Post/Pre Increment/Decrement ---------------------> Boon or Curse!

 Ah, 

so, We love Increment/Decrement operators ( well most of us ), don't we??

Be Cautious while using INCREMENT/DECREMENT operators when:

1) While Giving ARGUMENTS to a RECURSIVE FUNCTION:

Let's take an example of a recursive factorial()

int fact(int n)

{

    if(n==1)

    return 1;

    return n*fact(n--);

}

The above function may look straightforward but, the above function will call itself infinitely.

Why?

We have used post decrement ( -- ) while passing value to fact(). Lets recall, how does POST DECREMENT work?

--> It first assigns the value then decrement occurs.

so, we actually passed  N  instead of ( N-1 ) in argument, thus resulting in infinite recursion. 

We can instead use PRE DECREMENT OPERATOR.

Note: NEVER USE POST DECREMENT/INCREMENT OPERATORS while passing values to a function.

2) Don't use POST/PRE INCREMENT OPERATORS at all, when the recursion is not tail recursion:

Consider the below two functions:

int calc1(int a,int b)

{

    if(a>b)

        return 0;

    if(a%2==1)

    {

        return a + calc1(++a,b);

    }

    else

        return calc1(++a,b);

}

int calc(int a,int b)

{

    if(a>b)

        return 0;

    if(a%2==1)

    {

        return a + calc(a+1,b);

    }

    else

        return calc(a+1,b);

}

Draw the RECURSION TREE for both...( both the above functions are supposed to calculate the sum of all odd numbers in the given range RECURSIVELY )

let's take the range [ 1,2]..

We know that in the range [1,2], the sum of all odd numbers =1 ,.....

for function calc 1 :

a=1 ,

                return  a + calc1( ++a,b)            

                :     Now, this makes value of a=2 , so, actually the statement turns to 

                        

                            return 2 + calc1(2,2)

                        which will ultimately be 2 .. crazy 

Now, lets perform the same operation in function calc()

                         return  a + calc( a+1,b)            

                :     Now, this makes value of a=2 , so, this time 

                        

                            return 1 + calc(2,2)

                        which will ultimately be 1.

Thus, Always be careful with these operators while dealing with them in functions, Don't use any type of pre-increment/decrement operators when recursion is not TAIL RECURSION.