i have signal data.i apply fourier transform and multiply by one number and apply inverse fourier transform.finally i have complex data.
What i choose as output? real section or imaginary section or abs of complex data?
What will be sign of data? in input signal in some points data are positive and in some other are negative.in output data how to choose sign of data?
I guess your data are real-valued and you are performing the (inverse) Fourier transform numerically (via a computer). Getting complex (instead of real) output always happens due to issues related to numerical precision. You will notice that the magnitude of the imaginary components of the output signal is very small, of order around $10^{-15}$, which essentially means that you can safely ignore them, i.e., keep only the real part of the output.