I assume that what you mean is that you'd like matlab to display numbers as -0.0003 instead of 3e-4. If so,
will change the display to avoid scientific notation as much as possible.
If instead you're saying you want to store -0.0003 instead of 3e-4,. that makes no sense, they're the same numbers (and are stored internally in a completely different format). Furthermore, due to the way numbers are stored on a computer, it's actually impossible to store -0.0003 exactly (in ieee754 representation such as double or single). The actual number that is stored in memory is actually -0.0002999999999999999737189393389513725196593441069126129150390625, the nearest representable number to -0.0003. Any program will round that to -0.0003 for display.