Control Flow and Loops: if-else, switch, for, while, do-while with Traced Examples · কন্ট্রোল ফ্লো ও লুপ: if-else, switch, for, while, do-while ও ট্রেস করা উদাহরণ

1. Three ways a program can flow

Statements normally run one after another, top to bottom. That is sequence. Böhm and Jacopini showed in 1966 that any program can be written with just three structures, which is the basis of structured programming:

Structure	What it does	C, C++ and Java statements
Sequence	Runs statements in order	Ordinary statements
Selection (decision)	Chooses one path by a condition	if, if-else, else-if ladder, switch, ?:
Iteration (repetition)	Repeats a block while a condition holds	for, while, do-while
Jump (transfer)	Leaves a block or loop early	break, continue, goto, return

So "for, while and do-while" are all examples of iteration, and an exam that asks for "three types of control statements" wants selection, iteration and jump (or sequence, selection and iteration, if it means the structures).

2. if, if-else and the else-if ladder

if (marks >= 80)      grade = 'A';
else if (marks >= 60) grade = 'B';
else if (marks >= 40) grade = 'C';
else                  grade = 'F';

The conditions are tested from the top, and only the first true branch runs. Rules that exams test:

• In C, any non-zero value is true and 0 is false. So if (5) and if (-1) both run, and if (printf("0")) runs because printf returns 1, the number of characters it printed.
• = is not ==. if (n = 7) assigns 7 and is always true; if (n = 0) assigns 0 and is always false. Java refuses both, because an if condition must be boolean.
• Dangling else: an else belongs to the nearest unmatched if, whatever the indentation says.

int x = 2, y = 3;
if (x > 5)
    if (y > 1) printf("A\n");
else printf("B\n");      /* belongs to "if (y > 1)" */
printf("done\n");

This prints only done: x > 5 is false, and the else is part of the inner if, so nothing else runs. Use braces to make the intent clear.

• When x can only be 0 or 1, if (x == 0) x = 1; else x = 0; is the same as x = 1 - x; (a past MCQ).
• Python uses if, elif and else with a colon, and the indentation marks the block; wrong indentation is an IndentationError.

3. switch and fall-through

switch compares one value with constant case labels:

#include <stdio.h>
int main(void) {
    int i;
    for (i = 1; i <= 3; i++) {
        printf("s%d:", i);
        switch (i) {
            case 1: printf("one ");      /* no break: falls through */
            case 2: printf("two "); break;
            default: printf("other ");
        }
        printf("\n");
    }
    return 0;
}

Output:

s1:one two
s2:two
s3:other

When i is 1, control enters at case 1, prints "one", and falls through into case 2 because there is no break; the break after "two" then leaves the switch.

Rules of switch:

• In C the controlling expression and every case label must be integer types: int, char, enum and so on. float and double cannot be used, and neither can ranges or variables as labels.
• case labels must be constant and unique. default is optional, may appear anywhere, and runs when no case matches.
• Java allows char, byte, short, int, their wrapper classes, enums and String (since Java 7), but not long, float, double or boolean; fall-through works the same. Python has no switch; Python 3.10 added match/case, which does not fall through.

Point	if-else ladder	switch
Tests	Any condition, including ranges (x > 5 && x < 10)	Equality with constant values only
Types (C)	Any scalar, including float	Integer types only
Fall-through	No	Yes, unless break
Best for	Ranges and mixed conditions	A menu of fixed choices

4. The conditional operator ?:

condition ? value_if_true : value_if_false is the only ternary operator in C, C++ and Java. It is an expression, so it can sit inside other expressions, and it groups right to left:

int a = 7, b = 12, c = 9;
int m = a > b ? (a > c ? a : c) : (b > c ? b : c);   /* m = 12 */

a > b is false, so the right part runs: b > c is true, and m = 12. Python writes the same idea as x if condition else y.

5. The three loops

Loop	Syntax	Condition tested	Minimum runs	Typical use
for	for (init; condition; update) body	Before each pass (entry-controlled)	0	A known number of passes
while	while (condition) body	Before each pass (entry-controlled)	0	Repeat until something happens
do-while	do body while (condition);	After each pass (exit-controlled)	1	Menus, input validation

• Because the body of a do-while always runs once, it suits a menu: show the menu, read a choice, repeat until the user picks "exit". This is the keyed answer to "which loop is useful for processing a menu selection".
• Note the semicolon after while (condition); in a do-while; it is required there and wrong after for (…) or while (…) (see section 9).
• With int a = 10;, while (a < 5) {…} runs 0 times, but do {…} while (a < 5); runs once.
• Any for can be written as a while: for (i = 1; i <= n; i++) sum += i; becomes i = 1; while (i <= n) { sum += i; i++; } (one difference: continue in a for still runs the update, but in this while it would skip i++; another is that a variable declared in the for header exists only inside the loop).
• Two variables can be driven at once with the comma operator: for (i = 0, j = 9; i < j; i++, j--) runs 5 times (i = 0 … 4 while j = 9 … 5).
• All three parts of a for are optional: for (;;) (or while (1)) is an infinite loop, which ends only with break, return, goto or exit(). The usual keyed answer to "what can terminate for(;;)?" is break.
• Python has for item in sequence: (often range(start, stop, step), where stop is excluded) and while condition:, but no do-while. A Python loop may have an else block that runs only if the loop was not left by break.

6. break, continue and the other jumps

Statement	Effect
break	Leaves the innermost loop or switch at once
continue	Skips the rest of this pass and starts the next one (in a for, the update still runs)
goto label	Jumps to a label in the same function (legal, but avoid it)
return	Leaves the whole function
exit(n)	Ends the whole program (from <stdlib.h>)
break label; / continue label; (Java)	Leaves, or continues, the labelled outer loop: outer: for (…) { for (…) { if (x) break outer; } }

continue belongs only in loops; inside a switch that is not in a loop it is a compile error.

for (i = 1; i <= 10; i++) {
    if (i % 2 == 0) continue;   /* skip even numbers */
    if (i > 7) break;           /* stop after 7 */
    printf("%d ", i);
}
printf("| i=%d\n", i);

Output: 1 3 5 7 | i=9. Even values are skipped by continue; at i = 9 the break runs before the printf, so i stays 9 after the loop.

7. Counting passes and tracing loops

For for (i = a; i < b; i += s) with a < b, the loop runs ⌈(b − a) / s⌉ times; with <= it runs ⌊(b − a) / s⌋ + 1 times. So for (i = 0; i < n; i++) runs n times and for (i = 1; i <= n; i++) also n times, but for (i = 0; i <= n; i++) runs n + 1 times.

Code	Passes	Value after the loop (all checked by running)
for (i = 1; i <= 100; i *= 2)	7 (i = 1, 2, 4, 8, 16, 32, 64)	i = 128
for (i = 10; i > 0; i -= 3)	4 (i = 10, 7, 4, 1)	i = −2
x = 0; while (x < 100) x += 2;	50	x = 100
for (k = 0.0; k < 3.0; k++) with double k	3	k = 3.0
for (i = 0; i < 5; i++) for (j = 0; j < 5; j++) count++;	25	count = 25
for (i = n - 1; i > -1; i--)	n, the same indexes as for (i = 0; i < n; i++), in reverse	i = −1

Worked example 1: a triangular nested loop.

int i, j, count = 0;
for (i = 0; i < 5; i++)
    for (j = 0; j < i; j++)
        count++;
printf("%d", count);

The inner loop runs i times for each i:

i	Inner passes (j = 0 … i − 1)	count afterwards
0	0	0
1	1	1
2	2	3
3	3	6
4	4	10

Output: 10 (0 + 1 + 2 + 3 + 4 = 5 × 4 / 2). In general a loop like this runs n(n − 1)/2 times.

Which is not an infinite loop? (a past MCQ, with int i = 0;)

• for (;;) {} is infinite, and while ( ) {} does not even compile (a while needs a condition).
• while (++i < 0) { --i; }: ++i makes i = 1, 1 < 0 is false, so the loop ends at once. Not infinite.
• do { ++i; } while (--i <= 0);: each pass adds 1 and takes it away again, so i stays 0 and 0 <= 0 stays true for ever. Infinite.

8. Classic loop programs

These short programs are asked again and again in written papers. All outputs below were produced by running them.

Sum of digits and reverse of a number. Take the last digit with n % 10 and drop it with n / 10:

int n = 3426, sum = 0;
while (n > 0) { sum += n % 10; n /= 10; }        /* sum = 15 */

int m = 6789, rev = 0;
while (m > 0) { rev = rev * 10 + m % 10; m /= 10; } /* rev = 9876 */

The same pattern counts how often a digit occurs (digit 2 in 12223 → 3), and repeating the digit sum until one digit is left gives the digital root (9875 → 29 → 11 → 2).

Worked example 2: primes from 1 to n. A number n ≥ 2 is prime if no d with 2 ≤ d ≤ √n divides it. Testing up to √n is enough, because if n = a × b then one of a or b is at most √n.

#include <stdio.h>
int isPrime(int n) {
    int d;
    if (n < 2) return 0;            /* 0 and 1 are not prime */
    for (d = 2; d * d <= n; d++)
        if (n % d == 0) return 0;   /* found a divisor */
    return 1;
}
int main(void) {
    int i, n = 30;
    for (i = 1; i <= n; i++)
        if (isPrime(i)) printf("%d ", i);
    printf("\n");
    return 0;
}

Output: 2 3 5 7 11 13 17 19 23 29. Tracing isPrime(97): d runs from 2 to 9 (9 × 9 = 81 ≤ 97, but 10 × 10 = 100 > 97); none of 2 … 9 divides 97, so 97 is prime. For 91, d = 7 divides it (91 = 7 × 13), so it is not prime.

Factorial, leap year and Fibonacci:

long long f = 1;
for (i = 1; i <= 5; i++) f *= i;                       /* 5! = 120 */

int leap = (y % 4 == 0 && y % 100 != 0) || y % 400 == 0;

int a = 0, b = 1, t;
while (a <= 100) { printf("%d ", a); t = a + b; a = b; b = t; }

• Leap year: 2000 and 2024 are leap years; 1900, 2023 and 2100 are not (a century year must divide by 400).
• The Fibonacci loop prints 0 1 1 2 3 5 8 13 21 34 55 89.
• Other one-loop classics: sum 1 to 100 = 5050; 1² + 2² + … + 10² = 385; 153 is an Armstrong number (1³ + 5³ + 3³ = 153); the GCD of 12 and 18 is 6 and their LCM is 12 × 18 / 6 = 36.

Worked example 3: patterns with nested loops. The outer loop counts rows; the inner loop prints that row.

for (i = 1; i <= 4; i++) {
    for (j = 1; j <= i; j++) printf("%d", j);
    printf("\n");
}

1
12
123
1234

Print "*" instead of j for a star triangle. For a centred pyramid, row i prints 3 − i spaces and then 2i − 1 stars (for 3 rows: 1, 3 and 5 stars).

9. Tricky loop cases

• A semicolon after the header makes an empty loop: for (i = 0; i < 5; i++); runs five empty passes, and the block below it then runs once, with i = 5. The same mistake after while (…) usually gives an infinite loop.
• Unsigned counters never go below 0: for (unsigned u = 3; u >= 0; u--) never ends, because after 0 the value wraps to 4,294,967,295.
• Floating-point counters: for (f = 0.0f; f != 1.0f; f += 0.1f) never ends, because 0.1 is not exact in binary and f skips past 1.0. Count with an integer, or compare with <.
• Overflow inside a loop: 13! = 6,227,020,800 is larger than the 32-bit int maximum 2,147,483,647, so a factorial loop in int breaks from 13! (signed overflow is undefined behaviour in C). long long holds up to 20! = 2,432,902,008,176,640,000 but not 21!.
• Wrong starting value: a "find the maximum" loop that starts with max = 0 fails when every element is negative; it returns 0. Start with max = a[0] (a past Java MCQ).
• An invalid header: for (int x = 10; k <= 5; x / 9) is the invalid one in a past MCQ: k was never declared. (In Java, x / 9 as the update is also an error, "not a statement"; C accepts it with a warning, but it never changes x.)
• switch (f) with a float f does not compile in C or Java, and a break inside a switch that sits inside a loop leaves only the switch, not the loop.