substitute should be able of replacing an expression with a variable

[ufechner7]

The following code does not work:

# replace an expression with a variable
using Symbolics

@variables t
@syms ω(t) Pg(t) Pge(t) Pgc(t)
@variables A R a b Γ ρ U J Q

expr = (0.5A*R*a*Γ*ρ*(U^2)) / (J*ω(t)) - J*((0.5A*Γ*ρ*(U^3)*(b + (R*a*ω(t)) / U) - Pgc(t) - Pge(t)) / ((J^2)*(ω(t)^2)))
# Q = 0.5A*R*a*Γ*ρ

# how to substitute 0.5A*R*a*Γ*ρ by Q in expr?
expr2 = substitute(expr, Dict([0.5A*R*a*Γ*ρ => Q]))

Result:

(Pgc(t) + Pge(t) - 0.5A*Γ*ρ*(U^3)*(b + (R*a*ω(t)) / U)) / (J*(ω(t)^2)) + (0.5A*R*a*Γ*ρ*(U^2)) / (J*ω(t))

Expected result:

(Pgc(t) + Pge(t) - 0.5A*Γ*ρ*(U^3)*(b + (R*a*ω(t)) / U)) / (J*(ω(t)^2)) + (Q*(U^2)) / (J*ω(t))

Related: #988

Discussion on Discourse: https://discourse.julialang.org/t/how-to-simplify-a-symbolic-expression/104381

[shashi]

I can implement something for this.

[ufechner7]

That would be very kind! 👍