Practice · Arrays and Two-Dimensional Arrays

Q1

Which statement correctly creates an array of 10 integers?

A int a[10] = new int[]; B int[] a = new int[10]; C int[] a = new int(10); D array int a = 10;

Q2

What is the valid index range of an array created with new double[8]?

A 1 to 8 B 0 to 8 C 0 to 7 D 1 to 7

Q3

What is the default value of every element of new boolean[4]?

A true B false C 0 D null

Q4

What is the default value of every element of new String[4]?

A "" B null C "null" D It does not compile.

Q5

How do you obtain the number of elements of an array a?

A a.length() B a.length C a.size() D length(a)

Q6

What happens at run time when a program reads a[a.length]?

A It returns the last element. B An ArrayIndexOutOfBoundsException is thrown. C It returns 0. D The code does not compile.

Q7

Which declaration is not legal Java?

A int[] a = {1,2,3}; B int a[] = {1,2,3}; C int[] a = new int[]{1,2,3}; D int[] a; a = {1,2,3};

Q8

What is the output?

int[] a = {3, 6, 9};
int[] b = a;
b[1] = 99;
System.out.print(a[1]);

A 6 B 99 C 0 D It does not compile.

Q9

Which statement really copies the contents of a into a new array?

A int[] b = a; B int[] b = Arrays.copyOf(a, a.length); C int[] b = new int[a.length]; D int[] b = a.length;

Q10

What is the output of the following Java program?

public class TestArrayCopy {
    public static void main(String[] args) {
        int[] src  = {10, 20, 30, 40, 50};
        int[] dest = {1, 2, 3, 4, 5};
        System.arraycopy(src, 1, dest, 2, 2);
        for (int i : dest) System.out.print(i + " ");
    }
}

A 1 2 20 30 5 B 1 2 3 20 30 C 10 20 3 4 5 D 1 2 20 30 40

Q11

What is the output?

int[] a = {2, 4, 6, 8};
int sum = 0;
for (int x : a) sum += x;
System.out.print(sum);

A 4 B 20 C 8 D 0

Q12

Which task can a for-each loop not perform on an int[]?

A Printing every element. B Assigning new values to the array's elements. C Computing a total. D Finding the largest element.

Q13

What is printed?

int[] a = new int[4];
a[2] = 7;
System.out.print(a[0] + a[2] + a.length);

A 7 B 11 C 13 D It does not compile.

Q14

What does System.out.println(a) print for int[] a = {1,2,3}?

A 123 B [1, 2, 3] C Something like [I@1b6d3586 D It does not compile.

Q15

Which call prints the contents of a one-dimensional array in a readable form?

A Arrays.toString(a) B a.toString() C Arrays.deepToString(a) D String.valueOf(a)

Q16

A method with the heading static void reset(int[] a) sets every element to 0. After the call, the caller's array:

A is unchanged B contains only zeros C is null D has length 0

Q17

What is the output?

public class Grow {
    static void grow(int[] a) { a = new int[10]; a[0] = 5; }
    public static void main(String[] args) {
        int[] nums = {1, 2, 3};
        grow(nums);
        System.out.print(nums.length + " " + nums[0]);
    }
}

A 10 5 B 3 1 C 3 5 D 10 1

Q18

Linear search returns which value when the key is absent?

A 0 B −1 C null D The array length

Q19

Binary search may be used only when the array is:

A full B sorted C of primitive type D of even length

Q20

How many comparisons does binary search need, at most, on a sorted array of 1000 elements?

A About 10 B About 500 C Exactly 1000 D About 100

Q21

In the binary search code, what is the purpose of low = mid + 1?

A To restart the search. B To discard the lower half, because the key is larger than a[mid]. C To avoid an infinite loop only. D To sort the array.

Q22

Which algorithm repeatedly selects the smallest remaining element and swaps it into position?

A Bubble sort B Selection sort C Binary search D Insertion sort

Q23

What is the time complexity of selection sort on n elements?

A O(1) B O(log n) C O(n) D O(n²)

Q24

What is the output?

int[] a = {5, 1, 4};
int t = a[0]; a[0] = a[2]; a[2] = t;
System.out.print(a[0] + "" + a[1] + a[2]);

A 514 B 415 C 145 D 451

Q25

Which of the following statements correctly creates an empty two-dimensional array with dimensions 2 × 2 in Java?

A int[][] blue = new int[2, 2]; B int[][] blue = new int[2], [2]; C int[][] blue = new int[2][2]; D int[][] blue = new int[2 x 2];

Q26

For int[][] m = new int[3][5];, what are m.length and m[0].length?

A 3 and 5 B 5 and 3 C 15 and 15 D 3 and 3

Q27

Which line causes an ArrayIndexOutOfBoundsException?

String[][] matrix = new String[1][2];
matrix[0][0] = "Don't think you are, know you are.";   // m1
matrix[0][1] = "I'm trying to free your mind Neo";     // m2
matrix[1][0] = "Is all around you";                    // m3

A m1 B m2 C m3 D None

Q28

How many elements does new int[4][3] contain in total?

A 7 B 12 C 4 D 3

Q29

What is printed?

int[][] t = {{1,2,3},{4,5},{6}};
System.out.print(t.length + " " + t[1].length + " " + t[2][0]);

A 3 2 6 B 3 3 6 C 6 2 6 D It does not compile.

Q30

An array whose rows have different lengths is called:

A invalid B jagged C sparse D sorted

Q31

What is the output?

int[][] m = {{1,2},{3,4}};
int sum = 0;
for (int r = 0; r < m.length; r++)
    for (int c = 0; c < m[r].length; c++)
        if (r == c) sum += m[r][c];
System.out.print(sum);

A 4 B 5 C 10 D 6

Q32

What does the loop in the previous question compute?

A The sum of all elements B The sum of the main diagonal C The sum of the first row D The largest element

Q33

Which loop correctly totals the elements of the second row of int[][] m?

A for (int c = 0; c < m.length; c++) s += m[1][c]; B for (int c = 0; c < m[1].length; c++) s += m[1][c]; C for (int r = 0; r < m.length; r++) s += m[r][1]; D for (int x : m) s += x;

Q34

In a partially filled array with capacity 100 and count 12, where is the next value stored?

A a[100] B a[12] C a[11] D a[13]

Q35

Which call sorts an int[] in ascending order using the standard library?

A a.sort() B Arrays.sort(a) C Collections.sort(a) D Array.sort(a)

Q36 Short answer

Explain why int[] b = a; is not a copy, and write the loop that makes a genuine copy of a into a new array b.

Q37 Short answer

A method must return both the largest value of an array and its index. Describe two different ways to do this in Java, given that a method returns only one value.

Q38 Programming

Write a method public static int findMode(int[] array) that returns the mode (the most frequent value) of a sorted array. If several values tie, return the first one. Test it with {2,5,6,7,7,10,10,11,11,13,15,15,15,19,20,20}, whose mode is 15.

Q39 Programming

Write a method public static int leastFrequent(int[] arr) that returns the element occurring the fewest times; if several tie, return the one with the lowest index. For {1,3,2,1,2,2,3,1} the answer is 3, and for {10,20,30} it is 10.

Q40 Programming

Write a program that computes and prints the matrix sum C = A + B for A = [[1, 2, 3], [4, 5, 6], [7, 8, 9]] and B = [[9, 8, 7], [6, 5, 4], [3, 2, 1]], using two-dimensional arrays and nested loops, and printing C as a properly aligned grid.