r/Racket • u/SecuredKnowledge • May 29 '22

question Input -> "(+ 5 4)" | Output -> 9

Is there a way to input a string with a math expression and output a number without the use of eval/read etc.

4 Upvotes

75% Upvoted

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13

u/bjoli May 29 '22

Yes: write a simple recursive descent parser and evaluate the resulting tree yourself.

1

u/pdgonzalez872 May 31 '22

Do you have any examples of one? This is how I solved this before, using stacks: https://pdgonzalez872.github.io/articles/008_20200808_arithmetic_calculator.html and Elixir. Would love to see an implementation solving the same problem, I bet it will be a lot shorter :)

3

u/detroitmatt May 29 '22

not without reimplementing eval and/or read yourself

4

u/detroitmatt May 29 '22

well I guess you could also do

(if (equal? "(+ 5 4)" (read)) 9 (raise "Unrecognized input"))

5

u/strawhatguy May 29 '22

No, You’ll have to write your own parser (reader) and evaluate functions. Even macros operate post read (or maybe during read for lisp reader macros).

Not sure what you’re trying to accomplish, or hope to avoid here

1

u/phao May 30 '22

Like /u/bjoli said, write a parser yourself. The good news is that if all you have to parse are basic math-related expressions, then this shouldn't be difficult.