NSolve vs. N[Solve ]
- To: mathgroup at smc.vnet.net
- Subject: [mg103423] NSolve vs. N[Solve ]
- From: Helen Read <hpr at together.net>
- Date: Mon, 21 Sep 2009 05:51:07 -0400 (EDT)
- Reply-to: HPR <read at math.uvm.edu>
A while back (I forget what version), N was modified so that it will give output to any number of significant digits that you desire. Previously, N[x,k] would output 6 significant digits if k was anything less than machine precision. Asking for N[x,3] or N[x,15] or whatever would give output to 6 significant digits, while N[x,k] for k>=16 would give x to k significant digits. Lately my students have been using NSolve with a third argument (for the precision) -- which I hadn't told them about -- to solve a problem that I was actually expecting them to do a different way. What they did was a reasonable solution, but unfortunately they didn't notice that their results from NSolve were not coming out to the number of significant digits that they asked for. Evidently NSolve behaves like the old N, and differs from N[Solve ] For example, compare these: NSolve[E^x==10,x,3] (* result given to 6 significant digits, despite asking for only 3 *) N[Solve[E^x==10,x],3] (* result given to the desired precision *) NSolve[E^x == 10, x, 12] (* result given to only 6 significant digits, despite asking for 12 *) N[Solve[E^x==10,x],12] (* result given to the desired precision *) I hadn't noticed this unfortunate behavior until I started seeing it in my students' work. -- Helen Read University of Vermont
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