CODEWITHSUNDEEP
calgorithm
ThisClark
ThisClark

Reputation: 14823

How is this binary tree algorithm's conditional statement working?

I am looking at the implementation of an algorithm proposed at Build the binary search tree from inorder and preorder traversals.

I think the implementation is in c++ and I have a question about this one line Node * temp = (Node *)malloc(sizeof(Node)); and the evaluation of its following if condition as seen in a block like this :

Node * temp =  (Node *)malloc(sizeof(Node));
if(temp){
    //do some stuff
    //conditionally return a Node
}
return NULL

Node is presumably a binary tree node. Its implementation is not shown, but how is that if(temp) condition working such that the last return NULL is ever reached?

Upvotes: 0

Views: 422

Answers (5)

Sridhar Nagarajan
Sridhar Nagarajan

Reputation: 1105

Actually the code structure should be,

Node * temp =  (Node *)malloc(sizeof(Node));
if( temp != NULL ){
    //do some stuff
    //conditionally return a Node
}
else
{
    return NULL;
}

you should not follow the structure you shown for code readability.

Upvotes: 0

user3079266
user3079266

Reputation:

In C prior to C99, there wass no specially designated boolean type. A scalar type was, and still can be used instead, such as int or a pointer like, in your case, Node*. A zero value is equivalent to false, a nonzero value is equivalent to true.

So, in your example, the pointer temp is the condition in the if statement. That means that the code in the if will execute if the value of temp is true, i.e. doesn't equal zero (NULL).

Upvotes: 1

Eugene Sh.
Eugene Sh.

Reputation: 18371

the if block is executed whenever the condition statement (which is temp in your case) is not equal to zero. In C the NULL pointer has a numerical value of zero. So if temp is NULL, the if block will not be executed.

Upvotes: 1

FCo
FCo

Reputation: 505

if(temp) means if (temp!=NULL), which essentially means that if the Node* temp was successfully malloc'ed and returned from malloc, then enter the if block of code. Unless there is code to return something in the if block (which would then exit the program), the return NULL statement will execute.

Upvotes: 2

Drew Dormann
Drew Dormann

Reputation: 63912

The conditional in an if is considered false if it evaluates to 0, and true if it's non-zero.

how is that if(temp) condition working such that the last return NULL is ever reached?

It's reached if temp is NULL, which evaluates to 0.

Upvotes: 1

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