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.

int[] int_list = {0, 4, 51, 83, 92, 10, 123, 145}; // any list

System.out.println("For loop return:"); 
for (int i = 0; i < int_list.length; i++) {
    System.out.println(int_list[i]);
}

System.out.println(""); 

System.out.println("For each loop return: "); 
for (int item : int_list) { 
    System.out.println(item); 
}

System.out.println(""); 

System.out.println("Seperating Even and Odd numbers: "); 

ArrayList<Integer> evenNumbers = new ArrayList<>();
ArrayList<Integer> oddNumbers = new ArrayList<>();
for (int i = 0; i < int_list.length; i++) {
    int num = int_list[i];
    if (num % 2 == 0) {
        evenNumbers.add(num);
    } else {
        oddNumbers.add(num);
    }
}

System.out.println("Even numbers: " + evenNumbers);
System.out.println("Odd numbers: " + oddNumbers);

For loop return:
0
4
51
83
92
10
123
145

For each loop return: 
0
4
51
83
92
10
123
145

Seperating Even and Odd numbers: 
Even numbers: [0, 4, 92, 10]
Odd numbers: [51, 83, 123, 145]
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15


Iteration: 0

Current Velocity: 2, 2