This has nothing to do with numpy, the issue is the string "%" operator cannot convert list to a float (Line 3 in your example gives a list). In '"%f" % x' the x has to be a number or tuple.
Does not work: print "%f" % [1.0] Works: print "%f" % 1 print "%f" % (1, ) print "%f" % tuple([1]) print "%f" % a[0] Hope this helps, Pavol -- http://mail.python.org/mailman/listinfo/python-list