# MethodError: no method matching getindex(::VariableRef, ::Int64)

**URL:** <https://discourse.julialang.org/t/methoderror-no-method-matching-getindex-variableref-int64/56252>\
**Category:** Optimization (Mathematical)\
**Tags:** question, jump\
**Created:** [March 1, 2021, 3:10pm UTC](https://discourse.julialang.org/t/methoderror-no-method-matching-getindex-variableref-int64/56252 "2021-03-01T15:10:10Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![Kavitha](https://avatars.discourse-cdn.com/v4/letter/k/edb3f5/32.png) [@Kavitha](https://discourse.julialang.org/u/Kavitha)\
**Post date:** [March 1, 2021, 3:10pm UTC](https://discourse.julialang.org/t/methoderror-no-method-matching-getindex-variableref-int64/56252/1 "2021-03-01T15:10:10Z")

</div>

Hi,

I’m using JuMP to solve my optimization problem. Following is a part of the formulation and its causing the methoderror.

for t in 1:tp  
for j in 1:nt  
c10= @constraint(p, LT[j][t] - (l[j] \* Z[j][t]) == 0)  
set\_name(c10, “LTConstraint\_[$j],[$t]”)  
end  
end

The variables are defined as follows:  
@variable(p, Z[j in 1:nt, t in 1:tp] \>=0, Int)  
@variable(p, LT[j in 1:nt, t in 1:tp] \>=0, Int)

and l[j] is a one dimensional array; l =[1,6,4,3,1,10,8,2,8,5,5,5,1,4]

The constraint c10 gives me the error: MethodError: no method matching getindex(::VariableRef, ::Int64)  
I’m not sure what’s causing the error. Could anyone please help me understand what is causing the error.

Any help is deeply appreciated. Thanks in Advance!!

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<div class="post-metadata">

**Author:** ![Henrique\_Becker](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/henrique_becker/32/15443_2.png) [@Henrique\_Becker](https://discourse.julialang.org/u/Henrique_Becker)\
**Post date:** [March 1, 2021, 4:14pm UTC](https://discourse.julialang.org/t/methoderror-no-method-matching-getindex-variableref-int64/56252/2 "2021-03-01T16:14:58Z")

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I would guess that ` LT[j][t]` and `Z[j][t]` should instead be `LT[j, t]` and `Z[j, t]`. Otherwise you are getting a `VariableRef` from your matrix and then trying to index the variable.

Please, if possible use triple backticks to quote your code next time. When using triple backticks:

```  
for t in 1:tp  
for j in 1:nt  
c10= @constraint(p, LT[j][t] - (l[j] \* Z[j][t]) == 0)  
set\_name(c10, “LTConstraint\_[$j],[$t]”)  
end  
end  
```

becomes

```julia
for t in 1:tp
    for j in 1:nt
        c10= @constraint(p, LT[j][t] - (l[j] * Z[j][t]) == 0)
        set_name(c10, "LTConstraint_[$j],[$t]")
    end
end

```

What is more legible.

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<div class="post-metadata">

**Author:** ![Kavitha](https://avatars.discourse-cdn.com/v4/letter/k/edb3f5/32.png) [@Kavitha](https://discourse.julialang.org/u/Kavitha)\
**Post date:** [March 1, 2021, 4:51pm UTC](https://discourse.julialang.org/t/methoderror-no-method-matching-getindex-variableref-int64/56252/3 "2021-03-01T16:51:05Z")

</div>

Thank you @henrique_becker for getting back to me. It did solve the error. How ever, I do have one other error after making the suggested change. Here the indices of variables are separated by comma but I still get a method matching error. Could you please help me with this one as well.

MethodError: no method matching -(::VariableRef, ::Array{Int64,1})

```julia
for t in 1:tp
	for i in 1:NJ[t]
	 		c2= @constraint(p, B[t,i,1] - nums[t,i] <= 0)
	 		set_name(c2, "InitConstraint_[$i],[$t]")
	end
end

```

nums[t,i] is a 2D array.

```julia
tp = 3
NJ = [7,5,8]
nums = [[13,2,21,16,11,2,5], [29,30,26,28,8], [8,5,16,130,5,13,30,4]]

```

Also, sorry about the previous format of the code.  
Thank you in advance!!

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<div class="post-metadata">

**Author:** ![Henrique\_Becker](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/henrique_becker/32/15443_2.png) [@Henrique\_Becker](https://discourse.julialang.org/u/Henrique_Becker)\
**Post date:** [March 1, 2021, 5:46pm UTC](https://discourse.julialang.org/t/methoderror-no-method-matching-getindex-variableref-int64/56252/4 "2021-03-01T17:46:26Z")

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The way you are declaring `nums` make it an _array of arrays_ and not a _matrix_, so for `nums` the correct is `nums[t][i]`. I recommend reading the [arrays section of the manual](https://docs.julialang.org/en/v1/manual/arrays/), but as a basic rule:

1. If your data is an `Array{T, 2}` (i.e., a matrix) then it is indexed with `m[i, j]`, and it has a size `N` x `M`, in other words, some rows or columns cannot be smaller than other rows or columns.
2. If your data is an `Vector{Vector{T}}` (or `Array{Array{T, 1}, 1}`), then it is an array of arrays (or vector of vectors) and is indexed with `a[i][j]` (just `a[i]` will return the inner vector, then you index the inner vector with `[j]` for the integer inside it). Each element of the outer vector is a vector, and of these inner vectors can have a different sizes.

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<div class="post-metadata">

**Author:** ![Kavitha](https://avatars.discourse-cdn.com/v4/letter/k/edb3f5/32.png) [@Kavitha](https://discourse.julialang.org/u/Kavitha)\
**Post date:** [March 1, 2021, 5:55pm UTC](https://discourse.julialang.org/t/methoderror-no-method-matching-getindex-variableref-int64/56252/5 "2021-03-01T17:55:33Z")

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Thank you very much @Henrique_Becker for your help and suggestion!!