Conditional Control Statements

Overview — decision making & control flow

Programs are not only sequential. Conditions test expressions that are true (non-zero) or false (zero), then run the matching statements. That is the heart of control flow for decisions.

• Condition — a relational / logical expression (e.g. marks >= 40)
• Branch — the block that runs when the condition matches
• if family — flexible for ranges, compound logic, nested checks
• switch — clear when one value maps to many discrete alternatives

Simple if

A simple if runs a statement (or block) only when the condition is true. If the condition is false, that body is skipped and execution continues after the if.

Syntax:

if (condition) {
    /* statements when condition is true */
}

Example:

int n = 12;
if (n > 0)
    printf("Positive\n");

if-else

if-else chooses exactly one of two paths: the if body when true, or the else body when false.

Syntax:

if (condition) {
    /* true branch */
} else {
    /* false branch */
}

Example — even / odd:

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

Nested if

A nested if places an if inside another if (or else). Use it when a second decision depends on the first being true.

Syntax idea:

if (condition1) {
    if (condition2) {
        /* both true */
    }
}

Example:

if (age >= 18) {
    if (marks >= 40)
        printf("Eligible\n");
    else
        printf("Adult but marks low\n");
} else {
    printf("Underage\n");
}

if-else-if ladder

An if-else-if ladder checks conditions in order and runs the first true branch, then skips the rest. Ideal for mutually exclusive ranges such as grades.

Syntax:

if (condition1) {
    /* branch 1 */
} else if (condition2) {
    /* branch 2 */
} else {
    /* none matched */
}

Example — grades:

if (marks >= 90)
    printf("Grade A\n");
else if (marks >= 75)
    printf("Grade B\n");
else if (marks >= 60)
    printf("Grade C\n");
else
    printf("Grade F\n");

Comparison — when to use each if form

• Prefer else-if ladders over deep nesting for grade/slab problems
• Prefer nested if when later checks only make sense after an earlier gate
• Always use braces so structure stays clear

Switch statement — syntax, case, break, default

switch compares one integral expression against a list of constant case labels. break exits the switch; default handles values that match no case.

Syntax:

switch (expression) {
    case constant1:
        /* statements */
        break;
    case constant2:
        /* statements */
        break;
    default:
        /* optional */
        break;
}

Example — day of week:

switch (day) {
    case 1: printf("Monday\n"); break;
    case 2: printf("Tuesday\n"); break;
    case 3: printf("Wednesday\n"); break;
    case 4: printf("Thursday\n"); break;
    case 5: printf("Friday\n"); break;
    case 6: printf("Saturday\n"); break;
    case 7: printf("Sunday\n"); break;
    default: printf("Invalid day\n"); break;
}

Switch case rules (quick table)

Boolean expressions — relational & logical operators

Conditions use relational operators (== != < > <= >=) and logical operators (&& || !). In C, zero is false; any non-zero value is true.

• if (a > 0 && a < 100) — both must be true (short-circuit)
• if (ch == 'y' || ch == 'Y') — either is enough
• if (!done) — true when done is zero
• Prefer == for equality; never confuse with assignment =

The dangling else problem

An else always binds to the nearest unmatched if. Without braces, that can surprise you:

if (a)
    if (b)
        printf("both\n");
else
    printf("which if?\n");  /* belongs to if (b) */

Fix with braces so intent is unmistakable:

if (a) {
    if (b)
        printf("both\n");
} else {
    printf("a was false\n");
}

Nested if vs else-if ladder — clarity

• else-if ladder — exclusive alternatives; only one branch runs; easy to scan
• Nested if — later tests depend on earlier success; structure mirrors hierarchy
• Deep nesting without ladders often means you should flatten into else-if or extract functions
• For grades/slabs, ladders almost always read better than nested ifs

Switch fall-through — intentional vs accidental

Without break, control falls through to the next case. That can be intentional (shared logic) or accidental (bug).

Intentional fall-through:

switch (code) {
    case 1:
    case 2:
    case 3:
        printf("Low priority\n");
        break;
    default:
        printf("Other\n");
}

Switch limitations

• Controlling expression: integral types / char / enums — not float or double
• case labels must be constant expressions (not variables)
• No ranges like case 1 ... 10: in standard C (use if-else-if for ranges)
• Strings cannot be switched directly — compare with strcmp and if/else

switch vs if-else-if — when to choose

Common mistakes

• Using = instead of == inside conditions
• Missing break in switch (unwanted fall-through)
• Semicolon after if (cond); creating an empty body
• Forgetting braces → dangling else or wrong nesting
• Trying to switch on a float or a variable case label

Optional flowchart — if-else-if grade flow

Visual path for assigning a grade from marks: