Arrays Predictions (10)

← Module home · Predict on paper — draw the boxes and fill an index-column trace — then check the answers.

Questions

P1. (★)

int a[4] = {2};
for (int i = 0; i < 4; i = i + 1) std::cout << a[i] << ' ';

P2. (★)

int a[5] = {10, 20, 30, 40, 50};
for (int i = 4; i >= 0; i = i - 1) std::cout << a[i] << ' ';

P3. (★)

int a[6] = {4, 8, 15, 16, 23, 42};
std::cout << a[1] + a[4] << ' ' << a[a[0]] << '\n';

P4. (★)

int a[3] = {1, 2, 3};
for (int i = 0; i <= 3; i = i + 1) std::cout << a[i] << ' ';
std::cout << "\n(done)";

P5. (★★)

int a[5] = {3, 7, 2, 7, 9};
int best = 0;
for (int i = 1; i < 5; i = i + 1) {
    if (a[i] > a[best]) best = i;
}
std::cout << best << ' ' << a[best] << '\n';

P6. (★★)

int a[5] = {1, 2, 3, 4, 5};
int total = 0;
for (int i = 0; i < 5; i = i + 1) {
    if (a[i] % 2 == 0) continue;
    total += a[i];
}
std::cout << total << '\n';

P7. (★★)

int a[4] = {5, 6, 7, 8};
void twice(int data[]) {
    for (int i = 0; i < 4; i = i + 1) data[i] *= 2;
}
// ... in main: twice(a); then print a[0..3]

P8. (★★)

int tally[4] = {0};
int answers[6] = {1, 3, 2, 3, 1, 3};
for (int i = 0; i < 6; i = i + 1) tally[answers[i]] += 1;
for (int k = 0; k < 4; k = k + 1) std::cout << tally[k] << ' ';

P9. (★★)

int a[4] = {4, 8, 15, 16};
for (int i = 1; i < 4; i = i + 1) {
    a[i] = a[i] + a[i - 1];
}
for (int i = 0; i < 4; i = i + 1) std::cout << a[i] << ' ';

P10. (★★★)

int g[2][3] = { {1, 2, 3}, {4, 5, 6} };
int t = 0;
for (int c = 0; c < 3; c = c + 1) {
    for (int r = 0; r < 2; r = r + 1) {
        t += g[r][c];
    }
    std::cout << t << ' ';
}

Answers

A1. int a[4] = {2} fills box 0 with 2 and the rest with 0 — the one shortcut that does more than the first element. Output: 2 0 0 0 .

A2. Backward traversal from index 4 down to 0. Output: 50 40 30 20 10 .

A3. a[1] + a[4] = 8 + 23 = 31. a[a[0]] = a[4] = 23 (data-as-index: box 0’s value picks the box). Output: 31 23.

A4. The loop visits 0, 1, 2, 3 — and 4, one past the end. It prints 1 2 3 then whatever garbage sits at a[4] (or crashes — undefined). Then (done). The D1 bug in prediction form.

A5. Champion-by-index: a[0]=3 seeds; 7 > 3 → best 1; 2 < 7; 7 not > 7 → stays 1 (strict comparison, first winner kept); 9 > 7 → best 4. Output: 4 9 — the index 4, then the value 9.

A6. The continue skips evens; odds 1+3+5 = 6. Iteration P6’s filter, walking boxes.

A7. twice received a handle — the caller’s boxes double in place. Output: 10 12 14 16 . The handle rule in one call.

A8. tally[1] counts the 1s (2), tally[2] the 2 (1), tally[3] the 3s (3), tally[0] nothing (0). Output: 0 2 1 3 .

A9. Running-total in place: a[1] = 8+4 = 12; a[2] = 15+12 = 27 (the updated a[1]!); a[3] = 16+27 = 43. Output: 4 12 27 43 . The forward in-place dependency — D9’s mirror image.

A10. Column-major visit order (columns frozen in the outer loop): column 0 adds 1+4 = 5 → prints 5; column 1 adds 2+5 → t = 12 → prints 12; column 2 adds 3+6 → t = 21 → prints 21. Output: 5 12 21 . Two lessons: the frozen variable selects the line, and t was never reset — each print includes all previous columns (gallery M2 disguised as a column-sum question; a true per-column sum resets t inside the outer loop).

Scoring yourself: 9–10 — you trace arrays like tables, which is the point. 6–8 — reread the wrong ones’ index column, box by box. Below 6 — redo Lesson 1 §4’s dry run by hand for P1–P4 before moving on.