CODEWITHSUNDEEP
objective-cfor-loop
hinterbu
hinterbu

Reputation: 767

For loop with only a declaration?

This is a very basic question but please bear with me!

I got this code in a question as part of a quiz I was doing earlier and just didn't know if I might be missing something. I typed it into the editor and it would not run and it does appear to be incomplete. Had it been if (k) it would have made more sense.

But, as I have heard that you can leave out components of a for loop, I was just wondering if there is any time you would see the likes of for (k)?

int k = 0;
for (k) {
    printf ("hello");
}

Upvotes: 0

Views: 97

Answers (1)

Mathews Mathai
Mathews Mathai

Reputation: 1707

for(int k; ;)
    /*this is the correct syntax of a for loop without conditional statement and incrementation/decrementation statement*\

Remember,those semi-colons within the paranthesis is important(without that the program wouldn't compile).

Now,to answer some of the questions you asked me in the earlier answer-

for(int k; ;)
{
printf("infinite loop");
}

  • When will this loop come to an end? This loop will never come to an end.It is an infinite loop.It will keep printing infinite loop forever.

  • Is it possible to bring this loop to an end? Yes,it is.It can be brought to an end using break statement.

    for(int k; ;)  
//or for( ; ; )
{
printf("infinite loop");
break;
}

Prints infinte loop only once.It will encounter the break statement and the control will move outside the loop.

  • Possible application. It's used when you actually have no idea about when a loop should come to an end.

         int i=0; //to take user input
     for(int k; ;)
     {
     //accept the value of k from user.
     /*You want the user to enter 1 as the input*/
     if(k==1)
      {
      printf("entered 1,moving out of loop");
      break;
      }
      }

What is the meaning of above loop? - This loop keeps running until the user enters '1'.This is important in cases where you are giving the user options and the options are limited and so you don't want the user to give an invalid input.It runs until a valid input is entered(you can add more if statements with break statement).

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for(int k;k<10;k++)
 /* this is a finite loop and this isn't suitable for the above requirement because you are not sure if the user will give the valid input within 10 iterations.*/

When k becomes 10,the control will move out irrespective of whether the user has entered a valid input or not.What if the user inputs 8 when k=9? The control will move out of the loop at k=10.As a result,your program will not work efficiently because i=8 is not an input you expected.You wanted 1,2 or 3 as input.

So,an infinite loop is used when you are not sure about how many iterations are required.You will actually be using a break statement to exit such a loop.

  • Is this the only option for an infinite loop?Why not while() ?Isn't while() with no condition an infinite loop?

      while();// invalid in C.
  //objective-C  follows C-standards.

  while("condition"); //valid

  • Some valid for loop declarations in C

      for(k; ;) // infinite loop
  for(; ;) // infinite loop
  for(; k<0;)// valid

So,I think that sums up a small explanation. Remember,semi-colons are important(irrespective of whether a condition is given or not).

And of course,you have other options to keep running or taking user input unless a valid input is given.But above one was just an application I could figure out to show that an infinite loop could be cool! If you find any error or doubt,please comment. Well,even I am not too good in C.But yeah since java is somewhat similar,I figured it out.

Upvotes: 3

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