Exercises · T03 loops

Covers: while, do-while, for, sentinel loops, nested loops, accumulators, loop invariants, tracing, input-validation loops. Lectures L09–L10 (and validation threads in L04, L30). Outcomes PF-5.1–PF-5.4.

13 exercises · ladder 🟢 → 🔴.

PF-E-027 · Count-Up

Difficulty: Beginner · Lecture: L09 · Outcomes: PF-5.1 Prerequisites: E-003 Problem: Read n (1–20). Print the numbers 1 to n, one per line, with a for loop. Input: one integer · Output: n lines. Sample: 3 → 1 / 2 / 3 Hints: for (int i{1}; i <= n; ++i).

PF-E-028 · Count-Down Timer

Difficulty: Beginner · Lecture: L09 · Outcomes: PF-5.1 Prerequisites: E-027 Problem: Read a start count (1–20). Print the countdown to 1, then print LIFT OFF. Input: one integer · Output: countdown lines + final phrase. Sample: 3 → 3 / 2 / 1 / LIFT OFF Hints: decrement the counter — don't reuse E-027's loop unchanged.

PF-E-029 · Multiplication Table (Single)

Difficulty: Beginner · Lecture: L09 · Outcomes: PF-5.1 Prerequisites: E-027 Problem: Read a number (2–12) and print its multiplication table from 1× to 12×, each line n x k = product. Input: one integer · Output: 12 lines. Sample: for 3: 3 x 1 = 3 … 3 x 12 = 36 Hints: the loop variable is the multiplier, not n.

PF-E-030 · Sum of First n Naturals

Difficulty: Beginner · Lecture: L09 · Outcomes: PF-5.2 Prerequisites: E-027 Problem: Read n (0–10,000). Print the sum 1+2+…+n using a loop (no formula). Input: one integer · Output: one integer. Sample: 10 → 55 Hints: accumulate in a variable declared before the loop.

PF-E-031 · Sentinel-Triggered Averager

Difficulty: Foundational · Lecture: L09 · Outcomes: PF-5.2, PF-5.3 Prerequisites: E-030 Problem: Read integers until the user enters -1 (which is not data). Print how many values were read and their average (2 decimals). If the sentinel arrives first, print no data. Input: integers terminated by -1 · Output: count and average, or message. Sample: 80 90 70 -1 → count: 3 / average: 80.00 Hints: priming read before the loop, update read as the last statement; guard division by zero.

PF-E-032 · Digit Sum (again, on purpose)

Difficulty: Foundational · Lecture: L09 · Outcomes: PF-5.3 Prerequisites: E-007, E-030 Problem: Read a positive integer; print the sum of its digits with a while loop. Then print how many digits it has. Input: one positive integer · Output: two integers. Sample: 907 → digit sum: 16 / digits: 3 Hints: the digit-count loop can run in the same pass — count iterations.

PF-E-033 · Digit Reversal

Difficulty: Foundational · Lecture: L09 · Outcomes: PF-5.3 Prerequisites: E-032 Problem: Read a positive integer; print it reversed (e.g. 1234 → 4321) using arithmetic only. Input: one positive integer · Output: one integer. Sample: 1234 → 4321 Hints: rev = rev * 10 + n % 10; per pass — trace it on paper first.

PF-E-034 · Do-While Age Gate

Difficulty: Foundational · Lecture: L09 · Outcomes: PF-5.1 Prerequisites: E-031 Problem: Using do-while, repeatedly read an age until it is in 3–120. Print the accepted age. Count and print how many attempts were made. Input: integers (possibly invalid first) · Output: accepted age + attempts. Sample: 0 200 45 → accepted: 45 / attempts: 3 Hints: do-while fits because the body must run at least once.

PF-E-035 · Power Table

Difficulty: Foundational · Lecture: L09 · Outcomes: PF-5.2 Prerequisites: E-029 Problem: Read a base (2–5) and exponent limit (1–10). Print base^1 through base^k, using a loop that multiplies (no <cmath>). Input: two integers · Output: k lines. Sample: base 2, k 5 → 2 4 8 16 32 (one per line) Hints: keep a running power; multiply once per pass.

PF-E-036 · Factorial with Overflow Watch

Difficulty: Intermediate · Lecture: L09 · Outcomes: PF-5.2, PF-5.4 Prerequisites: E-035 Problem: Read n (0–20). Print n! using long long. State (in a printed note) the largest n whose factorial still fits in int — determine it experimentally, then print it. Input: one integer · Output: factorial + note. Sample: 10 → 3628800 / note: 13 is the largest safe n for int Hints: 0! = 1 is a boundary case; compare long long accumulation against int for the note.

PF-E-037 · Fibonacci Sequence

Difficulty: Intermediate · Lecture: L09 · Outcomes: PF-5.2, PF-5.4 Prerequisites: E-035 Problem: Read how many Fibonacci terms to print (1–45). Print them space-separated on one line. Then print the largest term that still fits in int. Input: one integer · Output: one line of terms + a note. Sample: 8 → 1 1 2 3 5 8 13 21 Hints: two rolling variables; 45 terms fits long long but not int.

PF-E-038 · Rectangle of Stars (nested)

Difficulty: Intermediate · Lecture: L10 · Outcomes: PF-5.2, PF-5.3 Prerequisites: E-029 Problem: Read rows (1–10) and columns (1–10). Print a filled rectangle of * with that shape, then print its blank outline (border only) of the same size. Input: two integers · Output: two rectangles. Sample: 3 5 → filled 3×5 block, then outline with spaces inside. Hints: outline test: row is first/last OR column is first/last.

PF-E-039 · Prime Checker

Difficulty: Intermediate · Lecture: L10 · Outcomes: PF-5.3, PF-5.4 Prerequisites: E-036 Problem: Read n (2–1,000,000). Print prime or composite, testing divisors only up to √n (integer arithmetic, no <cmath>: test i * i <= n). Input: one integer · Output: one word. Sample: 97 → prime / 1000000 → composite Hints: break on the first divisor; 2 is the only even prime — handle it before the loop.

PF-E-040 · Right-Angled Number Triangle

Difficulty: Intermediate · Lecture: L10 · Outcomes: PF-5.3 Prerequisites: E-038 Problem: Read n (1–9). Print a triangle where row r contains the digits 1..r. Then print the mirrored version (right-aligned). Compute alignment with loop-controlled spaces, not setw. Input: one integer · Output: two triangles. Sample: n = 4 → rows 1, 1 2, 1 2 3, 1 2 3 4, then the right-aligned twin. Hints: the mirror needs a spaces loop whose count depends on n − r.

PF-E-041 · Collatz Length

Difficulty: Intermediate · Lecture: L10 · Outcomes: PF-5.3, PF-5.4 Prerequisites: E-039 Problem: Read a starting value (1–10,000). Repeatedly apply: even → n/2, odd → 3n+1, until n = 1. Print the number of steps taken and the largest value reached. Print invalid for non-positive starts. Input: one integer · Output: steps + peak, or message. Sample: 6 → steps: 8 / peak: 16 Hints: track the peak in the same loop; the sequence always reaches 1 for these inputs.

PF-E-042 · Number Pyramid with Digit Rules

Difficulty: Advanced Introductory · Lecture: L10 · Outcomes: PF-5.3, PF-5.4 Prerequisites: E-040, E-041 Problem: Read rows (1–15). Print a centered pyramid where row r contains the digits of r² reversed, separated by single spaces (e.g. row 3 → 9 1 because 3² = 91 reversed... compute r², extract digits, print in reverse order). Center by padding with (rows − r) space-pairs. Validate rows; for rows > 9 the squares are multi-digit — your digit loop must handle that. Input: one integer · Output: the pyramid. Sample: rows = 4 → 1 / 4 / 9 1 / 61 6 Hints: reuse E-033's reversal logic per row; centering is a spaces loop.