Loops

자바의 반복문에 대해 정리하였습니다.

 

 

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< Introduction to Loops >

To repeat the same block of code until some condition is met.

We employ loops to easily scale programs - saving time and minimizing mistakes.

 

• Writing the same code over and over is time-consuming.
• Having less code means having less to debug.

 

 

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< While loop >

< Syntax >

while (condition) {
  // code block
}​

If the condition evaluates to true, the code block will run.
If the condition evaluates to false, the code block will stop.

 

Example)

int number = 1;

while (number <=6) {
  System.out.println(number);
  number += 1;
}

// 1 2 3 4 5 6

 

 

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< Incrementing While Loops >

To avoid Infinity Loops using counter.

 

• A counter (a.k.a iterator) is a variable used in the conditional logic of the loop and (usually) incremented in value during each iteration through the code.

 

Example)

int wishes = 0;
 
while (wishes < 3) {
  System.out.println("Wish granted.");
  wishes++;
}

// Wish granted.
// Wish granted.
// Wish granted.

: Counter is wishes variable

 

 

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< For Loops >

< Syntax >

for (int control-variable = 0; control-variable < 5; control-variable++) {
	// code block
}

A for loop is made up of the following three parts, each separated by a semicolon ;

1. The initialization of the loop control variable.
   => int control-variable = 0
2. A boolean expression.
   => control-variable < 5;
3. An increment or decrement statement.(On each iteration after initialization)
   => control-variable++

: The for loop will continue to execute until boolean expression is false

 

Example)

int[] secretCode = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
for (int i = 0; i < secretCode.length; i++) {
  // Increase value of element value by 1
  secretCode[i] += 1;
}
// [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

 

 

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< break and continue >

To exit a loop before it finishes or want to skip one of the iterations, we can use the break and 
continue keywords.

 

1. The break keyword is used to exit, or break, a loop.

: Once break is executed, the loop will stop iterating.

for (int i = 0; i < 10; i++) {
  System.out.println(i);
  if (i == 4) {
    break;
  }
}
// 1
// 2
// 3
// 4

 

2. The continue keyword is used inside of a loop if we want to skip an iteration.

: Once continue is executed, the current loop iteration will immediately end, and the next iteration will begin.

int[] numbers = {1, 2, 3, 4, 5};
for (int i = 0; i < numbers.length; i++) {
  if (numbers[i] % 2 == 0) {
    continue;
  }
  System.out.println(numbers[i]);
}

// 1
// 3
// 5

 

 

+. Loops can exist all throughout our code - including inside a method

If the return keyword was executed inside a loop contained in a method, then the loop iteration would be stopped and the method/constructor would be exited.

public static boolean checkForJacket(String[] lst) {
  for (int i = 0; i < lst.length; i++) {
    System.out.println(lst[i]);
    if (lst[i] == "jacket") {
      return true;
    }
  }
  return false;
} 
 
public static void main(String[] args) {
  String[] suitcase = {"shirt", "jacket", "pants", "socks"};   
  System.out.println(checkForJacket(suitcase));
}

// shirt
// jacket
// true

 

 

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< For-Each Loops >

To directly loop through each item in a list of items (like an array or ArrayList) and perform some action with each item.

For loop's Short Version!

 

< Syntax >

for (DataType element : list) {
  // code block
}

1. DataType : element's data type.
2. element : list's element's name.(anything you want)
3. list : list(like array or ArrayList) to iterate.

 

Example)

// For
for (int inventoryItem = 0; inventoryItem < inventoryItems.length; inventoryItem++) {
  System.out.println(inventoryItems[inventoryItem]);
}

// For-Each
for (String inventoryItem : inventoryItems) {
  System.out.println(inventoryItem);
}

 

 

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< Removing Elements During Loop >

When an element is removed from an ArrayList, all the items that appear after the removed element will have their index value shift by negative one — it’s like all elements shifted to the left. We’ll have to be very careful with how we use our counter variable to avoid skipping elements.

 

< 1. Removing An Element Using while >

When using a while loop, should not increment the counter variable.

We don’t need to increase the counter because all of the other elements have now shifted to the left.

int i = 0;

while (i < lst.size()) {
  // if value is odd, remove value
  if (lst.get(i) % 2 != 0){
    lst.remove(i);
  } else {
    // if value is even, increment counter
    i++;
  }
}

 

< 2. Removing An Element Using for >

When using a for loop, we can’t avoid increasing control variable.

So we should decrease the control variable whenever we remove an item.

for (int i = 0; i < lst.size(); i++) {
  if (lst.get(i) == "value to remove"){
    // remove value from ArrayList
    lst.remove(lst.get(i));
    // Decrease loop control variable by 1
    i--;    
  }
}