10.2 Control Flow

Once you can evaluate expressions, the next skill is control flow: deciding which lines execute and how often. Cyber Test items commonly show a short if ladder or a tiny loop and ask for a final counter, printed sequence, or boolean outcome. Treat every branch and every iteration as a checkpoint in your state table.

Branching: if / else

A conditional runs a block only when a boolean expression is true.

Language-agnostic pattern:

IF score >= 90 THEN
  SET grade TO "A"
ELSE IF score >= 80 THEN
  SET grade TO "B"
ELSE
  SET grade TO "C_or_below"
END IF

Python flavor:

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
else:
    grade = "C_or_below"

C++ flavor:

if (score >= 90) {
  grade = 'A';
} else if (score >= 80) {
  grade = 'B';
} else {
  grade = 'C';
}

Tracing Rules for Branches

1. Evaluate conditions top to bottom.
2. On the first true condition, run that body, then skip the remaining else if / else parts of that chain.
3. If none are true, run else when present; otherwise do nothing.

Worked example: score = 80.

• score >= 90? false
• score >= 80? true → grade becomes "B"
• The final else does not run

If the first condition had been score >= 80 alone without an else-if chain, a score of 95 would still get whatever that single true branch assigns — chain order and boundary values (>= vs >) are deliberate exam pressure points.

Nested if

An if inside another if means both conditions must succeed for the inner body to run. Draw boxes mentally: the inner box only exists when the outer box is entered.

IF user_ok THEN
  IF pin_ok THEN
    SET access TO "granted"
  ELSE
    SET access TO "bad_pin"
  END IF
ELSE
  SET access TO "unknown_user"
END IF

With user_ok = false, you never look at pin_ok; access is "unknown_user".

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Loops: while and for

A loop repeats a body while a condition holds.

while

SET i TO 0
SET total TO 0
WHILE i < 3 DO
  SET total TO total + i
  SET i TO i + 1
END WHILE

Iteration table:

Final: i = 3, total = 3. Notice the loop ran for i values 0, 1, 2 — three times — because the condition was i < 3, not i <= 3.

for

A for loop packages initialization, condition, and update. Same logic as the while above:

Python:

total = 0
for i in range(3):  # 0, 1, 2
    total += i

C++:

int total = 0;
for (int i = 0; i < 3; i++) {
  total += i;
}

range(3) and i < 3 starting at 0 are the usual "three iterations" pattern.

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Off-by-One Errors — High-Yield Trap

An off-by-one error means the loop runs one time too many or too few, or an index hits one past the last valid slot.

Worked trap: "Sum integers from 1 through 5."

Correct while:

SET s TO 0
SET i TO 1
WHILE i <= 5 DO
  SET s TO s + i
  SET i TO i + 1
END WHILE

Result s = 15. If the condition were i < 5, you would only add 1+2+3+4 = 10 — a classic wrong answer planted beside 15 on multiple-choice exams.

Another trap: counting down.

SET n TO 3
WHILE n > 0 DO
  SET n TO n - 1
END WHILE

Final n is 0. If the body decremented after a different check, or used >= 0, you might land on -1. Always ask: last value that still entered the loop, and value after the last update.

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break and continue

break example:

SET i TO 0
SET found TO -1
WHILE i < 10 DO
  IF i == 4 THEN
    SET found TO i
    BREAK
  END IF
  SET i TO i + 1
END WHILE

When i becomes 4, found is set to 4 and the loop ends. Without break, the loop would keep going to 10.

continue example (C++-style counting of odds):

int count = 0;
for (int i = 0; i < 5; i++) {
  if (i % 2 == 0) continue;
  count++;
}

For i = 0,2,4 the continue skips count++. Odds 1 and 3 increment → count = 2.

Python equivalent idea:

count = 0
for i in range(5):
    if i % 2 == 0:
        continue
    count += 1

Same result. Exam stems may ask whether a statement after continue runs on that iteration — it does not.

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Nested Loops (Brief)

An outer loop with an inner loop multiplies iterations: if outer runs 3 times and inner runs 4 times each, the inner body runs 12 times. Trace with a pair (outer, inner) in your table. A break only breaks the inner loop unless the language offers a labeled break (rarely tested at this level).

SET hits TO 0
FOR i FROM 0 TO 1 DO          # i = 0, 1
  FOR j FROM 0 TO 2 DO        # j = 0, 1, 2
    SET hits TO hits + 1
  END FOR
END FOR

hits ends at 6.

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Combined Worked Trace

SET n TO 0
FOR k FROM 1 TO 5 DO
  IF k % 2 == 0 THEN
    CONTINUE
  END IF
  SET n TO n + k
  IF n > 6 THEN
    BREAK
  END IF
END FOR

Assume FOR k FROM 1 TO 5 means k = 1,2,3,4,5.

Final n = 9. If you forget continue on evens, you would add 2 and 4 as well and hit the break earlier with a different sum — exact tracing beats intuition.

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Exam Habits

1. For every loop, list the sequence of loop-variable values that actually enter the body.
2. Boundary words matter: "through 5," "less than 5," "up to but not including."
3. On if/else if, stop at the first match.
4. break vs continue: exit entirely vs skip to next iteration.
5. Assignment vs equality still appears inside conditions — do not let loop syntax distract you from = vs ==.

Control flow sets up functions: a function body is just statements with branches and loops, entered by a call and left by a return — the topic of the next section.