# How to change my Mach number to stop at 2.89 instead of it going higher
**URL:** https://discourse.julialang.org/t/how-to-change-my-mach-number-to-stop-at-2-89-instead-of-it-going-higher/108699
**Category:** New to Julia
**Tags:** question, plotting
**Created:** [January 11, 2024, 6:08pm UTC](https://discourse.julialang.org/t/how-to-change-my-mach-number-to-stop-at-2-89-instead-of-it-going-higher/108699 "2024-01-11T18:08:58Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@Abood\_123](https://discourse.julialang.org/u/Abood_123)
#### Post date: [January 11, 2024, 6:08pm UTC](https://discourse.julialang.org/t/how-to-change-my-mach-number-to-stop-at-2-89-instead-of-it-going-higher/108699/1 "2024-01-11T18:08:58Z")
so the graph I’m getting here has a line that goes to up pretty much double the amount that I need it to go to. It’s supposed to stop at 2.893 as that’s the highest Mach number my nozzle goes up to but can’t seem to get it to end there. Can someone please have a look at the snippet and tell me where I’m going wrong.
```julia
R_star = 0.2476
A_star = R_star^2 * π
gamma = 1.1706
x_values = P_X:0.01:1.5
y_values = R_throat .+ x_values.^2
M = zeros(length(x_values))
for i in 1:length(x_values)
if x_values[i] == 0
M0 = 1.0
elseif x_values[i] < 0
M0 = 0.5
else
M0 = 5
end
function f(M)
return A_star / (y_values[i]^2 * π) * sqrt(((M^2 * γ - M^2 + 2) / (γ + 1))^((γ + 1) / (γ - 1))) - M
end
M[i] = find_zero(f, M0)
end
plot(x_values, M, xlabel="x", ylabel="Mach Number", title="Mach Analysis")
```
---
### Author:  [@mike.ingold](https://discourse.julialang.org/u/mike.ingold)
#### Post date: [January 11, 2024, 6:48pm UTC](https://discourse.julialang.org/t/how-to-change-my-mach-number-to-stop-at-2-89-instead-of-it-going-higher/108699/2 "2024-01-11T18:48:16Z")
Is there more code than what you provided here? You defined a variable `gamma` at the top but then reference a variable `γ` in your `f(M)` definition.
Other than that, nothing jumps out at me as an obvious error. It might help if you showed us a formatted/written version of the `f(M)` equation you’re trying to code to make sure that code actually says what you intended. The `for i in 1:length` loop with by-index writes/accesses is not very “Julian” code, so there’s probably a more elegant implementation that could be made, but that’s probably not central to the problem you’re currently trying to solve; feel free to ask for follow-up if interested in those tips.
---
### Author:  [@Abood\_123](https://discourse.julialang.org/u/Abood_123)
#### Post date: [January 11, 2024, 7:15pm UTC](https://discourse.julialang.org/t/how-to-change-my-mach-number-to-stop-at-2-89-instead-of-it-going-higher/108699/3 "2024-01-11T19:15:35Z")
Hello Mike, yes there’s some other coding lines above what I provided that include (\gamma) with the same value which is why i wrote it that way as its got a saved value in there. And can you explain what you meant by wanting to see a formatted/written version of the f(M) equation?
And sure i’d like to see what other more elegant/“Julian” way of writing the for ~~~for i in 1:length~~~ loop, as it would make my code better.
---
### Author:  [@mike.ingold](https://discourse.julialang.org/u/mike.ingold)
#### Post date: [January 11, 2024, 9:07pm UTC](https://discourse.julialang.org/t/how-to-change-my-mach-number-to-stop-at-2-89-instead-of-it-going-higher/108699/4 "2024-01-11T21:07:28Z")
> [@Abood\_123](#):
>
> And can you explain what you meant by wanting to see a formatted/written version of the f(M) equation?
If I’m not mis-reading something, I’m interpreting your code as implementing this equation:
f(M) = \frac{A^\*}{\pi y^2} \sqrt{ \left( \frac{\gamma M^2 - M^2 + 2}{\gamma+1} \right)^{(\gamma+1)/(\gamma-1)} } - M
Does this look right? It can be easy to make syntax errors in complicated equations.
I have to run right now but I’ll be back later with the other referenced tips.
---
### Author:  [@Abood\_123](https://discourse.julialang.org/u/Abood_123)
#### Post date: [January 11, 2024, 10:10pm UTC](https://discourse.julialang.org/t/how-to-change-my-mach-number-to-stop-at-2-89-instead-of-it-going-higher/108699/5 "2024-01-11T22:10:31Z")
Yes that’s how the function looks like when typing it on by hand.
And no worries take your time
---
### Author:  [@mike.ingold](https://discourse.julialang.org/u/mike.ingold)
#### Post date: [January 12, 2024, 2:46am UTC](https://discourse.julialang.org/t/how-to-change-my-mach-number-to-stop-at-2-89-instead-of-it-going-higher/108699/6 "2024-01-12T02:46:37Z")
So, in general, there’s a common code pattern in languages where you:
- Establish an empty vector
- Loop `for i in 1:length(vector)`
- Do something to `v[i]`
Julia has really great affordances for [array/vector operations and broadcasting](https://docs.julialang.org/en/v1/manual/arrays/#man-array-and-vectorized-operators-and-functions) and functional programming tools like [map](https://docs.julialang.org/en/v1/base/collections/#Base.map) that can usually simplify this code and improve performance. This usually means defining a function that operates per element and then broadcasting/map’ing it over the entire vector at once. Example:
```julia
x_values = P_X:0.01:1.5
y_values = R_throat .+ x_values.^2
function M(x, y)
# Transform your loop code into a single function of
# the corresponding x and y values that determine it.
return M
end
# This map generates pairs of (x,y) values from these input
# vectors and applies the M function to them, collecting the
# outputs into a new vector
M_values = map(M, x_values, y_values)
```