Weekly Quizzes — W09 to W12

10 questions each · ~15 min each · attempt ALL ten before opening the key · ← W05–W08 · Hub · W13–W16 →

W09 — Arrays & vectors (Week 9)

Q1. [Easy] int a[5]; creates…

a) five ints named a0…a4
b) one array of five ints, indices 0–4
c) an array of six ints, indices 0–5
d) a resizable list

Q2. [Easy] The first element of int a[4] is…

a) a[1]
b) a[0]
c) a[4]
d) a

Q3. [Easy] Which is true about vector<int> compared to a C array?

a) it cannot hold ints
b) it can grow at runtime and remembers its own size
c) it is always slower and larger
d) it has no indices

Q4. [Medium] What does this print?

int a[3] = {10, 20, 30};
cout << a[1] + a[2];

a) 30
b) 50
c) 60
d) an error

Q5. [Medium] What happens with int a[3]; cout << a[3];?

a) prints 0
b) out-of-bounds access — undefined behavior (no runtime check)
c) compile error
d) prints the size

Q6. [Medium] What does this print?

vector<int> v;
v.push_back(4);
v.push_back(9);
cout << v.size() << " " << v[0];

a) 2 4
b) 2 9
c) 1 4
d) 0 4

Q7. [Medium] Which range-based loop prints every element of vector<int> v?

a) for (int x : v) cout << x;
b) for (v : int x) cout << x;
c) for (int i : 0..v.size()) cout << v[i];
d) foreach (x in v) cout << x;

Q8. [Medium] What does this print?

int a[4] = {5, 2, 8, 1};
int m = a[0];
for (int i = 1; i < 4; i++)
    if (a[i] > m) m = a[i];
cout << m;

a) 1
b) 5
c) 8
d) 16

Q9. [Hard] What does this print?

vector<int> v = {1, 2, 3};
for (int i = 0; i <= v.size(); i++)
    cout << v[i] << " ";

a) 1 2 3
b) 1 2 3 then one garbage/out-of-bounds value — the classic <= bug
c) a compile error
d) nothing

Q10. [Hard] Why do functions receiving C arrays also receive the length — void f(int a[], int n)?

a) style only
b) the array parameter decays to a pointer; the function cannot know its size
c) arrays are copied without their metadata
d) C++ requires n as a reserved keyword

W09 — Answer key

Q1 — b. One object, five same-type elements, indexed 0–4. Declaration size is fixed forever — hence vector later.

Q2 — b. Array indices start at 0; the last of four is a[3].

Q3 — b. vector manages its own dynamic storage — push_back grows it, size() reports it. That management is exactly what C arrays lack.

Q4 — b. a[1] is 20, a[2] is 30 → 50. Index arithmetic before anything else.

Q5 — b. a[3] is past the end. C++ performs no bounds checking on raw arrays — the access compiles and misbehaves at runtime. This is the module’s central safety lesson.

Q6 — a. Two push_backs → size 2; v[0] is the first pushed value, 4.

Q7 — a. for (int x : v) binds x to each element in turn. The syntax in (b) and (c) does not exist; (d) is another language.

Q8 — c. Seed-from-first max scan → 8. The same three-line pattern will become max_element in the STL module.

Q9 — b. i <= v.size() touches v[3] — one past the last valid index (2). Range-for exists precisely so this off-by-one cannot be written.

Q10 — b. Arrays decay to a pointer to their first element when passed; the size is not part of what arrives. Hence the length parameter — the pointer-plus-count convention used across all C-family APIs.

W10 — Searching & sorting (Week 10)

Q1. [Easy] Linear search on n elements examines, worst case,…

a) about half the elements
b) all n elements
c) log₂ n elements
d) one element

Q2. [Easy] Binary search’s essential precondition is…

a) the array is small
b) the array is sorted
c) the array holds ints
d) the array is in a file

Q3. [Easy] Which sort repeatedly swaps adjacent out-of-order pairs?

a) selection sort
b) bubble sort
c) insertion sort
d) binary search

Q4. [Medium] Binary search on a sorted array of 16 elements examines at most…

a) 16
b) 8
c) 5 (⌈log₂ 16⌉ + 1 = 5 probes)
d) 1

Q5. [Medium] Searching for 7 in [2, 5, 8, 12, 16], binary search first compares 7 against…

a) 2
b) 5
c) 8 (the middle element)
d) 16

Q6. [Medium] One pass of selection sort on [4, 1, 3, 2] (ascending, select-min) produces…

a) [1, 4, 3, 2] — the minimum swapped into position 0
b) [1, 2, 3, 4] — fully sorted in one pass
c) [4, 1, 3, 2] — one pass changes nothing
d) [2, 1, 3, 4]

Q7. [Medium] What does this linear search return for key 9?

int a[5] = {3, 9, 1, 9, 4};
int pos = -1;
for (int i = 0; i < 5; i++)
    if (a[i] == key) { pos = i; break; }

a) 3 — the last match
b) 1 — the first match, thanks to the break
c) 9
d) −1

Q8. [Medium] Bubble sort’s early-exit optimization stops when…

a) the array is half sorted
b) a full pass completes with zero swaps
c) the first element is the smallest
d) the counter overflows

Q9. [Hard] What does this binary search print?

int a[5] = {2, 5, 8, 12, 16};
int lo = 0, hi = 4, key = 10;
while (lo <= hi) {
    int mid = (lo + hi) / 2;
    if (a[mid] == key) { cout << "at " << mid; return 0; }
    if (key < a[mid]) hi = mid - 1; else lo = mid + 1;
}
cout << "missing";

a) at 3
b) missing — 10 is not present
c) at 4
d) infinite loop

Q10. [Hard] Sorting student records by marks descending, then name ascending on ties, needs…

a) two separate sorts, one after the other, on separate arrays
b) a composite comparison: marks first, name as tie-breaker — on the records themselves, so swaps keep fields together
c) sorting only the marks array
d) binary search

W10 — Answer key

Q1 — b. Linear search stops early only when lucky; the worst case (absent key) examines all n. That linearity is its defining cost.

Q2 — b. Halving the search space is only valid if the middle comparison tells you which half to discard — which requires order. Unsorted input makes binary search silently wrong.

Q3 — b. Bubble sort’s pass compares neighbors and swaps inversions, bubbling the largest to the end each pass.

Q4 — c. Each probe halves the space: 16→8→4→2→1 — at most 5 probes. The logarithm in action; 1,000,000 elements need ≤ 20.

Q5 — c. lo=0, hi=4 → mid=2 → a[2]=8. The first comparison is always against the middle; 7 < 8 discards the right half immediately.

Q6 — a. Pass 1 selects the minimum (1) and swaps it with position 0. One pass fixes one position — that is selection’s contract.

Q7 — b. The break returns the first match. Without the break, pos would end at 3 — the last. Sentinel-initialized to −1 means “absent.”

Q8 — b. Zero swaps in a pass means every adjacent pair is ordered — proof of sortedness. This is bubble’s one adaptive virtue.

Q9 — b. Trace: mid=2 (8<10→lo=3), mid=3 (12>10→hi=2), lo>hi → exit → missing. The loop terminates because the window shrinks every pass.

Q10 — b. Composite comparisons — primary key, tie-breaker — on records: one swap moves all fields together. Sorting parallel arrays separately destroys the record alignment.

W11 — Strings & text processing (Week 11)

Q1. [Easy] Which reads a full line including spaces?

a) cin >> s;
b) getline(cin, s);
c) cin.get(s);
d) s.read();

Q2. [Easy] For string s = "Hello";, s.size() is…

a) 4
b) 5
c) 6
d) 0

Q3. [Easy] s[0] for s = "Hello" is…

a) 'H'
b) "H"
c) 'e'
d) 0

Q4. [Medium] What does this print?

string a = "data", b = "base";
cout << a + b;

a) data base
b) database
c) datab
d) an error

Q5. [Medium] What does this print?

string s = "banana";
cout << s.substr(1, 3);

a) ana
b) ban
c) nan
d) ana — hmm, check: index 1, length 3 → characters at 1, 2, 3

Q6. [Medium] Comparing strings with == compares…

a) their addresses
b) their contents, character by character
c) their lengths only
d) their first characters only

Q7. [Medium] What does this print?

string s = "hello";
s[0] = 'H';
cout << s;

a) hello
b) Hello — strings are mutable, index writes allowed
c) an error
d) H

Q8. [Medium] Which counts vowels in s (lowercase input)?

a) loop with if (s[i] == 'a' || 'e' || ...) — this is the trap
b) loop with if (s[i]=='a'||s[i]=='e'||s[i]=='i'||s[i]=='o'||s[i]=='u')
c) s.count("aeiou")
d) vowel(s)

Q9. [Hard] What does this print?

string s = "abc";
for (size_t i = 0; i < s.size(); i++)
    cout << (char)toupper(s[i]);
cout << "|" << s;

a) ABC|abc — toupper built output without touching s
b) ABC|ABC — s was modified
c) abc|ABC
d) ABC|

Q10. [Hard] After int n; cin >> n; getline(cin, line); with input 42 then hello world, line is…

a) hello world
b) "" — the getline consumed the newline left by >>
c) 42
d) undefined

W11 — Answer key

Q1 — b. getline reads to the newline and consumes it. >> stops at the first space.

Q2 — b. Five characters. size() returns the count; the last valid index is 4.

Q3 — a. Indexing yields a char — single quotes in the answer, and the first index is 0.

Q4 — b. + concatenates: no space is inserted unless you write one.

Q5 — a. substr(1, 3) = 3 characters starting at index 1: a, n, a. The second argument is a length, not an end index — the standard confusion.

Q6 — b. std::string’s == is value equality — the content. (C-style char* comparison compares pointers — a Week-11 contrast.)

Q7 — b. std::string is mutable: writing s[0] replaces the character.

Q8 — b. Each comparison must name s[i]. Option (a) parses as (s[i] == 'a') || 'e' || ... — 'e' is truthy, so every character “counts.” The most reused bug in student text-processing code.

Q9 — a. toupper(s[i]) returns a transformed value; unless it is assigned back, s is untouched. Transform-to-output vs modify-in-place are different programs.

Q10 — b. The mixing trap, third appearance: >> leaves the newline, getline is satisfied by it instantly. One cin.ignore() between them fixes it — now by reflex, one hopes.

W12 — File I/O (Week 12)

Q1. [Easy] Which opens a file for reading?

a) ofstream in("f.txt");
b) ifstream in("f.txt");
c) fstream out("f.txt");
d) open(f.txt);

Q2. [Easy] ofstream out("log.txt", ios::app); means…

a) create empty, discard old contents
b) append: new writes land after the existing content
c) read the log
d) binary mode

Q3. [Easy] After opening a file, the first thing to check is…

a) its size
b) whether the open succeeded — if (!in) ...
c) the last line
d) nothing; opens always succeed

Q4. [Medium] The read loop while (in >> x) { ... } stops when…

a) x is 0
b) the stream fails — end of data or bad content
c) x is negative
d) 100 iterations pass

Q5. [Medium] What does getline(in, line) return as a loop condition?

a) the line read
b) the stream — true until failure (e.g. end of file)
c) the line’s length
d) always true

Q6. [Medium] Writing with out << 42 << "\n"; produces…

a) the characters 42 followed by a newline — text formatting just like cout
b) the binary number 42
c) nothing until close
d) an error without flush

Q7. [Medium] marks.txt holds Ayesha 88\nBilal 72\n. What does this print?

ifstream in("marks.txt");
string name; int m;
while (in >> name >> m)
    cout << name << "-" << m << " ";

a) Ayesha-88 Bilal-72
b) Ayesha Bilal
c) 88 72
d) nothing — >> cannot read lines

Q8. [Medium] A program should read scores.txt, compute the average, then overwrite the file with the average. The safe sequence is…

a) open for read and write simultaneously and interleave
b) read fully (compute), close, reopen with ofstream (truncating) and write
c) append the average to avoid loss
d) it is impossible

Q9. [Hard] ifstream in("ghost.txt"); if (!in) { ... } — the check exists because…

a) opening a missing file throws an exception by default
b) a failed open leaves the stream in a fail state; reads do nothing and the program would silently “succeed” with empty data
c) the compiler detects missing files
d) Windows requires it

Q10. [Hard] A CSV line Ali Raza,88 is read with getline(in, line). Extracting the marks requires…

a) in >> marks — it will skip to the number
b) splitting on the comma: find(',') + substr + stoi — because the name contains a space, >>-based splitting is unsafe
c) atoi(line)
d) binary mode

W12 — Answer key

Q1 — b. ifstream = input file stream (reading); ofstream = output (writing). The names are the mnemonic.

Q2 — b. ios::app positions every write at the end, preserving prior content. Plain ofstream out("f") truncates — the data-loss footgun.

Q3 — b. Stream state after construction tells you whether the open worked. The missing-file program that “runs” is the silent failure this check prevents.

Q4 — b. The extraction operator converts into a boolean: success. Failure — EOF or garbage — ends the loop. Read-until-fail is the file twin of the sentinel loop.

Q5 — b. getline returns the stream reference; in a condition it converts to success/failure. The line itself arrives in the second argument.

Q6 — a. Output streams format values to text exactly as cout does — the same operators, different destination.

Q7 — a. >> skips whitespace including newlines, so name/marks pairs read across line boundaries until the data ends.

Q8 — b. Read-then-close-then-rewrite: one mode at a time, with the data safely in memory before the destructive truncating open. Interleaved read/write on the same stream is an advanced, error-prone pattern.

Q9 — b. C++ streams fail quietly. The if (!in) guard converts “no file” from silent-empty into an explicit, handled event.

Q10 — b. Fields with spaces force line-grain reading and explicit splitting. stoi converts the marks substring; finding one comma when names contain commas is the stated extension (hint: rfind).

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