Explain when you would use a while loop rather than a for loop and vice versa. Provide an example of one such instance.

Question

Explain when you would use a while loop rather than a for loop and vice versa. Provide an example of one such instance. How are looping statements different from selection statements?

Answer 1

In most languages, the for loop or foreach loop is for iterating over a list with a known number of elements. This is handy when iterating over arrays, where the compiler already knows the length of the array. This is common in nearly all languages I've encountered.

A while loop, however, can behave differently in various languages, but in general there's a simple rule to follow "while something is true, continue to do this thing here..." or, "while something remains false, keep looping over and over until I say otherwise", or even, "while this variable here says some value, keep running this block over and over". A good example is executing a simple block of code over and over, while some boolean remains true, until the block of code decides to set it to false, then the while loop will quit and your functional code will continue with its procedure.

Yes, it's true, in many languages you could use a while loop to iterate over an array, but that's not generally considered good form. However in some languages like Perl, there's an exception... while loops are preferred when iterating over very large arrays, since a for/foreach loop will load the entire array into memory, where as a while loop will load each element one at a time and then send it to garbage collection once it's moved on to the next element.

Answer 2

Quick Explanation: use a `while` loop in JavaScript when the number of iterations is unknown and a `for` loop when the iterations are predetermined. Looping statements are for repeating actions, while selection statements decide which code path to follow based on conditions.

For a more detailed answer ...

When to Use a While Loop vs. a For Loop in JavaScript

While Loop:

Use a `while` loop when you do not know how many times the loop should run. A `while` loop continues executing once a specified condition is considered true.

Example: If you are prompting a user for input until they type "exit," a `while` loop is appropriate because you don't know how many inputs the user will provide (how many times the user will input an answer that is not "exit").

let userInput = "";

while (userInput !== "exit") {

userInput = prompt("Enter something (type 'exit' to quit): ");

console.log(`You entered: ${userInput}`);

}

// The while loop will repeat and prompt the user for an entry, until the user enters "exit"

For Loop:

A `for` loop is ideal for situations where the number of iterations is predetermined, such as looping through each item in an array.

Example: A' for' loop is appropriate if you want to log each element of an array since you know the length.

const items = ["apple", "banana", "cherry"];

for (let i = 0; i < items.length; i++){

console.log(items[i]);

}

```

The for loop will iterate over "items" for the entire length (3). Each element (a fruit name) will be displayed

Output:

"apple"

"banana"

"cherry"

```

Difference Between Looping and Selection Statements

Looping Statements:

Looping statements (like `while` and `for`) execute a block of code multiple times as long as a specified condition is met or until a sequence is exhausted. They are used when you need to repeat the same operation multiple times.

Selection Statements:

Selection statements (like `if`, `else if`, and `else`) allow the code to make decisions and execute specific blocks of code based on conditions. They are used to choose between different code paths depending on whether a condition evaluates to true or false.

Example of Selection Statement in JavaScript:

let temperature = 75;

if (temperature > 80) {

console.log("It's hot outside.");

} else if (temperature > 60) {

console.log("It's warm outside.");

} else {

console.log("It's cold outside.");

}

```

Output: "It's warm outside."

Why:

75 is less than 80, so the "if" statement is fails, to the code moves to the "else if" statement.

The "else if" statement is true (75 is greater than 60), so the code stops and prints "It's warm outside."

Because the code stoped at the "else if" statement, it does not proceed to the "else" statement

In summary, use a `while` loop in JavaScript when the number of iterations is unknown and a `for` loop when the iterations are predetermined. Looping statements are for repeating actions, while selection statements decide which code path to follow based on conditions.

Answer 3

I agree with what has been said except for one thing. A while loop is more general that a for loop. So any for loop can be translated into a while but not all while loops can be translated into a non-degenerate for loop.

Yes, while (x > 0) is the same as for ( ; x>0; ) but I consider that degenerate.

A personal note: I learned assembly in 1961 and wrote many loops using "branch on condition" instructions. I was glad to step up to languages that handled loops more succinctly. I advise all learners to learn all the different iteration constructs in their language well. A "do { } while();" can simplify matters greatly, for example.

Answer 4

Hi Murshed, if you know how to use them, you can use either. As my fellow tutors pointed out they both can be written to give you the same result.

 

As a rule of thumb, if you find yourself copying and pasting the same code over and over, think for loop. If you want to break your loop based on a runtime value, think while loop.

 

 

Answer 5

Zakiyyah's explanations are spot on. Here are some examples in C++, but the concepts are the same for most languages.

 

Proper use of a While Loop

Here we are taking user input and insisting the user give us a positive number:

 

int num;

cout << "What is your favorite positive integer? ";

cin >> num

 

while(num <= 0)

{

    cout << "What is your favorite positive integer? ";

    cin >> num;

}

 

cout << "My favorite number is " << num << ", too!" << endl;

 

This can be written with a for loop, but it just looks ugly. The reason we use a while loop is because we don't know how many times the user will give us the wrong kind of number.

 

int num;
cout << "What is your favorite positive integer? ";
cin >> num

for(; num <=0 ; )
{
    cout << "What is your favorite positive integer? ";
    cin >> num;
}

 

Notice how I had to leave the initialization and incrementation parts blank -- I have no need for them! Why? Because I don't know how many times they will give me an incorrect value!

 

Proper Use of a For Loop

If I know how many times I want to loop, I use a for loop. For example, here I will print out the contents of a vector of ints.

 

vector<int> myVec;

myVec.push_back(1);

myVec.push_back(2);

myVec.push_back(3);

 

//The myVec variable is a vector containing 1, 2 and 3

 

for(int i = 0; i < myVec.size(); ++i)

{

   cout << "The number is: " << myVec[i] << endl;

}

 

I knew that exactly how many iterations I needed. I needed one for each element in the vector. I could do this as a while loop, but there are traps to doing so.

 

/* Initialize the vector same as above */

 

int i = 0;

while(i < myVec.size() )

{

    cout << "The number is: " << myVec[i]  << endl;

    ++i;

}

 

When using while loops in this fashion, one place you could make an error is forgetting to increment the loop counter (in this case the variable i). Doing so would give you an infinite loop and you wouldn't know it until run time. The for loop is better because it is designed with an incrementation portion so you won't forget it.

Answer 6

So in programming, the while loop and a for loop can be used interchangeably in some cases.

 

In other cases, it is better to use a while loop, and then again, others, a for loop is better.

 

Usually we'll use the for loop when we know how many time we want to execute the loop.  (for x<=100) whereas we normally will use a while loop when the number of iterations are not known.  (while x>0)

 

Since the for loop provides incremental change of the variable, it is better to use it when "counting"

 

Otherwise, if you want the the condition to just be true or false ex while (true) or while (false)

its better to use a while loop.

 

Keep in mind a while loop and be translated to for loop or vice versa;

 

for (x=0; x<10; x++)

{....}

can be translated to

int x=0

while (x<10)

{

.....

x++;

}

 

 

 

Sorry, I'll update tomorrow with some examples if I can but hopefully that's a good start.