Skip to main content

Using the value of the variable in the definition of the function



How do I take the current value of n in the second line in order for the answers to be the same in both cases?


n = 2;
f[x_] := x/n;

Print[f[8]];
n = 4;
Print[f[8]];

Now:


4
2

Want:


4

4

Basically, I want the function to be x/2 after my definition. (Please don't suggest defining it as x/2, I need to define a list of functions inside the cycle.)


UPDATE


The question I asked looks oversimplified, so suggested solutions do not really work for me. Here is a more realistic example.


I want to create a list of functions, each acting on a complicated argument:


ft = {};
For[n = 1, n <= 3, n++,
tmp[x_] := x[[1]]/n;
AppendTo[ft, tmp];

];
ft[[1]][{12, 1}]
ft[[2]][{12, 1}]
ft[[3]][{12, 1}]

The generated output is


3
3
3


The desired output is:


12
6
4

I cannot replace := with = since this produces an error.



Answer



First, it is much better to use Table to construct a list instead of using For and AppendTo. Second, in order to inject the value of n into your function definition, you need to use With. Finally, instead of defining tmp 3 times (so that each definition overrides the previous definition), you can define tmp[n] 3 times. Putting this together we have:


ft = Table[
With[{n=n},

tmp[n][x_] := x[[1]]/n;
tmp[n]
],
{n, 3}
]

ft[[1]][{12,1}]
ft[[2]][{12,1}]
ft[[3]][{12,1}]



{tmp[1], tmp[2], tmp[3]}


12


6


4



Address OP question


When you use SetDelayed, this means that the RHS is not evaluated when the definition is created. Example:


n=3;
f[x_] := x/n

DownValues[f]


{HoldPattern[f[x_]] :> x/n}



Notice how the definition has n instead of 3. In order to get the value of n inserted into your definition you need to use Set (although there are many cases where you can't use Set) or you need to use a method to insert the value of n, a common one being With:


With[{n = 3},
f[x_] := x/n
];
DownValues[f]



{HoldPattern[f[x$_]] :> x$/3}



Comments

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}]

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}]

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]],