Different outputs from recursive functions

Question

I just found out about recursive functions about a few minutes back. I was playing around with them and now I am getting different outputs from the following functions:

int function(int m)   {
    m = 2*m;
    std::cout<<"In f m = "<<m<<std::endl;
    if(m > 20)
    {
        return m;
    }
    function(m);
};

int function2(int n)   {
    n = 2*n;
    std::cout<<"In f2 n = "<<n<<std::endl;
    if(n < 20)
    {
        function2(n);
    }
    return n;
};

int main()  {
    int a = 2;
    std::cout <<"function(a) = "<<function(a)<<std::endl;
    std::cout <<"function2(a) = "<<function2(a);
    return 1;
}

To this I get the output:

In f m = 4
In f m = 8
In f m = 16
In f m = 32
function(a) = 32
In f2 n = 4
In f2 n = 8
In f2 n = 16
In f2 n = 32
function2(a) = 4

Shouldn't they both yield result of 32?

Your first function is actually exhibiting undefined behavior because it is not returning a value if the base case isn't triggered. — Charles Salvia
– Charles Salvia, Commented Jun 30, 2013 at 7:46

Some programmer dude · Accepted Answer · 2013-06-30 07:46:21Z

3

Because you only do return n and don't assign it from the recursive calls, so it will for the first call always be 4.

You should to e.g.

if(n < 20)
    n = function2(n);
return n;

answered Jun 30, 2013 at 7:46

Some programmer dude

411k36 gold badges420 silver badges655 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Colorless Photon Over a year ago

That fixed it! Don't know why I didn't notice that.

Colorless Photon Over a year ago

I should have noticed that n is local to every function call.

witrus · Accepted Answer · 2013-06-30 08:00:24Z

0

Actually in function2，whether n less than or greater than 20 or not,statement return n will always be executed as the if statement will not return.so when you pass the argument a=2 in function2, it will return 4.If u want both of them to return the same result. u can modify function2 like

if(n < 20)
{
    function2(n);
}
else
    return n;

answered Jun 30, 2013 at 8:00

witrus

2971 gold badge3 silver badges13 bronze badges

Collectives™ on Stack Overflow

Different outputs from recursive functions

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related