what is the condition of "if"?why it continue when it's false?

Question

I set condition that a number greater than surrounding,but it go a false.

for (var i = 0; i + 1 < n; i++) {
    if (arr[i] < arr[i + 1] || arr[i + 1] > arr[i + 2]) {
        count++;
        index1 = i + 1;
    }
}

arr=[1, 2, 3, 6, 5, 4, 7, 8],n=8.

You can see count plus when arr[i+1] smaller than arr[i+2].

this is the picture of problem

please add the missing variables, like `arr` and `n` and what you expect. — Nina Scholz, Apr 17 '17 at 08:09
What you are looking for is `&&` instead of `||` maybe. `true` or `false` returns `true` (as in your condition). — shahkalpesh, Apr 17 '17 at 08:11
according to the picture arr[i] < arr[i + 1] is interpreted to arr[0] < arr[1] i.e 1< 2 is true and by OR condition the if expression is true isn't it ? — Vinod Louis, Apr 17 '17 at 08:16
You are using OR condition so it will continue because one statement is true — kritikaTalwar, Apr 17 '17 at 08:19

score 0 · Answer 1 · answered Apr 17 '17 at 08:35

You should use AND condition instead of OR to check value is greater than left and right.

Refer below code:

var arr=[1, 2, 3, 6, 5, 4, 9, 8];
var n=8;
var count = 0;
var pos = [];
for (var i = 0; i + 1 < n; i++) {
    if (arr[i] < arr[i + 1] && arr[i + 1] > arr[i + 2]) {
        count++;
        index1 = i + 1;
        pos.push(index1);
    }
}
console.log("positions: "+pos);
console.log("count: "+count);

score 0 · Answer 2 · answered Apr 17 '17 at 09:26

The problem is in the condition of ||(OR) since

true || false will result in true

Also, &&(AND) will result in

true && false will reult in false

So modify your function using "&&" instead of "||"

var arr=[1, 2, 3, 6, 5, 4, 9, 8];
var n=8;
var count = 0;
for (var i = 0; i + 1 < n; i++) {
    if (arr[i] < arr[i + 1] && arr[i + 1] > arr[i + 2]) {
        count++;
        index1 = i + 1;
    }
}

Please refer to https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_Operators for further understanding

score -1 · Answer 3 · edited May 23 '17 at 11:54

-1

When i is reaching 6, you test array[i+2] with i+2=8, so the index i+2 is out of bound (the array contains 8 elements, with index starting from 0 to 7). In js, getting a value out of array returns undefined, which is evaluated as a falsy value (evaluated as false).

You can get an explanation about falsy values here:

https://stackoverflow.com/a/19839953/5520709

edited May 23 '17 at 11:54

Community

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answered Apr 17 '17 at 08:19

Damien Ferey

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what is the condition of "if"?why it continue when it's false?

3 Answers3