For loop

Let's go back to your original ambiguous post.

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I want to check an array.

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Check it for what? I'm going to assume you mean you want to examine each element of the array and compare it with another value to see if that value matches any elements.

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which one is quicker?

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Quicker in terms of what? Shortest propagation delay within one clock cycle? or least number of clocks cycles with the shortest clock period? Or maybe something else.

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using for loop or using a counter that counts up at the "posedge" of the clock.

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A for loop is a programming concept - it does not exist on an FPGA. When you synthesize it, the loop get unrolled into parallel hardware blocks, and it all depends on what is inside the for loop to know how that affects the timing.