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What is the return value of $f(p,p)$, if the value of $p$ is initialized to $5$ before the call? Note that the first parameter is passed by reference, whereas the second parameter is passed by value.
    

int f (int &x, int c) {
       c = c - 1;
       if (c==0) return 1;
       x = x + 1;
       return f(x,c) * x;
}

 

asked in Programming by Boss (17.8k points)
recategorized by | 3.1k views
0
@Arjun Sir.. If the first parameter were passed by address (using pointers), will it still be undefined behaviour??

1 Answer

+46 votes
Best answer

In GATE 2013 marks were given to all as the same code in C/C++ produces undefined behavior. This is because $*$ is not a sequence point in C/C++. The correct code must replace

return f(x,c) * x;
with
res = f(x,c); // ';' forms a sequence point 
//and all side-effects are guaranteed to be completed here 
//-- updation of the x parameter inside f is guaranteed 
//to be reflected in the caller from the next point onwards. 
return res * x;



In this code, there will be 4 recursive calls with parameters $(6,4), (7,3), (8,2)$ and $(9,1)$. The last call returns $1$. But due to pass by reference, $x$ in all the previous functions is now $9$. Hence, the value returned by $f(p,p)$ will be $9 * 9 * 9 * 9 * 1 = 6561$.

 

Good Read

answered by Veteran (339k points)
edited by
0

@Arjun Sir. This is in response to the above comment by @rajan 

The concerned statement is: 
return fun(a+a, b/2)+a;

If there was something like: func1 () + func2 () then there would have been ambuiguity since either of func1 and func2 can execute first. 
But what we have here is: function call() + some constant value  so there should not be any undefined behaviour here right? I mean, Atleast there should not be any effect on the output of the program isn't it?  
Can you please confirm if the ouput is 99? Please correct me if I'm wrong. 

 

+1
Not commenting as the question is not clear to me. The whole point of undefined behaviour for the question in post is due to "pass by reference" being used and this is not there in C though can be simulated via pointers.
0

Sir here is the question I am taking about.
https://www.facebook.com/groups/core.cs/permalink/1256338371065080/?comment_id=1256365671062350&comment_tracking=%7B%22tn%22%3A%22R2%22%7D

I have uploaded a picture in the comments about the solution. Can you please look into it?
 

+1
Moreover

int f (int &x, int c)
             $\Lambda$
it should be * not &. I may be wrong. But i never see such thing. So Give me example where i can use this.

Also
x = x + 1; // This is also wrong.
It should be.
*x = *x + 1.
0

" But due to pass by reference, x in all the previous functions is now 9 "

Why this happened ? Can Somone explain plz?

+1
@Pawan Kumar i.e how call by reference work,updated the value hand to hand.
0
Yes, there are many typing mistakes in the question.
0

Its not a mistake bro.. This concept is called aliasing which is used in C++ . Here you can directly use "x" without any de-referencing operator. Its like a extra name(nick name) to the same variable . 

0
Reference...?
0

Does this mean that int &x=p is a way to declare x as a pointer to p?

Also, would x=x+1 mean p=p+1?

Answer:

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