CODEWITHSUNDEEP
javascriptarrays
user3213563
user3213563

Reputation: 27

First non-negative element in an array

Hi I have an array that looks like:

hrarray = [-9,-7,0,3,7,8]

I've been trying to work out a function that selects the first non-negative (inc. 0) number in the array and returns the index (var elemid):

for (var i = 0, len=hrArray.length; i<len; i++) {
var num = hrArray[i];
if(num > 0) {
var elemid = i; //returns first non-negative element index
}
else if(num < 0) {}

Is my code logic correct?

Upvotes: 1

Views: 1411

Answers (3)

linstantnoodles
linstantnoodles

Reputation: 4400

I don't see you returning anything.

You can get the first non-negative with a single line:

for (var i = 0, len = arr.length; i < len && arr[i] < 0; i++);

Basically, we place the check for negatives in the for-loop guard. It will break once it finds a non-negative and hence return i of that first non-negative.

Update #1 (to use in a function):

function getFirstNonNegative(arr) {
  for (var i = 0, len = arr.length; i < len && arr[i] < 0; i++);
  // Use a ternary operator
  return (i === len) ? -1 : i; // If i === len then the entire array is negative
}

Upvotes: 1

p.s.w.g
p.s.w.g

Reputation: 148980

The main problem I see is that you're not breaking out of the loop once you find a non-negative value. This can be done with a break statement, or if you want to exit a function immediately, a return statement.

The function should look a bit like this:

function findNonNegative(arry) {
    for (var i = 0, len = arry.length; i < len; i++) {
        if(arry[i] >= 0)
            return i;
    }

    return -1;
}

Note that this method returns -1 if no non-negative elements are found.

And with a little tweaking, this can be shortened to:

function findNonNegative(arry) {
    for (var i = 0; arry[i] < 0; i++);
    return i < arry.length ? i : -1;
}

This version takes advantage of the behavior of the for loop to both test our values inside the loop and break out once a desired value is found. It also depends on the fact that x < y always returns false if x is undefined, thus ensuring we do not end up with an infinite loop. The last conditional expression is there to make the return value equal -1 if no non-negative elements were found; that can be reduced to just return i; if you are okay with the return value being arry.length in that case.

Upvotes: 0

rodrigovr
rodrigovr

Reputation: 454

Your code will assign to elemid the last non-negative in the array. Try this:

var elemid=-1; // elemid should be declared outside the for statement
for (var i = 0, len=hrArray.length; i<len; i++) {
    if(hrArray[i] > 0) {
        elemid = i; //returns first non-negative element index
        break;
    }
}

Upvotes: 0

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