For loops

Intermediate Java

Jim White

Java Developer

Loops

Let us repeat an action multiple times
Most common in Java: for loop
- Used when we know in advance how many repetitions we want

Image of three arrows forming a circle

for loop syntax

for (int i = 0; i < 5; i++) {

  // Do something

}

i: iterator
int i = 0: start with th i equal to 0
i < 5: continue while i is smaller than 5
i++: increase i by 1 after each loop
iterable: thing we loop over
- e.g., String or array

for loop example

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

H

e

l
l
o

i is iterator
"Hello" is iterable

for loop through an array

// Iterate through array and print each element
int[] num = {1, 2, 3, 4, 5};

for (int i = 0; i < num.length; i++) {
    System.out.println(num[i]);
}

Adding to array in a loop

// Iterate through array 
// and add i to each element
int[] num = {1, 2, 3, 4, 5};

for (int i = 0; i < num.length; i++) {
    System.out.println("Old: " + num[i]);
    num[i] += i;
    System.out.println("New: " + num[i]);
}

Old: 1
New: 1
Old: 2
New: 3
Old: 3
New: 5
Old: 4
New: 7
Old: 5
New: 9

Infinite loops

Causes:
- Not updating iterator
- Wrong stopping condition
- ...
Avoid infinite loops!
- Cause program to freeze or crash

Examples

// i is always bigger than 3
for (int i = 3; i > 2; i = i + 1) { 
  System.out.println("Forever...");
}

// Not updating i
for (int i = 0; i < 10; i = i - 0) {
  System.out.println("And ever...");

Break

Used to exit loop early
Not a good coding practice, use wisely
- while loops is usually better

for (int i = 0; i < 5; i++) {
  System.out.println(i);
  if (i == 1) {
    break; // Breaks the loop when i is 1
  }
}

0
1

Recap

// Example of a for loop
// Including the break keyword
String word = "Hey";

for (int i = 0; i < word.length(); i++){
  System.out.println(word.charAt(i));
  if (i == 2) {
    System.out.println("!");
    break;
  }
  System.out.println("-");
}

i: iterator
word: iterable
for loop runs for predetermined amount of loops
- Runs from 0 till the end of word
- Unless we use the break keyword

H
-
e
-
y
!

Let's practice!

Intermediate Java