In the last tutorial, we saw how to use a trap 
to build a zero checker. The zero checker releases a marble only if the number in its input stream is non-zero.
Let’s now modify it so that instead of checking for zero, it checks if two numbers are equal:
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Let’s break this down:
- The xor
checks each pair of digits. If they’re different, it emits 
through its central channel; if they’re the same, it emits 
. - If both digits were 1, we now have we need to dispose of coming out of the xor . We use a distributor
and a crossing 
to move them over to the right, where they’re out of the way. - We now route the xor central output to the trap . The first (indicating the first time the digits were different) will get held in the trap.
- Any subsequent will pass straight through the trap and be ignored.
- Every 4 steps, the timer sends into the trap. If there’s trapped there, it gets released. If not, the trap just emits .
- Therefore, every 4 steps, the trap emits if at least one pair of digits was different, and if they were all the same.
- Therefore, if is emitted, the numbers are not equal. If is emitted, they’re the same.
next
The ability to compare numbers will be a crucial component in any computer we build. For now, let’s get back to state. In the next tutorial, we’ll look at storing larger blocks of data efficiently.
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