Day 2: Loops

Loops

JavaScript Loops

Loops are a quick and easy way to repeatedly perform a series of instructions, and they are typically run a finite number of times. JavaScript has the following types of loops:

for
while
do-while
for-in
for-of

for

The for statement creates a loop that consists of three optional expressions, enclosed in parentheses and separated by semicolons, followed by one or more statements that will be executed in the loop.

Basic Syntax

for (initialization; condition; finalExpression) {
    statement(s);
}

Components

: An expression or variable declaration that is typically used to initialize a counter variable.
: This is the termination condition, which is an expression that's evaluated before each pass through the loop. If this expression evaluates to true, then is executed. If the expression evaluates to false, execution jumps to the first line of code after the end of the loop. If this statement is omitted, then always evaluates to true.
: An expression to be evaluated at the end of each loop iteration. This occurs before the next evaluation of .
: The statement (or statements) that is executed each time evaluates to true.

It's important to note that:

The , , and in the head of the for loop are optional, but are generally always used.
The head of a for loop typically looks like for (var i = 0; i < maxValue; i++), where is the maximum value you wish to iterate until.

EXAMPLE

Print all the integers in the range from to some number given as input.

process.stdin.on('data', function (data) {
    main(+(data));
});
/**** Ignore above this line. ****/

function main(input) {
    for (var i = 1; i <= input; i++) {
        process.stdout.write(i + " ");
    }
}

​x
 
process.stdin.on('data', function (data) {
    main(+(data));
});
/**** Ignore above this line. ****/
​
function main(input) {
    for (var i = 1; i <= input; i++) {
        process.stdout.write(i + " ");
    }
}

Run

Input

10

Output

Initialize

In this example, we omit the expression and instead initialize the variable used in and before our loop:

process.stdin.on('data', function (data) {
    main(+(data));
});
/**** Ignore above this line. ****/

function main(input) {
    var i = 1;

    for (; i <= input; i++) {
        process.stdout.write(i + " ");
    }
}

 
process.stdin.on('data', function (data) {
    main(+(data));
});
/**** Ignore above this line. ****/
​
function main(input) {
    var i = 1;
​
    for (; i <= input; i++) {
        process.stdout.write(i + " ");
    }
}

Run

Input

10

Output

Condition

In this example, we omit the expression and instead add an if statement inside the loop that terminates the loop once a the condition i > input is satisfied:

process.stdin.on('data', function (data) {
    main(+(data));
});
/**** Ignore above this line. ****/

function main(input) {

    for (var i = 1;; i++) {
        if (i > input) {
            break;
        }

        process.stdout.write(i + " ");
    }
}

 
process.stdin.on('data', function (data) {
    main(+(data));
});
/**** Ignore above this line. ****/
​
function main(input) {
​
    for (var i = 1;; i++) {
        if (i > input) {
            break;
        }
​
        process.stdout.write(i + " ");
    }
}

Run

Input

10

Output

Infinite Loop

If we omit all three blocks, our loop will run infinitely or until such a time as we call break; from inside the loop. In this example, we do just that:

process.stdin.on('data', function (data) {
    main(+(data));
});
/**** Ignore above this line. ****/

function main(input) {
    var i = 1;

    for (;;) {
        if (i > input) {
            break;
        }

        process.stdout.write(i + " ");
        i++;
    }
}

 
process.stdin.on('data', function (data) {
    main(+(data));
});
/**** Ignore above this line. ****/
​
function main(input) {
    var i = 1;
​
    for (;;) {
        if (i > input) {
            break;
        }
​
        process.stdout.write(i + " ");
        i++;
    }
}

Run

Input

10

Output

while

The while statement creates a loop that executes its internal statement(s) as long as the specified evaluates to true. The condition is evaluated before executing the statement.

Basic Syntax

while (condition) {
    statement(s);
}

: This is the termination condition, which is an expression that's evaluated before each pass through the loop. If this expression evaluates to true, then is executed; if it evaluates to false, execution jumps to the first line of code after the end of the loop.
: The statement (or statements) that is executed each time evaluates to true.

EXAMPLE

Print all the integers from to .

process.stdin.on('data', function (data) {
    main(+(data));
});
/**** Ignore above this line. ****/

function main(input) {
    var i = 1;

    while (i <= input) {
        process.stdout.write(i + " ");

        i++;
    }
}

 
process.stdin.on('data', function (data) {
    main(+(data));
});
/**** Ignore above this line. ****/
​
function main(input) {
    var i = 1;
​
    while (i <= input) {
        process.stdout.write(i + " ");
​
        i++;
    }
}

Run

Input

10

Output

do-while

The do-while statement creates a loop that executes its internal statement(s) until the specified evaluates to false. The condition is evaluated after executing the internal statement(s), so the contents of the loop always execute at least once.

Basic Syntax

do {
    statement(s);
} while (condition);

: This is the termination condition, and it's evaluated after each pass through the loop (meaning the loop will always run at least once). Once the statement(s) inside the loop is executed, is evaluated. If this expression evaluates to true, then is executed again; if it evaluates to false, execution jumps to the first line of code after the end of the loop.
: The statement (or statements) that is executed each time evaluates to true.

EXAMPLE

Print all the integers in the range from to some number given as input.

process.stdin.on('data', function (data) {
    main(+(data));
});
/**** Ignore above this line. ****/

function main(input) {
    var i = 1;

    do {
        process.stdout.write(i + " ");

        i++;
    } while (i <= input);
}

 
process.stdin.on('data', function (data) {
    main(+(data));
});
/**** Ignore above this line. ****/
​
function main(input) {
    var i = 1;
​
    do {
        process.stdout.write(i + " ");
​
        i++;
    } while (i <= input);
}

Run

Input

10

Output

for-in

This loop iterates (in an arbitrary order) over the name of each enumerable property in an object, allowing statements to be executed for each distinct property.

Basic Syntax

for (var variable in object) {
    // insert code that uses variable here
}

: A variable that refers to a different property name during each iteration of the loop. You can declare this with var or let.
: The object whose enumerable properties are being iterated through.

EXAMPLE

In the code below, we create an object (referenced by the variable) and iterate over its enumerable properties:

var actress = {
    firstName: "Julia",
    lastName: "Roberts",
    dateOfBirth: "October 28, 1967",
    nationality: "American",
    firstMovie: "Satisfaction"
};

for (var property in actress) {
    console.log("actress." + property + " = " + actress[property]);
}

 
var actress = {
    firstName: "Julia",
    lastName: "Roberts",
    dateOfBirth: "October 28, 1967",
    nationality: "American",
    firstMovie: "Satisfaction"
};
​
for (var property in actress) {
    console.log("actress." + property + " = " + actress[property]);
}

Run

Output

The code above produces the following output:

actress.firstName = Julia
actress.lastName = Roberts
actress.dateOfBirth = October 28, 1967
actress.nationality = American
actress.firstMovie = Satisfaction

In this code, we create a Monster object named , then print the object followed by its individual properties.

Input Format

The first line contains a string, , denoting the type of monster.
The second line contains a string, , denoting the location where the monster lives.
The third line contains a string, , describing the monster.

'use strict';
process.stdin.on('data', function (data) {
    main(String(data).trim().split(new RegExp("[\n]+")));
});
/**** Ignore above this line. ****/

class Monster {
    constructor(name, home, description) {
        this.name = name;
        this.home = home;
        this.description = description;
    }
}

function main(input) {
    var monster = new Monster(input[0], input[1], input[2]);

    // Print array
    console.log(monster);

    // Print each of its elements on a new line
    for (let property in monster) {
        console.log(property + ": " + monster[property]);
    }
}

 
'use strict';
process.stdin.on('data', function (data) {
    main(String(data).trim().split(new RegExp("[\n]+")));
});
/**** Ignore above this line. ****/
​
class Monster {
    constructor(name, home, description) {
        this.name = name;
        this.home = home;
        this.description = description;
    }
}
​
function main(input) {
    var monster = new Monster(input[0], input[1], input[2]);
​
    // Print array
    console.log(monster);
​
    // Print each of its elements on a new line
    for (let property in monster) {
        console.log(property + ": " + monster[property]);
    }
}

Run

Input

Minotaur
Labyrinth
Bull head, man body.

Output

The code above produces the following output for the given input:

Monster {
  name: 'Minotaur',
  home: 'Labyrinth',
  description: 'Bull head, man body.' }
name: Minotaur
home: Labyrinth
description: Bull head, man body.

for-of

This loop iterates over iterable objects such as an Array, Map, Set, String, TypedArray, arguments object, etc. It essentially iterates over the value of each distinct property in the structure, such as each letter in a word or each element in an array.

Basic Syntax

for (let variable of iterable) {
    statement(s);
}

: A variable that refers to a different property value during each iteration of the loop. You can declare this with var or let.
: The object whose enumerable properties are being iterated through.

EXAMPLE

The code below splits the input into an array and prints it. It then iterates over each element of the array and prints it on a new line.

Input Format
Space and/or newline-separated words.

'use strict';
process.stdin.on('data', function (data) {
    main(String(data).trim());
});
/**** Ignore above this line. ****/

function main(input) {
    // Split the words read as input into an array of words
    var array = input.split(new RegExp("[ \n]+"));

    // Print array
    console.log(array);

    // Print each of its elements on a new line
    for (let value of array) {
        console.log(value);
    }
}

 
'use strict';
process.stdin.on('data', function (data) {
    main(String(data).trim());
});
/**** Ignore above this line. ****/
​
function main(input) {
    // Split the words read as input into an array of words
    var array = input.split(new RegExp("[ \n]+"));
​
    // Print array
    console.log(array);
​
    // Print each of its elements on a new line
    for (let value of array) {
        console.log(value);
    }
}

Run

Input

hi bye 
hello goodbye

Output

The code above produces the following output:

[ 'hi', 'bye', 'hello', 'goodbye' ]
hi
bye
hello
goodbye

In this code, we iterate over the set of Key-Value pairs in a Map, first printing each Key-Value pair and then printing each individual Key and its paired Value.

'use strict';

let actress = new Map([
    ["firstName", "Julia"],
    ["lastName", "Roberts"],
    ["dateOfBirth", "October 28, 1967"],
    ["nationality", "American"],
    ["firstMovie", "Satisfaction"]
]);

// Print each Key-Value pair in the map
for (let info of actress) {
    console.log(info);
}

// Print each Key and Value as "Key: Value"
console.log();
for (let info of actress) {
    console.log(info[0] + ": " + info[1]);
}

 
'use strict';
​
let actress = new Map([
    ["firstName", "Julia"],
    ["lastName", "Roberts"],
    ["dateOfBirth", "October 28, 1967"],
    ["nationality", "American"],
    ["firstMovie", "Satisfaction"]
]);
​
// Print each Key-Value pair in the map
for (let info of actress) {
    console.log(info);
}
​
// Print each Key and Value as "Key: Value"
console.log();
for (let info of actress) {
    console.log(info[0] + ": " + info[1]);
}

Run

Output

The code above produces the following output:

[ 'firstName', 'Julia' ]
[ 'lastName', 'Roberts' ]
[ 'dateOfBirth', 'October 28, 1967' ]
[ 'nationality', 'American' ]
[ 'firstMovie', 'Satisfaction' ]

firstName: Julia
lastName: Roberts
dateOfBirth: October 28, 1967
nationality: American
firstMovie: Satisfaction

Table Of Contents

for
while
do-while
for-in
for-of

Go to Top

Something went wrong!

Day 2: Loops

Loops

JavaScript Loops

for

Basic Syntax

Components

Initialize

Condition

Infinite Loop

while

Basic Syntax

do-while

Basic Syntax

for-in

Basic Syntax

for-of

Basic Syntax

Challenge Walkthrough

Review the problem statement

Choose a language

Enter your code

Test your code

Submit to see results