Unit 2 — Control Statements

Topics Covered

Syllabus topics: Decision-making statements — if, if-else, else-if ladder, switch. Loop control statements — while, for, do-while. Jump control statements — break, continue, goto.

This is the shortest unit and the easiest to score on. Almost every question is either "write a program to…" or "what is the output of…". Both reward practice rather than memorisation.

2.1 Decision making

if

if (marks >= 40)
    printf("Pass\n");

if-else

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

else-if ladder

Conditions are tested top to bottom; the first true one runs and the rest are skipped.

if (marks >= 90)       grade = 'A';
else if (marks >= 75)  grade = 'B';
else if (marks >= 60)  grade = 'C';
else if (marks >= 40)  grade = 'D';
else                   grade = 'F';

Order matters. Reverse this ladder — testing >= 40 first — and every passing student gets a D, because the first condition is true for all of them.

Nested if

if (age >= 18) {
    if (has_id)
        printf("Allowed\n");
    else
        printf("ID required\n");
}

The dangling else. An else binds to the nearest unmatched if, not to the one your indentation suggests:

if (a > 0)
    if (b > 0)
        printf("both positive\n");
else                             /* binds to the INNER if, despite indentation */
    printf("a is not positive\n");   /* ... so this is wrong */

Always use braces on nested ifs. This is a favourite "find the error" question.

switch

switch (choice) {
case 1:
    printf("Addition\n");
    break;
case 2:
    printf("Subtraction\n");
    break;
default:
    printf("Invalid choice\n");
}

Rules — all examinable:

• The controlling expression must be an integer or character type. float and string will not compile.

• Each case label must be a constant, not a variable or a range.

• break is essential. Without it, control falls through into the next case and keeps going.

• default is optional and may appear anywhere, though convention puts it last.

• Duplicate case labels are a compile error.

Deliberate fall-through is occasionally useful:

switch (grade) {
case 'A':
case 'B':
case 'C':
    printf("Pass\n");   /* all three land here */
    break;
case 'F':
    printf("Fail\n");
}

if-else ladder vs switch

Use switch for a menu; use a ladder for marks >= 90-style ranges.

2.2 Loops

while — entry-controlled

int i = 1;              /* initialise */
while (i <= 5) {        /* test BEFORE the body */
    printf("%d ", i);
    i++;                /* update -- forget this and the loop never ends */
}

If the condition is false at the start, the body runs zero times.

for — entry-controlled, all three parts in one line

for (int i = 1; i <= 5; i++)
    printf("%d ", i);

for (initialise; condition; update). Any part may be left empty; for (;;) is an infinite loop.

do-while — exit-controlled

int i = 1;
do {
    printf("%d ", i);
    i++;
} while (i <= 5);       /* the semicolon here is REQUIRED */

The body runs at least once, because the test comes after it. That is the whole difference, and it is asked constantly.

int i = 10;
while (i < 5)   { printf("while\n");    i++; }   /* prints nothing   */
do              { printf("do-while\n"); i++; } while (i < 5);  /* prints once */

Comparison

Nested loops

for (int i = 1; i <= 4; i++) {
    for (int j = 1; j <= i; j++)
        printf("* ");
    printf("\n");
}

Output:

*
* *
* * *
* * * *

Pattern-printing questions appear in nearly every paper. The trick is always the same: the outer loop controls rows, the inner loop controls columns, and the inner loop's bound usually depends on the outer variable.

2.3 Jump statements

break

Leaves the innermost loop or switch immediately.

for (int i = 1; i <= 10; i++) {
    if (i == 5) break;
    printf("%d ", i);       /* 1 2 3 4 */
}

In nested loops, break escapes only one level.

continue

Skips the rest of this iteration and jumps to the next one.

for (int i = 1; i <= 10; i++) {
    if (i % 2 == 0) continue;
    printf("%d ", i);       /* 1 3 5 7 9 */
}

int i = 0;
while (i < 10) {
    if (i == 5) continue;   /* BUG: i never increments again */
    printf("%d ", i);
    i++;
}

goto

    if (error) goto cleanup;
    /* ... */
cleanup:
    printf("Cleaning up\n");

goto jumps to a label in the same function. Avoid it. It destroys the readability that structured programming exists to provide, and every goto can be rewritten with loops and flags. Exams ask you to define it and then explain why it is discouraged — the accepted answer cites Dijkstra's 1968 letter "Go To Statement Considered Harmful" and the resulting "spaghetti code".

The one defensible use is jumping to a single cleanup block from deep inside nested error handling, which is common in the Linux kernel.

break vs continue

int i, j;
for (i = 1; i <= 3; i++) {
    for (j = 1; j <= 3; j++) {
        if (j == 2) continue;
        if (i == 3) break;
        printf("%d%d ", i, j);
    }
}

Exam questions from this unit

Two marks

1. Differentiate while and do-while.
2. What is the purpose of break and continue?
3. Why is goto discouraged?
4. Can switch be used with a floating-point expression? Explain.

Five marks

1. Explain the switch statement with syntax and an example. What happens if break is omitted?

2. Compare entry-controlled and exit-controlled loops with examples.

3. Write a program to print the Floyd's triangle pattern.

Ten marks

1. Explain all decision-making statements in C with syntax, flowcharts and examples.

2. Explain loop control statements in C, comparing all three loops.

Mistakes that cost marks