hi everybody,   i've got no idea to solve my problem so you are my last hope! the solution may be quite simple, but not for me. so...this is the task: an array with x columns and y rows with a variety of values. i just want to replace the variety of values by three simple values. for this purpose, i defined three conditions. here an example: value = fltarr(cols, rows) seg = fltarr(cols,rows) mean = 3.4 sd = 1.4 K=0.82 a = where(value gt -5) b=where((value lt -5) and (value gt -7.5) and (value-mean/sd gt K)) c=where(value lt -75) seg[a]=1 seg[b]=0.5 seg[c]=0 so....the compilation is ok, but the run of the script results in an error massage: attempt to subscript seg with b isout of range...as i take it, there must be an error in b=where(...). can anybody tell me, how to use three conditions in where(...).   thanks in advance, cwesche