Diodes,when will it be undefined
Maybe because for Iin &lt; 0 both the diodes are surely off, because the current is forced to flow in a direction opposite to that permitted by the diode. 
In that case D1 and D2 would be open circuits and the current could not flow nor in the first branch (with R1 and D1), neither in the second (with D2): Vout would be then the voltage across the Iin generator and if Iin is ideal, voltage is not defined across it (only the current is defined).