Rattanak Chea

Posted on Oct 6, 2017 • Updated on Nov 22, 2018

For Loop in different programming languages

#learn #javascript #java #loop

Programming often involves working on redundant tasks. The for loops help shorten the code and reduce tedious tasks. But the way for is used can be different for different languages. This post aims to provide some for loop examples for Java, Javascript and PHP working on String, Array, and Object.

Loop a String

Java

String str = "hello";
for (int i=0; i < str.length(); i++){
    System.out.print(str.charAt(i));
}
//another less optimal solution is to convert the str into an char array
//using str.toCharArray();
//see loop an Array section in Java below

Note:
length() and charAt(index) are methods of String object class.

JavaScript

var str = "hello";
for (var i=0; i < str.length; i++){
    console.log(str.charAt(i));
}

Note:
In JavaScript, we can declare string in two ways:

var str1 = 'primitive';  //datatype is primitive: string
var str2 = new String('string object');  //string as object

Since primitive has no methods and property, str1 was autoboxed to wrapper class String (as in s2). Then str1 becomes a String object with length as property and charAt as its method, and so on.

PHP

It is not as simple as Java, and JavasScript looping a string. One way is to convert the string to an array, then we can loop that array. Another way to use helper method, substr() to get each character of the string.

//method 1: use for loop
for($i=0; $i < count($array); $i++){
    echo $array[$i];
}

//method 2: convert a string to an array first, then we can loop the array
//use str_split() function to split a string by character
$str = "hello";
$array = str_split($str);  //split by character into an array
foreach($array as $value){
    echo $value;
}

Loop an Array

Java

int[] nums = new int[5];
for (int i=0; i < nums.length; i++){
    nums[i] = i; }
    System.out.print(Arrays.toString(nums)); //[0, 1, 2, 3, 4]
    //or use for (:) as for each loop
for(int i : nums){
    System.out.print(i);  //01234
}
//you may compare for(:) loop with foreach loop in PHP and other language.

Note: An array is a container object with a fixed size. The length of an array is established when the array is created. Array has a length property instead of length method in Object. In fact, length is a public final field of Array.
Read more here Chapter 10. Arrays (http://docs.oracle.com/javase/specs/jls/se7/html/jls-10.html#jls-10.7)

Javascript

var nums = ["hi", "hello", "you"];
//sequential loop
for (var i=0; i < nums.length; i++){
    console.log(num[i]));   //hihelloyou
}

Note: javascript has another loop for-in which is commonly used in Javascript object loop.

var obj = {  "a": 1,  "b": 2,  "c": 3};
for (var prop in obj) { 
  if (obj.hasOwnProperty(prop)) {
// or if (Object.prototype.hasOwnProperty.call(obj,prop)) for safety..
     alert("prop: " + prop + " value: " + obj[prop])  }
}

Read more: Loop through array in JavaScript (http://stackoverflow.com/questions/3010840/loop-through-array-in-javascript)

Loop an Object

Java

In Java, to loop an array of objects, we can use sequential loop or iterator or for (:) loop ##

ArrayList<String> list = new ArrayList<String>();
list.add("a");
list.add("b");
Iterator<String> itr = list.iterator();
while(itr.hasNext()){
    //do something with itr.next();
    //for example itr.remove();
}

//using foreach loop
for( String s : list){
    //do something with s
    // s is local String variable
    //modify s does not modify the list
}

PHP

In PHP, loop an object use foreach loop like in array.

foreach ($objects as $obj){
    echo $obj->property;
}
//or below
foreach ($obj as $key => $value) {
    echo "$key => $value\n";
}

References
What is the difference between string literals and String objects in JavaScript? (http://stackoverflow.com/questions/17256182/what-is-the-difference-between-string-literals-and-string-objects-in-javascript)

str_split - Manual (http://us.php.net/str_split)
Arrays (The Java™ Tutorials > Learning the Java Language > Language Basics) (https://docs.oracle.com/javase/tutorial/java/nutsandbolts/arrays.html)

Top comments (29)

Ben Halpern • Oct 6 '17

Love the apples-to-apples comparison.

Here's Ruby:

for num in 0..max_num do
   puts "The number is #{num}"
end

But in practice, nobody uses for loops.

Here's a while:

while num < max_num  do
   puts("The number is #{num}" )
   num +=1
end

But nobody uses those either.

People are way more likely to do

(0..max_num).each do |num|
   puts "The number is is #{num}"
end

Or using a collection of objects, like an array or an ActiveRecord collection.

pizza_toppings.each do |topping|
   puts "The topping is #{topping.name}"
end

Rattanak Chea • Oct 6 '17

Thank you for adding Ruby syntax. This post was written a few year ago when I found myself always scrambling to write loop when working with different programming languages. Nowadays, a functional programming approach is preferable. I love Python and Ruby for their clean syntax.

Rattanak Chea • Oct 6 '17

Do you mind if I include your Ruby code into the post? I will put credit where it belongs.

Ben Halpern • Oct 6 '17

Go for it. And no need to credit if it effects the reading in any way. 🙂

Harry • Oct 7 '17

There's a few other ways to iterate in JavaScript:

for-of

const array = [1,2,3];
for (const num of array) {
  console.log(num);
}

forEach

const array = [1,2,3];
array.forEach(num => {
  console.log(num);
};
// or
array.forEach(console.log);

Functional programming - map

const array = [1,2,3];
array.map(console.log);

Harry • Oct 7 '17 • Edited

Note: forEach(console.log) will print the item, index and the array - developer.mozilla.org/en-US/docs/W...

Array#forEach((item, index, array) => { /*...*/ })

Unknown • Oct 12 '17

There is no need to use breckets in lambda:

const array = [1,2,3];
array.forEach(num => console.log(num));

Antonin J. (they/them) • Oct 6 '17

Here's one for Go:

myStr := "My string"

for i := 0; i < len(myStr); i++ {
  fmt.Printf("%c", myStr[i])
}

You can also use a while loop:

for _, char := range myStr {
  fmt.Printf("%c", char)
}

Note how a "while loop" is basically the same as a for loop.

Jeremy Mill • Oct 6 '17

Or in rust

let my_str = "Hello World!";    
for c in my_str.chars() { 
    print!("{}",c);
}

David Multer • Oct 6 '17

In Python, this:

for num in range(max_num):
    print "The number is {}".format(num)

or this:

print '\n'.join(["The number is {}".format(num) for num in range(max_num)])

Louis-Saglio • Nov 2 '17

It's why i use Python.

Dan Ross • Oct 6 '17 • Edited

Your PHP string foreach example seems wrong - PHP strings can be referenced like an array. The following works on PHP, for example.

$str = "hello";
for($i = 0; $i < strlen($str); $i++){
    echo $str[$i];
}

Rattanak Chea • Oct 7 '17

I did include that too, but it was down. Now i moved it to the top. Thanks man.

Everton Agner • Oct 10 '17 • Edited

Let's be fair with Java. The language has seen quite a few enhancements since Java 8 release, 3 years ago, and very recently on Java 9, both on API and syntax. Now using the new stuff...

Loop a string:

String str = "hello";
str.chars() // Produces a stream of integers... yeah, that's odd
    .mapToObj(i -> (char) i) // Now we have a stream of chars
    .forEach(System.out::println);

Loop a collection:

List<String> list = List.of("a", "b");
list.forEach(System.out::println);

Steven Salka • Oct 7 '17

Nice post! When working with JavaScript/TypeScript, I usually go for the forEach array method, which is pretty convenient for usage with functions:

const fruit = ['apple', 'banana', 'orange'];

fruit.forEach(console.log);
// apple
// banana
// orange

If using a library like lodash, you can also iterate over objects (though iteration order is not guaranteed):

const funcs = {
  square: x => x ** 2,
  abs: x => Math.abs(x),
  reciprocal: x => 1 / x
};

_.forEach(funcs, (fn, fnName) => {
  console.log(`${fnName}(-2) = ${fn(-2)}`);
});
// square(-2) = 4
// abs(-2) = 2
// reciprocal(-2) = -0.5

Miff • Oct 7 '17 • Edited

Here's the ways to loop a string and an array in C# with a for loop, they're pretty much the same as any other language.

    string str = "hello";
    for (int i = 0; i < str.Length; i++){
        Console.WriteLine(str[i]);
    }

    int[] nums = new int[5];
    for (int i = 0; i < nums.length; i++){
        nums[i] = i;
        Console.WriteLine(i);
    }

However, C# really likes iterators, so those tasks are most commonly done with a foreach loop.

    string str = "hello";
    foreach (char c in str){
        Console.WriteLine(c);
    }

    foreach (int num in Enumerable.Range(0, 5)){
        Console.WriteLine(num);
    }

    // Borrowing from Ben Halpern's example for looping over a collection
    foreach (var topping in pizzaToppings){
        Console.WriteLine("The topping is {0}", topping.Name);
    }

Chris Shepherd • Oct 7 '17

Then resharper tells you that you can replace that with one line of linq... if my experience is anything to go by 😀

Dmytro Pylypenko • Oct 7 '17 • Edited

Swift 2, 3, 4:

// 1
for index in 0..<9 {
    print(index)
}

// 2
(0..<9).forEach { index in
    print(index)
}

// 3
let arr = ["foo", "bar"]
for (index, value) in arr.enumerated() {
    print("index: \(index), value: \(value)")
}

Rattanak Chea • Oct 7 '17

I never learn/write Swift. Look interesting. Thanks for sharing.

View full discussion (29 comments)

DEV Community

For Loop in different programming languages

Loop a String

Java

JavaScript

PHP

Loop an Array

Java

Javascript

Loop an Object

Java

PHP

Top comments (29)

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