How input current affects circuit? Is it ignored?
There are voltage and current sources. In the case of an ideal voltage source, the source maintains a constant voltage, and the load draws whatever current it needs. 

\$I=\dfrac{V}{R}\$

In the case of an ideal current source, the source maintains a constant current, and the load voltage is induced by that current.

\$V=IR\$

Your example falls in the voltage source category. No one is ignoring the input current. They solve for it because it is determined by the load.