CODEWITHSUNDEEP
c++arraysrecursion
gallardoelise
gallardoelise

Reputation: 342

Recursively finding a negative in an array of doubles

My homework assignment requires me to go through an array of doubles and return true or false depending on whether or not it contains a negative number or not. The catch is that I have to use a recursive function and I can't use loops. I also can't use access any functions or variables outside of the function I am given.

The function takes two parameters: the array, and the number of elements to be inspected.

I'm having trouble making it stop recursing once it has inspected the specified number of elements.

//what I have so far
bool anyNegative(const double a[], int n)
{
    if(n <= 0)
        return false;

    if(*a < 0)
        return true;

    anyNegative(a + 1, n);
}

First, I thought of using counter, but that doesn't work since it gets reset every time the function recurses.

I also tried to compare pointer indexes with

if(currentElement == &a[n])

where currentElement is a pointer to the first element of array a.

However, I THINK the reason my program didn't work when I did that is because "a" is set to a new value every time the function is recursed so that &a[n] will always be n elements ahead of currentElement.

I'm stuck and if someone could give me a hint, that would be great.

Upvotes: 1

Views: 1081

Answers (4)

Vlad from Moscow
Vlad from Moscow

Reputation: 311068

Size of an array can not be negative. So that do not do such a superfluous check as

if(n <= 0)
    return false;

it is better to define the second parameter as having type size_t.

Your function has undefined behaviour because it returns nothing when the control will achieve statement

anyNegative(a + 1, n);

and moreover instead of the call above must be

anyNegative(a + 1, n -1);

I would write the function the following way

bool anyNegative( const double a[], size_t n )
{
   if ( n == 0 ) return false;
   return ( *a < 0 ? true : anyNegative( a + 1, n - 1 ) );
}

Upvotes: 0

Ammar
Ammar

Reputation: 233

You are missing two things, first you are not decrementing value of n so that terminating condition can be reached. Secondly you are not returning the result of sub-executions to the upper level.

Here is the modified code:

//what I have so far
bool anyNegative(const double a[], int n)
{
    if(n <= 0)
        return false;

    if(*a < 0)
        return true;

    return anyNegative(a + 1, n - 1);
}

Upvotes: 1

nothrow
nothrow

Reputation: 16178

You need to decrease n, and also return the value of recursive call.

return anyNegative(a + 1, n - 1);

Upvotes: 1

StoryTeller - Unslander Monica
StoryTeller - Unslander Monica

Reputation: 170193

Decrease n, since the array you pass is smaller

anyNegative(a + 1, n - 1);

Upvotes: 1

Related Questions