# Mathematica - (Very) Simple Question

by Sosi
Tags: mathematica, simple
 P: 11 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!
 P: 37 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.
P: 11
 Quote by jpreed 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!

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