What condition is being checked by the if statement

Question

So I'm looking up code in regards to 2D Haar Wavelet Transformation. And there's this if statement that I am confused about.

So part of the code looks like this:

unsigned char indexMask[4]; // the '4' here supposed to be a variable but I'm going to keep it simple here

for (int k = 0; k < 4; k++) {
indexMask[k] = 0;
}

for (int j = 1; j <= 5; j+=2) {
    if (indexMask[j/2]) {
        //some codes here
    }
}

My confusion is, what does the if statement here checks? This is my first time seeing a if statement structured this way so I sort of confused. Thanks alot

463035818_is_not_an_ai · Accepted Answer · 2019-08-29T08:06:19.020

2

From cppreference (quoting only the relevent parts):

if ( condition ) statement-true

condition     -   expression which is contextually convertible to bool

In layman terms: Numbers can convert to bool. 0 is converted to false and everything else is converted to true.

Hence, the condition could also be written as

if (indexMask[j/2] != 0) {
    //some codes here
}

edited Aug 29 '19 at 08:06

answered Aug 29 '19 at 08:00

463035818_is_not_an_ai

109,796
11
89
185

Thanks for the answer, appreciate it – Hensley Aug 30 '19 at 02:30

Bathsheba · Answer 2 · 2019-08-29T08:05:18.340

2

The expression indexMask[j / 2] is implicitly convertible to true or false. (It's an integral type, which will convert to false if 0 and true if any other value).

It's more readable than the hideous

if (indexMask[j / 2] == true)

or other unnecessarily long variants. Note also that if the initialisation was

unsigned char indexMask[4] = {};

then you wouldn't need that loop to set the elements to 0.

edited Aug 29 '19 at 08:05

answered Aug 29 '19 at 08:02

Bathsheba

231,907
34
361
483

well, I'd say `if (indexMask[j / 2] != 0) ` – Moia Aug 29 '19 at 08:05
@Moia or `if (indexMask[j / 2] != 0 ? true : false)` ! – Bathsheba Aug 29 '19 at 08:05
I was supposing to make the statement more clear – Moia Aug 29 '19 at 08:07
@Moia: It doesn't, not to me anyway. – Bathsheba Aug 29 '19 at 08:08
2

Some people might cry with `cond ? true : false` instead of `cond`... – Jarod42 Aug 29 '19 at 08:08
2

well you can always add more layers of confusion, what about `if (((indexMask[j / 2] == true) == true) == true)` ? :P – 463035818_is_not_an_ai Aug 29 '19 at 08:09
1

"operator" `!!` :) `!!indexMask[j / 2]`. – Jarod42 Aug 29 '19 at 08:11
@formerlyknownas_463035818: You could probably get a lambda function in there somewhere too. – Bathsheba Aug 29 '19 at 08:11
1

@Bathsheba my favourite tool for obfuscation still is the comma operator – 463035818_is_not_an_ai Aug 29 '19 at 08:14
Thanks for the answer! – Hensley Aug 30 '19 at 02:31

score 0 · Answer 3 · answered Aug 29 '19 at 17:10

What happens here is that indexMask[j / 2] is contextually converted into a boolean value. So basically every int or char value that is not 0 will be contextually be converted into true, while 0 will be converted into false.

Note that this is how C programmers use conditionals, as C does not have a boolean type, they use integers to return true / false values instead.

Also note that the conversion is not implicit, but is rather a contextual conversion, there is a large difference between them. But I would rather not explain them as it is most likely not inside the scope of this question.

What condition is being checked by the if statement

3 Answers3