# How to access variables and expressions faster in jump

**URL:** <https://discourse.julialang.org/t/how-to-access-variables-and-expressions-faster-in-jump/94123>\
**Category:** Optimization (Mathematical)\
**Tags:** question, jump, dataframes\
**Created:** [February 6, 2023, 8:34am UTC](https://discourse.julialang.org/t/how-to-access-variables-and-expressions-faster-in-jump/94123 "2023-02-06T08:34:53Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Betristor](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/betristor/32/45264_2.png) [@Betristor](https://discourse.julialang.org/u/Betristor)\
**Post date:** [February 6, 2023, 8:34am UTC](https://discourse.julialang.org/t/how-to-access-variables-and-expressions-faster-in-jump/94123/1 "2023-02-06T08:34:53Z")

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I established a very large model using julia with plenty of variables and expressions. Though not all of these will be meaningful for accessing, I still wrote all into results in dataframe and then saved to csv files. I’d like to know how could I access a column or row of a large matrix, let’s say in two or three dimensions with index length over hundred, and fill them into a dataframe with fast speed.

For example, I have an expression named `NeededExpr` in JuMP with index f,z,t, the length of them is 6, 31 and 168 (or larger). And I need `NeededExpr[f, z, t]` with given f and z, should I use `value.(NeededExpr[f, z, t])` or `value.(NeededExpr)[f, z, t]`?

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**Author:** ![jd-foster](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jd-foster/32/35824_2.png) [@jd-foster](https://discourse.julialang.org/u/jd-foster)\
**Post date:** [February 6, 2023, 8:42am UTC](https://discourse.julialang.org/t/how-to-access-variables-and-expressions-faster-in-jump/94123/2 "2023-02-06T08:42:48Z")

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Moving this to the Optimization category (you’re more likely to get people who know the answers to your question).

In the meantime, have a look at this section of the JuMP docs:

> **[Containers · JuMP](https://jump.dev/JuMP.jl/stable/manual/containers/#Tables)**
>
> Documentation for JuMP.

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<div class="post-metadata">

**Author:** ![jd-foster](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jd-foster/32/35824_2.png) [@jd-foster](https://discourse.julialang.org/u/jd-foster)\
**Post date:** [February 6, 2023, 10:02am UTC](https://discourse.julialang.org/t/how-to-access-variables-and-expressions-faster-in-jump/94123/3 "2023-02-06T10:02:52Z")

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> [@Betristor](#):
>
> And I need `NeededExpr[f, z, t]` with given f and z, should I use `value.(NeededExpr[f, z, t])` or `value.(NeededExpr)[f, z, t]`?

If you’re asking which is faster for access to the single value, it’s the former and the dot . is not needed. If you want to obtain a whole column of values, it’s the latter.

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<div class="post-metadata">

**Author:** ![Betristor](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/betristor/32/45264_2.png) [@Betristor](https://discourse.julialang.org/u/Betristor)\
**Post date:** [February 6, 2023, 3:00pm UTC](https://discourse.julialang.org/t/how-to-access-variables-and-expressions-faster-in-jump/94123/4 "2023-02-06T15:00:17Z")

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Typically, the last index represents time and is the biggest. In practice we need to store and access it for many times. When I discussed with others about how julia store and access data, we came into a claim that julia stores and accesses data in column order, so if we place t at the first position like `NeededExpr[t, f, z]` and execute `value.(NeededExpr)[:, f, z]`, will this be faster than `NeededExpr[f, z, t]` and `value.(NeededExpr)[f, z, :]` with given f and z?

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**Author:** ![odow](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/odow/32/28685_2.png) [@odow](https://discourse.julialang.org/u/odow)\
**Post date:** [February 6, 2023, 7:42pm UTC](https://discourse.julialang.org/t/how-to-access-variables-and-expressions-faster-in-jump/94123/5 "2023-02-06T19:42:58Z")

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> [@Betristor](#):
>
> And I need `NeededExpr[f, z, t]` with given f and z, should I use `value.(NeededExpr[f, z, t])` or `value.(NeededExpr)[f, z, t]`?

`value.(NeededExpr[f, z, t])`.

```Julia
# value.(NeededExpr[f, z, t]) is equivalent to
a = NeededExpr[f, z, t]
value.(a)

# value.(NeededExpr)[f, z, t] is equivalent to

a = value.(NeededExpr)
a[f, z, t]

```

The second approach computes the value of **all** variables, and then subsets that. Computing the value of every variable only to pick a few is expensive.

> we came into a claim that julia stores and accesses data in column order

Ignore this suggestion (for now). It is almost certainly not something that will make a material difference to the runtime of your function.

The real answer for what you should do depends on how you’re going to use the results.

- If you want only a few individual values once, then use `value.(NeededExpr[f, z, t])`.
- If you’re going to eventually access the value of every variable, and some indices multiple times, then store the solution once in `solution = value.(NeededExpr)` and query from that.

The most common bottleneck is getting solutions out of the solver with the `value` call.

It’s hard to provide more specific advice without a reproducible code example of exactly what you’re trying to do and achieve.