# Writing matrix in Julia

**URL:** <https://discourse.julialang.org/t/writing-matrix-in-julia/100253>\
**Category:** General Usage\
**Created:** [June 13, 2023, 12:59am UTC](https://discourse.julialang.org/t/writing-matrix-in-julia/100253 "2023-06-13T00:59:24Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![Umishra](https://avatars.discourse-cdn.com/v4/letter/u/dc4da7/32.png) [@Umishra](https://discourse.julialang.org/u/Umishra)\
**Post date:** [June 13, 2023, 12:59am UTC](https://discourse.julialang.org/t/writing-matrix-in-julia/100253/1 "2023-06-13T00:59:24Z")

</div>

I wanted to write matrix for the total coin angular momentum with the following code

mvalues = -j:1:j

cardinality = length(mvalues)

# Jminus is zero except for directly above the diagonal

# There is an implicit \hbar & square root

Jminus = zeros(cardinality)

for i=2:cardinality  
m = mvalues[i]  
Jminus[i-1, i] = sqrt(j\*(j+1)-m\*(m-1))  
end  
After compiling, I am getting the following error  
BoundsError: attempt to access 9-element Vector{Float64} at index [1, 2]

I am new to Julia, therefore any suggestion would be very helpful.

---

<div class="post-metadata">

**Author:** ![Norman](https://avatars.discourse-cdn.com/v4/letter/n/97f17d/32.png) [@Norman](https://discourse.julialang.org/u/Norman)\
**Post date:** [June 13, 2023, 1:19am UTC](https://discourse.julialang.org/t/writing-matrix-in-julia/100253/2 "2023-06-13T01:19:18Z")

</div>

`Jminus = zeros(cardinality)` creates a vector of length `cardinality`. Maybe you want `Jminus = zeros(cardinality, cardinality)` for a square matrix?