Unit 4.2 - For Loops

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);
}

0
1
2
3
4

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
}

0
1
2
3
4

#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.

import java.util.ArrayList;
int[] int_list = {0, 4, 51, 83, 92, 10, 123, 145};
ArrayList<Integer> even = new ArrayList<>();
ArrayList<Integer> odd = new ArrayList<>();

private static void isEven(int num)
{
    switch (num%2)
    {
        case 0:
            even.add(num);
        break;

        default:
            odd.add(num);
        break;
    }
}

for(int i=0;i<int_list.length;i++)
{
    isEven(int_list[i]);
}
System.out.println("Even " + even);
System.out.println("Odd " + odd);

even.clear();
odd.clear();

for(int value : int_list)
{
    isEven(value);
}
System.out.println("Even " + even);
System.out.println("Odd " + odd);

Even [0, 4, 92, 10]
Odd [51, 83, 123, 145]
Even [0, 4, 92, 10]
Odd [51, 83, 123, 145]