# Evaluate string based on dictionary values

**URL:** <https://discourse.julialang.org/t/evaluate-string-based-on-dictionary-values/40135>\
**Category:** New to Julia\
**Created:** [May 25, 2020, 3:24pm UTC](https://discourse.julialang.org/t/evaluate-string-based-on-dictionary-values/40135 "2020-05-25T15:24:05Z")\
**Posts on this page:** 8\
**Page:** 1

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**Author:** ![catosp](https://avatars.discourse-cdn.com/v4/letter/c/91b2a8/32.png) [@catosp](https://discourse.julialang.org/u/catosp)\
**Post date:** [May 25, 2020, 3:24pm UTC](https://discourse.julialang.org/t/evaluate-string-based-on-dictionary-values/40135/1 "2020-05-25T15:24:05Z")

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Hello!

Is possible to evaluate a string something like “x + y” or “x \> y” based on values of x and y taken from dictionary?  
In python works if I write eval(“x+y”, dict), but in julia I can’t do it.

Thank you!

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**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [May 25, 2020, 4:08pm UTC](https://discourse.julialang.org/t/evaluate-string-based-on-dictionary-values/40135/2 "2020-05-25T16:08:51Z")

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Yes, it’s possible. But what problem are you trying to solve here? Why are you passing expressions as strings rather than as functions?

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**Author:** ![catosp](https://avatars.discourse-cdn.com/v4/letter/c/91b2a8/32.png) [@catosp](https://discourse.julialang.org/u/catosp)\
**Post date:** [May 25, 2020, 4:20pm UTC](https://discourse.julialang.org/t/evaluate-string-based-on-dictionary-values/40135/3 "2020-05-25T16:20:46Z")

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I need this for evaluation of some expressions from text file.  
The text file content is something like this:

Parameters: a, b, c  
If a+b \>c {  
Print “ok”  
}

I managed to write an if, while and for parser but I don’t know how to parse the a+b\>c or other expression, so i need to use julia parser and eval but with values from dictionaries.

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**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [May 25, 2020, 4:27pm UTC](https://discourse.julialang.org/t/evaluate-string-based-on-dictionary-values/40135/4 "2020-05-25T16:27:06Z")

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So, you are defining a custom input language? Why not have the input language be Julia, possibly with macros to add domain-specific syntax if needed?

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**Author:** ![catosp](https://avatars.discourse-cdn.com/v4/letter/c/91b2a8/32.png) [@catosp](https://discourse.julialang.org/u/catosp)\
**Post date:** [May 25, 2020, 4:30pm UTC](https://discourse.julialang.org/t/evaluate-string-based-on-dictionary-values/40135/6 "2020-05-25T16:30:49Z")

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Well, this is the problem… I don’t want to define in julia program x and y, I want to evaluate an expression with x and y defined in dictionary. The values from dictionary will be introduced from readline() in console.

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**Author:** ![catosp](https://avatars.discourse-cdn.com/v4/letter/c/91b2a8/32.png) [@catosp](https://discourse.julialang.org/u/catosp)\
**Post date:** [May 25, 2020, 4:33pm UTC](https://discourse.julialang.org/t/evaluate-string-based-on-dictionary-values/40135/7 "2020-05-25T16:33:55Z")

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@stevengj yes this is a pseudolanguage for data aquisition using serial, usb and gpib protocol.

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**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [May 25, 2020, 4:34pm UTC](https://discourse.julialang.org/t/evaluate-string-based-on-dictionary-values/40135/8 "2020-05-25T16:34:51Z")

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If you want to define local variables from a dictionary, you can do:

```julia
function parse_eval_dict(s::AbstractString, locals::Dict{Symbol})
    ex = Meta.parse(s)
    assignments = [:($sym = $val) for (sym,val) in locals]
    eval(:(let $(assignments...); $ex; end))
end

```

hence

```julia
julia> parse_eval_dict("x + y", Dict(:x => 30, :y => 47))
77

```

But if I were you I would re-think your whole approach. Rather than defining and parsing a brand new “pseudolanguage”, it would be vastly easier, more flexible, and faster to just let users write Julia code. If you want to provide a compact domain-specific syntax you can do it with macros.

Note that if you are worried about safety/security (i.e. if the user inputs aren’t trusted), you will need to filter the `Meta.parse` output to allow only a small set of allowed expressions.

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**Author:** ![catosp](https://avatars.discourse-cdn.com/v4/letter/c/91b2a8/32.png) [@catosp](https://discourse.julialang.org/u/catosp)\
**Post date:** [May 25, 2020, 4:46pm UTC](https://discourse.julialang.org/t/evaluate-string-based-on-dictionary-values/40135/9 "2020-05-25T16:46:46Z")

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Ok, thank you very much! I want to write a pseudo-language for users who don’t know how to program verry well and to interact with measurement intruments. I want that most of the work to be done under the hood, and the final user only to write some intuitive lines of code. I will think about your idea! Thanks!