Interchange loop variable with if statement - c++

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

Answer 1

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.

Answer 2

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;

Answer 3

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