Rattanak Chea

Posted on Oct 6, 2017 • Edited 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)

Latest comments (29)

Thomas Graf • Dec 31 '17 • Edited

Java 9 IntStream has a new static method:

iterate(int seed, IntPredicate hasNext, IntUnaryOperator next)

IntStream.iterate(0, i -> i < 10, i -> i + 1)
            .forEach(System.out::println)

is equivalent to the following for-i loop, but more in a functional style

for(int i = 0; i < 10; i++) {
    System.out.println(i);
}

Jason C. McDonald • Dec 8 '17

Very nice, but it's missing my favorite, C++!

std::string str = "hello";
for (int i=0; i < str.length(); ++i)
{
    std::cout << str[i] << std::endl;
}

Note I'm using the prefix increment in my for loop, for pedantic (and habitual) performance reasons. (In reality, your compiler usually optimizes this itself.)

Given more time later, I'll come back and add the other two.

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);

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);

Unknown • Oct 12 '17

There is no need to use breckets in lambda:

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

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) => { /*...*/ })

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 😀

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

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.

KhoPhi • Oct 7 '17

I built a website ago just to keep how different programming languages approach concepts in their different ways.

Like the Fibonacci in 6 languages: code.khophi.co/codes/-KL7i0Vj9WSsh...

code.khophi.co

Rattanak Chea • Oct 7 '17

I checked it out. That's a good idea. Keep it up.

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.

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.

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