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

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 3

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 4

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.