks);

    if (marks >= 40)
        printf("Pass");
    else
        printf("Fail");

    return 0;
}

6. Grade Based on Marks

Problem 6: Print the grade based on marks.

Input: One integer marks from 0 to 100.

Output: Print Grade A, Grade B, Grade C, Grade D, or Fail.

Official C Program

#include <stdio.h>

int main()
{
    int marks;
    scanf("%d", &marks);

    if (marks >= 90)
        printf("Grade A");
    else if (marks >= 75)
        printf("Grade B");
    else if (marks >= 60)
        printf("Grade C");
    else if (marks >= 40)
        printf("Grade D");
    else
        printf("Fail");

    return 0;
}

7. Leap Year

Problem 7: Check whether a year is a leap year.

Input: One integer year.

Output: Print Leap Year or Not a Leap Year.

Official C Program

#include <stdio.h>

int main()
{
    int year;
    scanf("%d", &year);

    if (year % 400 == 0 || (year % 4 == 0 && year % 100 != 0))
        printf("Leap Year");
    else
        printf("Not a Leap Year");

    return 0;
}

8. Calculator Using switch

Problem 8: Create a calculator using switch.

Input: An integer a, an operator (+, -, *, /), and an integer b.

Output: Print the integer result, Invalid operator, or Division by zero is not allowed.

Official C Program

#include <stdio.h>

int main()
{
    int a, b;
    char op;
    scanf("%d %c %d", &a, &op, &b);

    switch (op)
    {
        case '+': printf("%d", a + b); break;
        case '-': printf("%d", a - b); break;
        case '*': printf("%d", a * b); break;
        case '/':
            if (b != 0) printf("%d", a / b);
            else printf("Division by zero is not allowed");
            break;
        default: printf("Invalid operator");
    }

    return 0;
}

9. Menu-Driven Arithmetic

Problem 9: Create a menu-driven program for addition, subtraction and multiplication.

Input: choice, a and b. Use 1 for addition, 2 for subtraction and 3 for multiplication.

Output: Print the result or Invalid choice.

Official C Program

#include <stdio.h>

int main()
{
    int choice, a, b;
    scanf("%d %d %d", &choice, &a, &b);

    switch (choice)
    {
        case 1: printf("%d", a + b); break;
        case 2: printf("%d", a - b); break;
        case 3: printf("%d", a * b); break;
        default: printf("Invalid choice");
    }

    return 0;
}

10. Eligibility with Two Conditions

Problem 10: Check whether a person is eligible based on age and another condition.

Input: Two integers: age and hasID, where hasID is 1 for yes and 0 for no.

Output: Print Eligible only when age >= 18 and hasID == 1; otherwise print Not Eligible.

Official C Program

#include <stdio.h>

int main()
{
    int age, hasID;
    scanf("%d %d", &age, &hasID);

    if (age >= 18 && hasID == 1)
        printf("Eligible");
    else
        printf("Not Eligible");

    return 0;
}

11. Vowel or Consonant

Problem 11: Find whether a character is a vowel or consonant.

Input: One alphabetic character.

Output: Print Vowel or Consonant.

Official C Program

#include <stdio.h>

int main()
{
    char ch;
    scanf(" %c", &ch);

    switch (ch)
    {
        case 'a': case 'e': case 'i': case 'o': case 'u':
        case 'A': case 'E': case 'I': case 'O': case 'U':
            printf("Vowel");
            break;
        default:
            printf("Consonant");
    }

    return 0;
}

12. Valid Triangle

Problem 12: Check whether three sides can form a valid triangle.

Input: Three integers a, b and c representing side lengths.

Output: Print Valid Triangle or Invalid Triangle.

Official C Program

#include <stdio.h>

int main()
{
    int a, b, c;
    scanf("%d %d %d", &a, &b, &c);

    if (a > 0 && b > 0 && c > 0 &&
        a + b > c && a + c > b && b + c > a)
        printf("Valid Triangle");
    else
        printf("Invalid Triangle");

    return 0;
}

8.18 Key Takeaway

🎯 Decision making is the bridge between understanding a problem and writing a program.

Remember:

Problem → Condition → Decision → Output

Master if, if-else, else-if, nested if and switch before moving to loops.

INTERVIEW PREPARATION

🎤 Decision Making — Interview Questions

1.What is the difference between if and if-else?

2.Why does the order of conditions matter in an else-if ladder?

3.When is switch preferred over if-else?

4.What happens when break is omitted in a switch case?

5.What is the role of default in switch?

6.What is a nested if statement?

7.Why are boundary cases important in decision-making programs?

8.What common mistake occurs with = and == inside conditions?

PLACEMENT TIPS

💡 Decision Making — Placement Tips

For an else-if ladder, write the most restrictive or highest-range conditions first when ranges overlap.
Always test the exact boundary values around a condition: for a pass mark of 40, test 39, 40 and 41.
Use switch for discrete menu values; use if-else for ranges, relational tests and compound logical conditions.
In output-prediction questions, trace only the branch that actually executes and remember that an else-if ladder stops at the first true condition.
When using switch, check whether fall-through is intentional before omitting break.
Keep nested decisions readable. If nesting becomes deep, simplify the conditions or split the logic into smaller functions later.

EXTRA PRACTICE

✍️ Decision Making — Extra Practice Questions

Write conditions to classify a temperature as cold, moderate, or hot.
Write an else-if ladder to classify a score into five ranges and test every boundary.
Predict the output of a switch program when one case intentionally omits break.
Rewrite a simple menu program once using if-else and once using switch, then compare readability.
Write a nested decision that allows entry only when age is sufficient and an ID is available.
Create test cases for a leap-year program that include century years such as 1900, 2000 and 2100.

🔀 8. Decision Making

8.1 What Is Decision Making?

8.2 Conditions in C — What Counts as True?

8.3 Simple if — Run Something Only When Needed

8.4 if-else — Choose Between Two Alternatives

8.5 else-if Ladder — Check Conditions in Order

8.6 Nested if — A Decision Inside Another Decision

8.7 The Dangling else — A Common Beginner Trap

8.8 switch — Multi-Way Selection

8.9 break and Fall-Through in switch

8.10 if-else vs switch — Choose by the Problem

8.11 Conditional Operator ?: — Choose a Value

8.12 goto and Labels — Unconditional Transfer

8.13 Boundary Conditions Are Part of the Algorithm

8.14 Decision-Design Method — Think Before Writing if

8.15 Quick Revision

🎬 Decision Making — else-if Path

🎬 Grade Decision Visualizer

🔎 Program Tracing — Decision Making

8.16 Quick MCQs

8.17 🎯 Practice Problems

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8.18 Key Takeaway

🎤 Decision Making — Interview Questions

💡 Decision Making — Placement Tips

✍️ Decision Making — Extra Practice Questions

8.3 Simple `if` — Run Something Only When Needed

8.4 `if-else` — Choose Between Two Alternatives

8.5 `else-if` Ladder — Check Conditions in Order

8.6 Nested `if` — A Decision Inside Another Decision

8.7 The Dangling `else` — A Common Beginner Trap

8.8 `switch` — Multi-Way Selection

8.9 `break` and Fall-Through in `switch`

8.10 `if-else` vs `switch` — Choose by the Problem

8.11 Conditional Operator `?:` — Choose a Value

8.12 `goto` and Labels — Unconditional Transfer

8.14 Decision-Design Method — Think Before Writing `if`