How Do I Make Decisions with If, Else, and Switch?

After this lesson, you will be able to:

• Write if, else if, and else chains for multi-branch decision logic
• Identify the dangling else problem and prevent it with braces
• Write switch statements with break and explain fall-through behavior
• Explain why switch cannot be used with strings in C
• Use the ternary operator (? :) for concise conditional assignments

Same Syntax, Different Dangers

Good news: C’s if/else looks almost identical to Java’s. Bad news: the differences are subtle and dangerous. C won’t stop you from writing if (x = 5) (assignment, always true) or leaving out braces and hitting the dangling else problem.

Let’s use these statements to build real logic — starting with a number classifier and ending with the water bill calculator you’ll need for Lab 4.

Branching in C

The if Statement

int temperature = 35;

if (temperature > 100)
{
    printf("Boiling!\n");
}

No surprises — same as Java. But always use braces, even for single statements:

The Dangling Else

// What does this do?
if (x > 0)
    if (x > 100)
        printf("Big positive\n");
else
    printf("Negative?\n");     // WRONG — this else belongs to the inner if!

Without braces, else attaches to the nearest unmatched if. The indentation is misleading.

The Trick: Always use braces, even for single-statement bodies. It prevents the dangling else bug and makes your code unambiguous. This is standard professional practice.

if/else if/else Chains

Classify a number:

int num;
printf("Enter a number: ");
scanf("%d", &num);

if (num > 0)
{
    printf("%d is positive\n", num);
}
else if (num < 0)
{
    printf("%d is negative\n", num);
}
else
{
    printf("%d is zero\n", num);
}

From Java: This is identical to Java syntax. The only difference is that C’s condition can be any integer expression (zero = false, non-zero = true), while Java requires a boolean.

The switch Statement

For multiple discrete values:

char grade;
printf("Enter grade (A-F): ");
scanf(" %c", &grade);      // Note: space before %c eats whitespace

switch (grade)
{
    case 'A':
        printf("Excellent!\n");
        break;
    case 'B':
        printf("Good job!\n");
        break;
    case 'C':
        printf("Satisfactory\n");
        break;
    case 'D':
    case 'F':
        printf("See your instructor\n");
        break;
    default:
        printf("Invalid grade\n");
        break;
}

Common Pitfall: Forgetting break in a switch is one of the most common C bugs. Without break, execution falls through to the next case. This is sometimes intentional (like cases ‘D’ and ‘F’ above sharing the same action) but usually a bug.

Common Pitfall: C’s switch only works on int and char values. You cannot switch on strings, floating-point values, or other non-integer types. For string comparison, use if/else if chains with strcmp.

The Ternary Operator

A compact if/else for simple assignments:

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

// Equivalent to:
int max;
if (a > b)
{
    max = a;
}
else
{
    max = b;
}

Water Bill Logic (Lab 4 Preview)

Lab 4 asks you to compute a water bill with tiered rates. The logic looks like:

double bill;

if (gallons <= 1000)
{
    bill = gallons * 0.006;
}
else if (gallons <= 2000)
{
    bill = 1000 * 0.006 + (gallons - 1000) * 0.008;
}
else
{
    bill = 1000 * 0.006 + 1000 * 0.008 + (gallons - 2000) * 0.010;
}

Each tier builds on the previous one. This is a classic if/else if pattern — and understanding how the tiers accumulate is the key to getting Lab 4 right.

Decisions Are the Same, Discipline Is Different

The syntax for decisions in C is almost identical to Java. The difference is discipline: C won’t catch = vs == mistakes, won’t enforce braces, and won’t warn about switch fall-through (without -Wall). Your safety net is consistent style — always brace, always use -Wall, always break.

Next: loops. You’ll learn for, while, and do-while — plus the input validation pattern you’ll use in every interactive program.

Big Picture: Decisions plus operators give you the ability to write programs that respond to input. Combined with loops (next lesson), you’ll have all the control flow tools needed for every lab in this course. The tiered water bill logic is the kind of real-world conditional reasoning that shows up everywhere — tax brackets, shipping rates, grade scales.