eval() function for Arduino?

Question

I plan to do the following:

void setup(){
  String term = "12 + (2 * 5)";
  int result = eval(term); // eval function (from Python)
  Serial.println(result); // expecting 22
}

However there is no eval function. Are there any alternatives?

Answer 1

TinyExpr does what you want, and more.

TinyExpr is a very small recursive descent parser and evaluation engine for math expressions. It's handy when you want to add the ability to evaluation math expressions at runtime without adding a bunch of cruft to you project.

In addition to the standard math operators and precedence, TinyExpr also supports the standard C math functions and runtime binding of variables.

Your example

You'd just need to save tinyexpr.h and tinyexpr.c next to your Sketch.

#include "tinyexpr.h"

void setup() {
  Serial.begin(115200);
}

void loop() {
  char term[] = "12 + (2 * 5)";
  Serial.println(term);
  int error;
  double result = te_interp(term, &error);
  if (error){
    Serial.println("Problem with expression.");
  } else {
    Serial.printf(" = %.10g\n", result);
  }
  delay(1000);
}

It outputs:

12 + (2 * 5)
 = 22

Since term is known at compile-time, this example is not very useful, and it simply outputs 22 in an endless loop.

Basic interactive console

If you use the input from Serial, you can write a very basic interactive console. A String has been used for conciseness, but this example could be rewritten with cstrings:

#include "tinyexpr.h"

void setup() {
  Serial.begin(115200);
}

void loop() {
  if (Serial.available() > 0) {
    String expression = Serial.readStringUntil('\n');
    Serial.print("> ");
    Serial.println(expression);

    int error;
    double result = te_interp(expression.c_str(), &error);

    if (error) {
      Serial.print(" ");
      for (int i = 0; i < error; i++) {
        Serial.print(" ");
      }
      Serial.println("↑");
      Serial.println("I didn't understand this part.");
    } else {
      Serial.printf(" = %.10g\n", result);
    }
  }

  delay(50);
}

TinyExpr knows the order of operations, and it understands many other mathematical expressions:

> 12 + 2 * 5
 = 22
> 3^5
 = 243
> exp(7)
 = 1096.633158
> (1 + sqrt(5)) / 2
 = 1.618033989
> 1 + 2 * 3 + 4 * 5
 = 27
> sin(pi/2)
 = 1
> sin(pi/3)
 = 0.8660254038

If there's a syntax error somewhere, error tells you the position of the first encountered error:

> 3 + 
      ↑
I didn't understand this part.
> sin() * 5
      ↑
I didn't understand this part.
> 3 / (4 + 5 * 7
                ↑
I didn't understand this part.

Answer 2

No, there are no alternatives. You need to parse the string and perform the operations yourself.

Personally I prefer to use RPN since it is so much easier to parse. Your string could be re-written then as:

2,5,*,12,+

Tokenise on , and look to see if it's a number (push on a stack) or an operator (pop operands off stack, do operation, push result on stack).

Answer 3

I Googled and the news isn't good. There isn't an off the shelf function that does this in C/C++. There are plenty of links to algorithms that you could implement. But would you have the memory to do it on an Arduino?

So, looking at the problem for a different perspective why don't you run python on the Arduino? There is a version called pymite which runs on a mega.