C program on number sequence without loops

Question

1.00/5 (2 votes)

See more:

how does this code work??

C++

int main()
{
  static int n=1;

  printf("%d ", n) && (n++<100) && main();

  return 0;
}

Posted 1-Sep-12 7:42am

User 9352593

Updated 1-Sep-12 7:47am

Kenneth Haugland

v3

Add a Solution

Comments

Kenneth Haugland 1-Sep-12 13:50pm

Did you compile it and run debug, breakepoints etc? And if so what did ytou find out? I have an idea what it does but it looks like a spagetti code if Im right....

[no name] 25-Sep-12 11:27am

but it does work! im a newbie sorry if ive abused d language...plz correct me

[no name] 1-Sep-12 13:51pm

When you stepped through the code using a debugger, what did you observe?

Kenneth Haugland 1-Sep-12 14:00pm

He should put a breakepoint on one of these comments too :laugh:

pasztorpisti 1-Sep-12 16:03pm

btw, what is breakpoint? :-)

Zoltán Zörgő 1-Sep-12 14:07pm

This is a C easter egg: recursion on main. But I think, this is the big classical one: http://research.microsoft.com/en-us/um/people/tball/papers/xmasgift/

pasztorpisti 1-Sep-12 16:10pm

Thats what I call abuse! :-) I've seen such code only in perl with regex magic.

3 solutions

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

**Zoltán Zörgő** · Accepted Answer · 2012-09-01T08:13:00

Solution 1

It does not use loop, because it is using recursion. And as C is using short circuit evaluations by default, the second condition will stop the recursion. It is using a static variable to pass the current iteration between the recursive calls. This later approach is even better in this case - than using a parameter -, since it is stored only once in the stack - the parameter would be stored as many times the recursion is called. The static variable is initialized only at the first declaration, will not be reset to one on every call, thus can store the postfix incremented value embedded in the second condition.

Posted 1-Sep-12 8:13am

Zoltán Zörgő

Updated 1-Sep-12 8:15am

v2

Comments

Philip Stuyck 1-Sep-12 14:42pm

+5 good explanation. recursion, static variable, alle key points are there

Zoltán Zörgő 1-Sep-12 14:44pm

Thank you.

ridoy 1-Sep-12 15:13pm

hesitated whether i will give a solution or read your solution!!..but surely a broader explanation than me and also nice..so accept it and +5 from me:)

Zoltán Zörgő 1-Sep-12 15:16pm

Thank you too.

pasztorpisti 1-Sep-12 15:29pm

Just wanted to correct this: "since it is stored only once in the stack"
A static variable isn't stored on the stack, its a global variable with special initialization/finalization code. Looking at the question I suspect the OP doesn't even know what the stack is in this context. :-)

Zoltán Zörgő 1-Sep-12 15:31pm

You'r right. In both. :)

Joan M 1-Sep-12 15:56pm

+5, good explanation.

Andreas Gieriet · Accepted Answer · 2012-09-01T13:04:00

Two things that are not mentioned in the other solutions but might be of interest to the beginner:

It works since printf(...) returns a non-0 value (the number of printed characters) and since any value not equal to 0 is interpreted as true in C.
The only purpose of the whole x && y & z is to evaluate one after the other: first x, if x evaluates to true, do y, if y evaluates to true, do z. The result is not stored, and therefore, it does not matter that the tail call of main() returns 0 = false - it does the recursion internally before it can return the 0 value.

Cheers
Andi

pasztorpisti · Accepted Answer · 2012-09-01T09:35:00

Because of the short-circuit evaluation[^] your code is equivalent with this:

C++

int n=1; // in case of non-pod "initializable" types this
         // isn't equivalent with declaration inside a function
         // even if we overlook the difference in accessiblity
int main()
{
  if (printf("%d ", n))
  {
    if (n++ < 100)
    {
      main();
    }
  }
 
  return 0;
}

EDIT: Recursion mentioned by Zoltán is also very important if you want to understand this: http://en.wikipedia.org/wiki/Recursion_%28computer_science%29[^].
An important stuff (in my opinion) to study if you don't know what the stack is: http://en.wikipedia.org/wiki/Call_stack[^].

C program on number sequence without loops

3 solutions

Solution 1

Solution 3

Solution 2

Add your solution here

Preview 0