Interchange loop variable with if statement

Question

I'm trying to change a for loop's start variable to use an if statement instead. However, they don't seem to be equivalent..

I have a loop that looked like:

int K = 0;
std::uint32_t CheckSum = 0;
const std::uint8_t* BuffPos = static_cast<const std::uint8_t*>(Data);
int Start = Height < 12 ? 1 : 12;

for (std::size_t I = Start; I < Height; ++I)
{
    for (std::size_t J = 0; J < Width; ++J, ++K)
    {
        BuffPos += 3;  //skip RGB and move to alpha pixel.
        CheckSum += *(BuffPos++);  //Checksum = count of alpha pixels.
    }
}

std::cout<<CheckSum;

However, I don't want my loop to start with I = Start. I'd rather do that in an if statement. I tried the following:

int K = 0;
std::uint32_t CheckSum = 0;
const std::uint8_t* BuffPos = static_cast<const std::uint8_t*>(Data);
int Start = Height < 12 ? 1 : 12;

for (std::size_t I = 0; I < Height; ++I)
{
    for (std::size_t J = 0; J < Width; ++J, ++K)
    {
        BuffPos += 3; //Skip RGB and move to alpha pixel.

        if (I >= Start)  //The if statement.
            CheckSum += *(BuffPos++);  //Checksum = count of alpha pixels.
    }
}

std::cout<<CheckSum;

However, the second version with the if statement prints a totally different number than the first for the exact same bitmap.

Is there a reason why it does this? How can I fix it? I can't see why it would be any different :S

Ed Swangren · Accepted Answer · 2013-09-14 05:51:22Z

3

As you noticed, it is no longer the same.

Sure, you segregated the checksum bit, but now you are incrementing BufPos a different number of times. Your loop is now iterating more times, so BuffPos has a different value once you start creating the checksum than it did in the first version.

Look at it this way; the first version starts accumulating the checksum at BuffPos + 3. The second version begins accumulating the checksum from BuffPos + (Start * Width) + 3.

Honestly, the first version is better. I can't tell you which one is correct, but assuming the first version is (iterating from Start) then why introduce a branch at all? It just muddles up the code.

If you want the second version to be the same as the first you'll need to initialize BuffPos to Data + Start * Width or place both statements within the if. Of course, if you do that, you're just doing nothing at all until I == Start, and that should tell you something.

edited Sep 14, 2013 at 5:51

answered Sep 14, 2013 at 5:46

Ed Swangren

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9 Comments

Brandon Over a year ago

The branch was so that the first part would calculate a checksum of all pixels and within the branch, I'd calculate a checksum of all except the first 12 rows.

Ed Swangren Over a year ago

@CantChooseUsernames: then why are you surprised that it now gives a different value? You changed the checksum. Just set BuffPos to point to the correct place to begin with (as I showed).

Brandon Over a year ago

I was trying to use the two loops to calculate two checksums rather than looping the image with 4 loops (one checksum starting at row 12. Another starting at row 0).

Ed Swangren Over a year ago

@CantChooseUsernames: Ok, but your example code doesn't do that. We could only answer based on what you show us.

necromancer Over a year ago

"I can't tell you which one is correct." deserves a -1 but since this is a recreational question, I'll refrain!

|

Ed Swangren · Accepted Answer · 2013-09-14 06:07:31Z

1

how's this version?

int K = 0;
std::uint32_t CheckSum = 0;
const std::uint8_t* BuffPos = static_cast<const std::uint8_t*>(Data);
int Start = Height < 12 ? 1 : 12;

for (std::size_t I = 0; I < Height; ++I)
{
    if (I < Start)  //The if statement.
        continue; 
    for (std::size_t J = 0; J < Width; ++J, ++K)
    {
        BuffPos += 3; //Skip RGB and move to alpha pixel.
        CheckSum += *(BuffPos++);  //Checksum = count of alpha pixels.
    }
}

std::cout<<CheckSum;

edited Sep 14, 2013 at 6:07

Ed Swangren

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answered Sep 14, 2013 at 5:54

necromancer

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7 Comments

Ed Swangren Over a year ago

"how's this version?" - pretty useless. I know it's what the OP is asking for, but in this case, the OP wants the wrong solution.

necromancer Over a year ago

hehe... who am i to deny the OP's desire. did you undownvote? i was mainly wondering what people would do in response. this was a rhetorical answer if there's such a thing. and also a sociological experiment. props for the undownvote - you pass :)

necromancer Over a year ago

@EdS. i spoke too soon! either somebody else downvoted, or you unundownvoted.

Ed Swangren Over a year ago

I downvoted, removed it... and then downvoted again. I was a bit conflicted because it's technically what the OP was asking for. However, it's brain-dead code. Why introduce cycles which do absolutely nothing? I would refactor that code in a second, so I don't think it's a good answer.

necromancer Over a year ago

@EdS. well, there's might still be time to ununundownvote, lol

|

Gibbs · Accepted Answer · 2013-09-14 05:50:28Z

0

Both statements(Buffpos and cheksum) should come under if statement .

answered Sep 14, 2013 at 5:50

Gibbs

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