# One line updating of rows of a matrix
**URL:** https://discourse.julialang.org/t/one-line-updating-of-rows-of-a-matrix/63845
**Category:** New to Julia
**Created:** [June 30, 2021, 4:59pm UTC](https://discourse.julialang.org/t/one-line-updating-of-rows-of-a-matrix/63845 "2021-06-30T16:59:39Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@Mark\_Williams](https://discourse.julialang.org/u/Mark_Williams)
#### Post date: [June 30, 2021, 4:59pm UTC](https://discourse.julialang.org/t/one-line-updating-of-rows-of-a-matrix/63845/1 "2021-06-30T16:59:40Z")
I have a simple recursive procedure to populate a matrix which can be made easier by populating row by row. i.e. calculating the row and copying it to the matrix. I can’t find a way of making it work and returning the matrix of stacked rows. I can do the calculation elementwise but that’s rather unnatural given the theory.
```julia
P = [0.8 0.1 0.1; 0.1 0.7 0.2; 0.05 0.05 0.9]
init_vec = [0.1 0.8 0.1]
function Find_L(init_vec, P)
L = zeros(3,3)
L[1,:] = init_vec
[L[t,:] = transpose(P)*L[t-1,:] for t in 2:size(P,2)]
return L
end
```
Is there a nice way of doing something like this that returns a matrix?
---
### Author:  [@DNF](https://discourse.julialang.org/u/DNF)
#### Post date: [June 30, 2021, 5:10pm UTC](https://discourse.julialang.org/t/one-line-updating-of-rows-of-a-matrix/63845/2 "2021-06-30T17:10:29Z")
> [@Mark\_Williams](#):
>
> ```julia
> P = [0.8 0.1 0.1; 0.1 0.7 0.2; 0.05 0.05 0.9]
> init_vec = [0.1 0.8 0.1]
>
> function Find_L(init_vec, P)
> L = zeros(3,3)
> L[1,:] = init_vec
> [L[t,:] = transpose(P)*L[t-1,:] for t in 2:size(P,2)]
> end
>
> ```
You forgot to return `L` from the function.
---
### Author:  [@sijo](https://discourse.julialang.org/u/sijo)
#### Post date: [June 30, 2021, 5:14pm UTC](https://discourse.julialang.org/t/one-line-updating-of-rows-of-a-matrix/63845/3 "2021-06-30T17:14:23Z")
As @DNF says, though I think that’s an abuse of array comprehension. It creates a useless array that is immediately thrown away, and it obfuscates the meaning of the code. What’s wrong with a plain old `for` loop?
```julia
function Find_L(init_vec, P)
L = zeros(3,3)
L[1,:] = init_vec
for i = 2:size(P,2)
L[i,:] = transpose(P)*L[i-1,:]
end
return L
end
```
---
### Author:  [@Mark\_Williams](https://discourse.julialang.org/u/Mark_Williams)
#### Post date: [June 30, 2021, 5:19pm UTC](https://discourse.julialang.org/t/one-line-updating-of-rows-of-a-matrix/63845/4 "2021-06-30T17:19:24Z")
Mostly a supervisor who likes my code to read in a certain way. I agree there shouldn’t be anything wrong with the above.
---
### Author:  [@Mark\_Williams](https://discourse.julialang.org/u/Mark_Williams)
#### Post date: [June 30, 2021, 5:20pm UTC](https://discourse.julialang.org/t/one-line-updating-of-rows-of-a-matrix/63845/5 "2021-06-30T17:20:02Z")
Yep that’s a typo. Thanks
---
### Author:  [@DNF](https://discourse.julialang.org/u/DNF)
#### Post date: [June 30, 2021, 5:25pm UTC](https://discourse.julialang.org/t/one-line-updating-of-rows-of-a-matrix/63845/6 "2021-06-30T17:25:57Z")
Also, keep in mind that Julia arrays are column major, so it is faster and more natural to work with columns than rows.
---
### Author:  [@DNF](https://discourse.julialang.org/u/DNF)
#### Post date: [June 30, 2021, 5:29pm UTC](https://discourse.julialang.org/t/one-line-updating-of-rows-of-a-matrix/63845/7 "2021-06-30T17:29:05Z")
The loop @sijo wrote is _much_ clearer and easier to read. Change the orientation to avoid `transpose`, use a loop, and stand up to your supervisor 😉 They will respect you more if you do 😃