Skip to main content

programming - When to use Get vs Needs


When should I use << vs Needs? I just always use Needs as << seems less safe. I tried to find the answer in the documentation, but all it really seems to say is that Needs uses Get. My understanding is that Needs will only do the import once, whereas Get will happily do it again. So is this just a style issue? Or are there reasons to use one or the other?




Answer



Needs versus Get



< reads in a file, evaluating each expression in it, and returning the last one.



(<< is shorthand for Get.)



Needs["context`"] loads an appropriate file if the specified context is not already in $Packages.



Needs is normally appropriate for making sure that a package has been loaded, but when you need to force the package to reload you want Get. One simple example of the difference is if you have loaded the Notation package but closed the Palette. Calling Needs["Notation`"] will not reload the package, and will not cause the Palette to be re-displayed, whereas Get["Notation`"] will.




Get can be used for loading several different formats containing data or definitions, in addition to formal packages. See this answer for an overview of such methods.



A related function that deserves mentioning is DeclarePackage. While Get does a hard load and Needs does a soft one, DeclarePackage does a delayed load. That is, it only loads the package when a certain Symbol (from a list of one or more) is first used. For example if you might want to use functions from the Calendar package you do not need to load it into memory, but rather may use:


DeclarePackage["Calendar`", {"DayOfWeek", "DaysPlus", "DaysBetween"}]

Now if any of those three functions are used the Calendar package will be transparently loaded and the function will work. Be aware that if other function names from the package are used before one of these three you may create a shadowing problem. You could add all package Symbols to the DeclarePackage statement, or you could just be aware of the problem and act accordingly.


Comments

Post a Comment

Popular posts from this blog

plotting - Filling between two spheres in SphericalPlot3D

Manipulate[ SphericalPlot3D[{1, 2 - n}, {θ, 0, Pi}, {ϕ, 0, 1.5 Pi}, Mesh -> None, PlotPoints -> 15, PlotRange -> {-2.2, 2.2}], {n, 0, 1}] I cant' seem to be able to make a filling between two spheres. I've already tried the obvious Filling -> {1 -> {2}} but Mathematica doesn't seem to like that option. Is there any easy way around this or ... Answer There is no built-in filling in SphericalPlot3D . One option is to use ParametricPlot3D to draw the surfaces between the two shells: Manipulate[ Show[SphericalPlot3D[{1, 2 - n}, {θ, 0, Pi}, {ϕ, 0, 1.5 Pi}, PlotPoints -> 15, PlotRange -> {-2.2, 2.2}], ParametricPlot3D[{ r {Sin[t] Cos[1.5 Pi], Sin[t] Sin[1.5 Pi], Cos[t]}, r {Sin[t] Cos[0 Pi], Sin[t] Sin[0 Pi], Cos[t]}}, {r, 1, 2 - n}, {t, 0, Pi}, PlotStyle -> Yellow, Mesh -> {2, 15}]], {n, 0, 1}]
Post a Comment
Read more

plotting - Plot 4D data with color as 4th dimension

I have a list of 4D data (x position, y position, amplitude, wavelength). I want to plot x, y, and amplitude on a 3D plot and have the color of the points correspond to the wavelength. I have seen many examples using functions to define color but my wavelength cannot be expressed by an analytic function. Is there a simple way to do this? Answer Here a another possible way to visualize 4D data: data = Flatten[Table[{x, y, x^2 + y^2, Sin[x - y]}, {x, -Pi, Pi,Pi/10}, {y,-Pi,Pi, Pi/10}], 1]; You can use the function Point along with VertexColors . Now the points are places using the first three elements and the color is determined by the fourth. In this case I used Hue, but you can use whatever you prefer. Graphics3D[ Point[data[[All, 1 ;; 3]], VertexColors -> Hue /@ data[[All, 4]]], Axes -> True, BoxRatios -> {1, 1, 1/GoldenRatio}]
Post a Comment
Read more

plotting - Adding a thick curve to a regionplot

Suppose we have the following simple RegionPlot: f[x_] := 1 - x^2 g[x_] := 1 - 0.5 x^2 RegionPlot[{y < f[x], f[x] < y < g[x], y > g[x]}, {x, 0, 2}, {y, 0, 2}] Now I'm trying to change the curve defined by $y=g[x]$ into a thick black curve, while leaving all other boundaries in the plot unchanged. I've tried adding the region $y=g[x]$ and playing with the plotstyle, which didn't work, and I've tried BoundaryStyle, which changed all the boundaries in the plot. Now I'm kinda out of ideas... Any help would be appreciated! Answer With f[x_] := 1 - x^2 g[x_] := 1 - 0.5 x^2 You can use Epilog to add the thick line: RegionPlot[{y < f[x], f[x] < y < g[x], y > g[x]}, {x, 0, 2}, {y, 0, 2}, PlotPoints -> 50, Epilog -> (Plot[g[x], {x, 0, 2}, PlotStyle -> {Black, Thick}][[1]]), PlotStyle -> {Directive[Yellow, Opacity[0.4]], Directive[Pink, Opacity[0.4]],
Post a Comment
Read more