Is there any way of writing the edge function code cleaner and more efficient for a beginer

In the blur function it took me waaaaaay to long to make all the for loops work and I almost gave up. Is there a good way to find write this type of loops easier if I clearly know what to do(ie: I know I need to access the i - 1 to i + 1 element but how do I write this without going out of bounds)

Is there any way of writing the edge function code cleaner and more efficient for a beginer

In the blur function it took me waaaaaay to long to make all the for loops work and I almost gave up. Is there a good way to find write this type of loops easier if I clearly know what to do(ie: I know I need to access the i - 1 to i + 1 element but how do I write this without going out of bounds). I could probably rewrite the function using the 0 padding as well, like for the edge one but this seems cleaner.