Arrays: the four operations

Try it: predict the output

int[] stock = {12, 0, 7, 3, 9};
int low = 0;
for (int i = 0; i < stock.length; i++) {
  if (stock[i] < 5) {
    System.out.println("Restock item " + i);
    low++;
  }
}
System.out.println("Items to restock: " + low);

Write exactly what this prints. Careful: what does it print, i or stock[i]?

Restock item 1
Restock item 3
Items to restock: 2

Quick check (on paper)

1. Given marks = {64, 90, 77, 81, 59}, what is marks[4]? What happens with marks[5]?
2. Insert 15 at index 1 of [5, 10, 20, 25, _]. Which values move, and in what order?
3. Delete index 0 of [8, 16, 24, 32, _]. Which values move, and in what order?
4. Which of these would a linked list make easier, and why?

Coding practice

A canteen freezer should stay at −10 °C or colder. Its thermometer saved five readings: int[] temps = {-12, -5, -18, -3, -9};. Write a program that:

1. prints all five readings using a loop;
2. finds and prints the warmest (highest) reading;
3. prints only the readings warmer than −10 (too warm for the freezer);
4. counts and prints how many readings were too warm.

Checklist: Did you use a loop? Did warmest start with temps[0]? (Starting at 0 would print 0, a reading that never happened: the trap from Part 3.) Did you use > -10, so that exactly −10 counts as cold enough? You can run it in the Programiz online compiler.

public class Main {
  public static void main(String[] args) {
    int[] temps = {-12, -5, -18, -3, -9};

    // 1. print all five readings
    for (int i = 0; i < temps.length; i++) {
      System.out.println(temps[i]);
    }

    // 2. find the warmest (highest) reading
    int warmest = temps[0];
    for (int i = 1; i < temps.length; i++) {
      if (temps[i] > warmest) {
        warmest = temps[i];
      }
    }
    System.out.println("Warmest: " + warmest);

    // 3 and 4. print the readings warmer than -10, and count them
    int tooWarm = 0;
    for (int i = 0; i < temps.length; i++) {
      if (temps[i] > -10) {
        System.out.println(temps[i]);
        tooWarm++;
      }
    }
    System.out.println("Too warm: " + tooWarm);
  }
}

Your coding task: remove a value

This task joins Part 3 and Part 4: find a value, then delete it, like crossing a sold-out item off a canteen list. Finish removeValue so it removes the first copy of target and returns the new size. indexOf is finished for you and finds the index. Write about 5 lines where it says YOUR CODE, then run it: the four printed lines must match the comments.

 1public class Main {
 2  // Removes the first copy of target. Returns the new size.
 3  static int removeValue(int[] arr, int size, int target) {
 4    int k = indexOf(arr, size, target);
 5    // YOUR CODE:
 6    // 1. If target is not there (k is -1), return size unchanged.
 7    // 2. Shift every value after index k one slot left.
 8    // 3. Return the new size.
 9
10    return size;
11  }
12
13  // Finished for you: the index of target, or -1 if it is not there.
14  static int indexOf(int[] arr, int size, int target) {
15    for (int i = 0; i < size; i++) {
16      if (arr[i] == target) {
17        return i;
18      }
19    }
20    return -1;
21  }
22
23  // Finished for you: prints only the counted values.
24  static void show(int[] arr, int size) {
25    for (int i = 0; i < size; i++) {
26      System.out.print(arr[i] + " ");
27    }
28    System.out.println();
29  }
30
31  public static void main(String[] args) {
32    int[] arr = {10, 25, 30, 40, 0};   // capacity 5
33    int size = 4;                      // 4 values are counted
34
35    size = removeValue(arr, size, 25);
36    show(arr, size);                   // expected: 10 30 40
37    size = removeValue(arr, size, 99);
38    show(arr, size);                   // expected: 10 30 40
39    size = removeValue(arr, size, 40);
40    show(arr, size);                   // expected: 10 30
41    size = removeValue(arr, size, 10);
42    show(arr, size);                   // expected: 30
43  }
44}

▶ This program runs as is. Try it in the Programiz online compiler: tap Copy, paste it there, and press Run.

 1static int removeValue(int[] arr, int size, int target) {
 2  int k = indexOf(arr, size, target);
 3  if (k == -1) {
 4    return size;
 5  }
 6  for (int i = k; i < size - 1; i++) {
 7    arr[i] = arr[i + 1];
 8  }
 9  return size - 1;
10}