Control Statements

Programming Language & Its Applications — Introduction to C Programming, NEC licence examination syllabus (Nepal Engineering Council).

Control Statements: making the program choose

if, switch, and the dangling-else trap that has bitten every C programmer.

A program that runs the same instructions every time is a calculator, not software. Control statements let execution take different paths, and C gives you two mechanisms: if-else chains for arbitrary conditions, and switch for dispatching on a single integer value. Choosing between them matters for both readability and speed.

🌍 Where this lives: the dangling-else bug here is not a museum piece. In 2014 Apple shipped the "goto fail" bug — a duplicated line inside an unbraced if that silently skipped SSL certificate validation on every iPhone and Mac, so anyone on your network could impersonate your bank. One missing pair of braces, in exactly this pattern. Look up "Apple goto fail SSL"; it is the most expensive brace in history and it makes "always use braces" stop feeling pedantic.

The if family

if (condition) statement; if (condition) statement; else statement; if (c1) s1; else if (c2) s2; else if (c3) s3; else s4; Ternary shorthand: result = (condition) ? value_if_true : value_if_false; In C, ANY non-zero value is true. Zero is false. if (5) → always true if (0) → always false if (ptr) → true if the pointer is not NULL

grading.c

#include <stdio.h>

int main(void) {
    int marks;
    printf("Marks: ");
    if (scanf("%d", &marks) != 1) return 1;

    /* order matters: check highest first */
    if (marks >= 80)      printf("Distinction\n");
    else if (marks >= 60) printf("First division\n");
    else if (marks >= 45) printf("Second division\n");
    else if (marks >= 32) printf("Pass\n");
    else                  printf("Fail\n");

    /* the same thing as a ternary */
    printf("Result: %s\n", marks >= 32 ? "PASS" : "FAIL");
    return 0;
}
Input:  72
Output: First division
        Result: PASS

The dangling else

An else always binds to the NEAREST unmatched if. if (a > 0) if (b > 0) printf("both positive"); else printf("a is not positive"); ← WRONG! The else actually pairs with the INNER if, so this prints "a is not positive" when a>0 and b<=0 — the opposite of what the indentation suggests. FIX — use braces always: if (a > 0) { if (b > 0) printf("both positive"); } else { printf("a is not positive"); }

Indentation means nothing to a C compiler. The dangling-else bug exists precisely because programmers trust the layout instead of the braces. The habit worth forming: always write braces, even for single statements. It costs two characters and eliminates an entire class of bug.

switch

switch (expression) { /* must be int or char */ case constant1: statements; break; /* WITHOUT break, it falls through */ case constant2: statements; break; default: statements; } Restrictions: · the expression must be an integer type (int, char, enum) — NOT float, NOT string · case labels must be compile-time CONSTANTS · no duplicate case values · no ranges (case 1..5 is not standard C)

calculator.c

#include <stdio.h>

int main(void) {
    double a, b;
    char op;
    printf("Enter e.g. 12 * 4 : ");
    if (scanf("%lf %c %lf", &a, &op, &b) != 3) return 1;

    switch (op) {
        case '+': printf("%.2f\n", a + b); break;
        case '-': printf("%.2f\n", a - b); break;
        case '*': printf("%.2f\n", a * b); break;
        case '/':
            if (b == 0) printf("Cannot divide by zero\n");
            else        printf("%.2f\n", a / b);
            break;
        default:
            printf("Unknown operator '%c'\n", op);
    }
    return 0;
}
Input:  12 * 4
Output: 48.00

Deliberate fall-through

fallthrough.c

#include <stdio.h>

int main(void) {
    char grade = 'B';

    /* Several cases sharing one action - fall-through is
       intentional and useful here */
    switch (grade) {
        case 'A':
        case 'B':
        case 'C':
            printf("Passed\n");
            break;
        case 'D':
        case 'F':
            printf("Failed\n");
            break;
    }

    /* ACCIDENTAL fall-through - the classic bug */
    int day = 2;
    switch (day) {
        case 1: printf("Mon\n");      /* no break! */
        case 2: printf("Tue\n");      /* no break! */
        case 3: printf("Wed\n");
                break;
    }
    return 0;
}
Output:
Passed
Tue
Wed        <-- fell through from case 2!

Worked example — switch vs if-else efficiency

For dispatching on many discrete values, a compiler can turn switch into a JUMP TABLE: if-else chain of n cases: average comparisons = n/2, worst case = n → O(n) time switch with a jump table: compute an index, jump once → O(1) time, regardless of how many cases For 100 cases: if-else: up to 100 comparisons switch: 1 indexed jump But the jump table needs the case values to be reasonably DENSE. For sparse values (1, 500, 99999) the compiler falls back to a binary search or comparison chain. Use switch when: dispatching on one integer with several discrete values Use if-else when: conditions are ranges, involve floats, or test different variables

💡 Exam angle: the dangling else and missing break are the two favourite trap questions — expect to be given code and asked for its output. Trace it literally, ignoring the indentation. Know the switch restrictions (integer only, constant labels) and be able to state why switch can be O(1) via a jump table while if-else is O(n).

Syllabus points

if, if-else, nested if, switch

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