How Do You Substitute Variable Values in Mathematica?

[Sosi]

Hi!
I'm doing my first steps in Mathematica and I've wanted to do something simple like assigning a variable to the output and then use it. Something like this:

In:Vp = vprx == kprx2*X1;
In:dX1 = kp X0 + X1 (-cat kcatl - kgpx gpx - kprx2 prx2);
In:rX1 = Solve[dX1 == 0, X1];
Out:{{X1 -> (kp X0)/(catalase kcatl + gpx kgpx + kprx2 prx2)}}

then i want to Sbustitute X1 in the expression Vp, with the value of the output of rX1.I'm using the function Replace but this doesn't do what I want. The output is:

In:Replace[Vp, X1 -> rX1]
Out:vprx == kprx2 X1

when it should be

Out:vprx==(kprx2kp X0)/(catalase kcatl + gpx kgpx + kprx2 prx2)

Thanks a lot for all the help!

 

[jpreed]

There are two problems, one is simple, one is more complicated.

First, use "ReplaceAll" instead of "Replace", it uses a deeper algorithm that will properly do what you want it to do.

Second, look at the result of "rX1". It is NOT just a number that can be plugged in. It is a list that has one component with a variable, "X1", being transformed to a larger expression.

Try using:

ReplaceAll[Vp, rX1[[1]] ]

This takes the first component of the rX1 list and uses it in the ReplaceAll function.

 

[Sosi]

Try using:

ReplaceAll[Vp, rX1[[1]] ]

This takes the first component of the rX1 list and uses it in the ReplaceAll function.

Awsome! It works just as I wanted! Thanks a lot!