How to iterate arrays?

[evil-shrike]

Hi.

I'm trying to figure out how I can iterate array in expressions.
Array.map doesn't seem to work as mathjs doesn't understand lambda functions:

const node = mathjs.parse("entries.map(t => t.topic).join(';')");
let res = node.evaluate({entries: [ {topic: '1'}, {topic: '2'}]});

fails with the following error (about the => in map):

/Users/segy/work/samples/mathjs-sandbox/node_modules/mathjs/lib/cjs/expression/parse.js:1582
      throw createSyntaxError(state, 'Value expected');
      ^

SyntaxError: Value expected (char 16)
    at createSyntaxError (/Users/segy/work/samples/mathjs-sandbox/node_modules/mathjs/lib/cjs/expression/parse.js:1615:19)
    at parseEnd (/Users/segy/work/samples/mathjs-sandbox/node_modules/mathjs/lib/cjs/expression/parse.js:1582:13)
    at parseParentheses (/Users/segy/work/samples/mathjs-sandbox/node_modules/mathjs/lib/cjs/expression/parse.js:1569:12)
    at parseNumber (/Users/segy/work/samples/mathjs-sandbox/node_modules/mathjs/lib/cjs/expression/parse.js:1542:12)
    at parseObject (/Users/segy/work/samples/mathjs-sandbox/node_modules/mathjs/lib/cjs/expression/parse.js:1524:12)
    at parseMatrix (/Users/segy/work/samples/mathjs-sandbox/node_modules/mathjs/lib/cjs/expression/parse.js:1456:12)
    at parseString (/Users/segy/work/samples/mathjs-sandbox/node_modules/mathjs/lib/cjs/expression/parse.js:1350:12)
    at parseSymbol (/Users/segy/work/samples/mathjs-sandbox/node_modules/mathjs/lib/cjs/expression/parse.js:1248:12)
    at parseCustomNodes (/Users/segy/work/samples/mathjs-sandbox/node_modules/mathjs/lib/cjs/expression/parse.js:1221:12)
    at parseLeftHandOperators (/Users/segy/work/samples/mathjs-sandbox/node_modules/mathjs/lib/cjs/expression/parse.js:1145:12) {
  char: 16
}

[josdejong]

The expression parser of mathjs has a different syntax for lambda functions than JavaScript, see docs:

https://mathjs.org/docs/expressions/syntax.html#functions

In your case, you can do:

const node = mathjs.parse("map(entries, f(t) = t.topic).join(';')");
let res = node.evaluate({entries: [ {topic: '1'}, {topic: '2'}]}); 
// res = "1;2"

[evil-shrike]

thank you!