4.2 For Loops

Similar to while loops, for loops run until a condition is false. Format of for loop below:

for (int i = 0; i < 5; i++) {
    System.out.println(i);
}

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Explanation

  • in the above for loop:
    • int i = 0 defines the iterating variable
    • i < 5 is the condition (once i < 5 is false, this loop breaks)
    • i++ is the incrementation of the iterating variable
    • System.out.println(i); is the body code that runs every time the loop iterates

For Each Loops:

  • Apart from iterating using an incrementing variable, you can also iterate through items in a list.

    Example of For Each loop below

int[] list = {1, 2, 3, 4, 5}; // any list
for (int item : list) { // for each item in the list, execute the body
    System.out.println(item); // body code
}

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#python version
array=[1, 2, 3, 4, 5]
for i in array:
    print(i)

Explanation:

  • in the above loop:
    • int item : list - this line is saying that for each item in the list, execute code below
    • System.out.println(num); - this line is the body code.

Fun For Loop Hack:

Create a program that iterates through a list of numbers (int_list = {0, 4, 51, 83, 92, 10, 123, 145}) using both a for loop and a for each loop, then split the numbers in the list into even/odd lists, and output them.

public class Main {
    public static void main(String[] args) {
        int[] int_list = {0, 4, 51, 83, 92, 10, 123, 145};
        
        ArrayList<Integer> evenNumbers = new ArrayList<>();
        ArrayList<Integer> oddNumbers = new ArrayList<>();

        System.out.println("Using for loop:");
        for (int i = 0; i < int_list.length; i++) {
            System.out.println("Number: " + int_list[i]);
            if (int_list[i] % 2 == 0) {
                evenNumbers.add(int_list[i]);
            } else {
                oddNumbers.add(int_list[i]);
            }
        }
        
        System.out.println("Even numbers: " + evenNumbers);
        System.out.println("Odd numbers: " + oddNumbers);
        
        evenNumbers.clear();
        oddNumbers.clear();
        
        System.out.println("\nUsing for-each loop:");
        for (int num : int_list) {
            System.out.println("Number: " + num);
            if (num % 2 == 0) {
                evenNumbers.add(num);
            } else {
                oddNumbers.add(num);
            }
        }
        
        System.out.println("Even numbers: " + evenNumbers);
        System.out.println("Odd numbers: " + oddNumbers);
    }
}
Main.main(null)
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Iteration: 0

Current Velocity: 2, 2