# Could double summation be implemented without JuMP?

Hello, I encounter a problem when trying to convert these(Python) to Julia:

``````delta_TRC = [sum((i - TRC)**2) for i in TRC]
return sum(delta_TRC)
``````

I found that the syntax of sum function provided by JuMP is very similar to these, but it is possible to implement only with the standard function ( whose syntax: sum(f, itr; [init]) )?
I am new to both Python and Julia, so I would appreciate if you explain the issue in detail…
Thank you.
The following is for reference, if needed:

``````from scipy.optimize import minimize
class Portfolio:
def __init__(self,port_a,port_b,port_c,port_d):
self.port_a=port_a
self.port_b=port_b
self.port_c=port_c
self.port_d=port_d

self.df=data[["Date",port_a,port_b,port_c,port_d]].dropna().reset_index(drop=True)
ret=self.df[[port_a,port_b,port_c,port_d]]
self.Rcov=ret.cov()
self.cov=np.array(ret.cov())

def risk_budget_objective(self,weights,cov):
weights = np.array(weights) #one-dimension array
sigma = np.sqrt(np.dot(weights, np.dot(cov, weights)))
MRC = np.dot(cov,weights)/sigma
TRC = weights * MRC
delta_TRC = [sum((i - TRC)**2) for i in TRC]
return sum(delta_TRC)
``````

Hello and welcome to Julia!

In short, the answer is: yes, you can do the same thing in base Julia than what Python+Numpy allows, with almost the same syntax.

The only syntactic difference lies in `sum((TRC - i)**2)`. In this expression, `i` is a number (a scalar), `TRC` is an array of such numbers, and in this context the computation is supposed to mean:

• `tmp1 = TRC - i` build the array obtained by subtracting the number `i` to every element of `TRC`
• `tmp2 = tmp1**2` build the array obtained by squaring `tmp1` elementwise
• `sum(tmp2)` compute the sum of all elements in `tmp2`

You see here that the arithmetic operations used to build `tmp1` and `tmp2` above are to be understood as elementwise operations; they are not simple operations between scalar numbers. This is where languages differ:

• plain python lists do not allow this
• `[1,2,3] - 1` is an error
• for numpy arrays, arithmetic operations are implicitly considered to be elementwise
• `numpy.array([1, 2, 3]) - 1 == numpy.array([0, 1, 2])`
• Julia allows to do this without any library, but you need to be more explicit by using dot operators
• `[1,2,3] .- 1 == [0,1,2]`

Apart from the need to “dot” the operators (and the fact that power is denoted by `^` in Julia), the same syntax is supported:

``````julia> TRC = [1.0, 2.1, 3.2]
3-element Vector{Float64}:
1.0
2.1
3.2

julia> delta_TRC = [sum((i .- TRC).^2) for i in TRC]
3-element Vector{Float64}:
6.050000000000001
2.4200000000000004
6.050000000000001

julia> sum(delta_TRC)
14.520000000000001
``````

You can even write the double summation in one expression, which could be more performant:

``````julia> sum(sum((i .- TRC).^2) for i in TRC)
14.520000000000001
``````
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