What is a Finite-State Machine for Changing Every Other Bit in an Input String?

[livcon]

Homework Statement

Construct a finite-state machine that changes every other bit, starting with the second bit, of an input string, and leaves the other bits unchanged.

Homework Equations

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The Attempt at a Solution

Say I have the string 1001101, then I am supposed to make a finite-state machine that turns it into 1100111 (every other bit changed, starting from the 2nd). I know how to change the bits in a machine, what I need help for is how to determine the index of the input, that is how to let the finite-state machine know i.e. that 0 which happens to be the second bit of the string, is in fact the second.
Thanks.

 

[livcon]

I realized that this problem is quite easy and requires only two states, $$s_{0}$$ and $$s_{1}$$, using the first one as initial state. When the machine receives either 0 or 1 in the initial state, the output function simply returns the same bit as the input as it changes state from $$s_{0}$$ to $$s_{1}$$. From this state the bit changes on its way back to state $$s_{0}$$.