Both functions are differentiable almost everywhere. Think about what the functions look like.
round() is a step function 59 so it has derivative zero almost everywhere. Although it’s differentiable (almost everywhere), it’s not useful for learning because of the zero gradient.
                    
            
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clamp() is linear, with slope 1, inside (min, max) and flat outside of the range. This means the derivative is 1 inside (min, max) and zero outside. It can be useful for learning as long as enough of the input is inside the range.
                    
            
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