freeCodeCamp/curriculum/challenges/english/08-coding-interview-prep/rosetta-code/s-expressions.english.md

title, id, challengeType, forumTopicId

title	id	challengeType	forumTopicId
S-Expressions	59667989bf71cf555dd5d2ff	5	302303

Description

S-Expressions are one convenient way to parse and store data.

Instructions

Write a simple reader/parser for S-Expressions that handles quoted and unquoted strings, integers and floats. The function should read a single but nested S-Expression from a string and return it as a (nested) array. Newlines and other whitespace may be ignored unless contained within a quoted string. "()" inside quoted strings are not interpreted, but treated as part of the string. Handling escaped quotes inside a string is optional; thus "(foo"bar)" may be treated as a string "foo"bar", or as an error. For this, the reader need not recognize "\" for escaping, but should, in addition, recognize numbers if the language has appropriate data types. Note that with the exception of "()"" ("\" if escaping is supported) and whitespace there are no special characters. Anything else is allowed without quotes. The reader should be able to read the following input

((data "quoted data" 123 4.5)
(data (!@# (4.5) "(more" "data)")))

and turn it into a native data structure. (See the Pike, Python and Ruby implementations for examples of native data structures.)

Tests

tests:
  - text: <code>parseSexpr</code> is a function.
    testString: assert(typeof parseSexpr === 'function');
  - text: <code>parseSexpr('(data1 data2 data3)')</code> should return <code>['data1', 'data2', 'data3']</code>
    testString: assert.deepEqual(parseSexpr(simpleSExpr), simpleSolution);
  - text: <code>parseSexpr('(data1 data2 data3)')</code> should return an array with 3 elements.
    testString: assert.deepEqual(parseSexpr(basicSExpr), basicSolution);

Challenge Seed

function parseSexpr(str) {
  // Good luck!
  return true;
}

After Test

const simpleSExpr = '(data1 data2 data3)';
const simpleSolution = ['data1', 'data2', 'data3'];

const basicSExpr = '((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))';
const basicSolution = [["data","\"quoted data\"",123,4.5],["data",["!@#",[4.5],"\"(more\"","\"data)\""]]];

Solution

function parseSexpr(str) {
  const t = str.match(/\s*("[^"]*"|\(|\)|"|[^\s()"]+)/g);
  for (var o, c = 0, i = t.length - 1; i >= 0; i--) {
    var n,
      ti = t[i].trim();
    if (ti == '"') return;
    else if (ti == '(') t[i] = '[', c += 1;
    else if (ti == ')') t[i] = ']', c -= 1;
    else if ((n = +ti) == ti) t[i] = n;
    else t[i] = `'${ti.replace('\'', '\\\'')}'`;
    if (i > 0 && ti != ']' && t[i - 1].trim() != '(') t.splice(i, 0, ',');
    if (!c) if (!o) o = true; else return;
  }
  return c ? undefined : eval(t.join(''));
}