{"id":20142,"date":"2022-11-08T21:04:13","date_gmt":"2022-11-08T15:34:13","guid":{"rendered":"https:\/\/jassweb.com\/solved\/solved-is-there-an-alternative-to-slice-in-javascript-closed\/"},"modified":"2022-11-08T21:04:13","modified_gmt":"2022-11-08T15:34:13","slug":"solved-is-there-an-alternative-to-slice-in-javascript-closed","status":"publish","type":"post","link":"https:\/\/jassweb.com\/solved\/solved-is-there-an-alternative-to-slice-in-javascript-closed\/","title":{"rendered":"[Solved] Is there an alternative to slice in Javascript? [closed]"},"content":{"rendered":"

[ad_1]
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You could write tokenize<\/code> which consumes your text input and produces a stream of tokens –<\/p>\n<\/p>\n

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const input = \n  [ '7', '3', '+', '6', 'x', '8', '+', '5', '4', 'x', '2' ]\n  \nfunction* tokenize (es) {\n  let r = 0, n\n  for (const e of es) {\n    switch (e) {\n      case \"+\":\n      case \"x\":\n        yield { number: r }\n        yield { operation: e }\n        r = 0\n        break\n      default:\n        n = Number.parseInt(e, 10)\n        if (Number.isNaN(n))\n          throw Error(`unexpected input: ${e}`)\n        else\n          r = r * 10 + n\n        break\n    }\n  }\n  if (r > 0) yield { number: r }\n}\n          \nfor (const t of tokenize(input))\n  console.log(t)<\/code><\/pre>\n<\/div>\n<\/div>\n
{\"number\":73}\n{\"operation\":\"+\"}\n{\"number\":6}\n{\"operation\":\"x\"}\n{\"number\":8}\n{\"operation\":\"+\"}\n{\"number\":54}\n{\"operation\":\"x\"}\n{\"number\":2}\n<\/code><\/pre>\n
Followed by parse<\/code> which consumes a stream of tokens and produces a abstract syntax tree –<\/p>\n
function parse (ts) {\n  let s = e => e\n  for (const t of ts) {\n    if (t?.number) {\n      let r = s(t)\n      s = _ => r\n    }\n    else if (t?.operation) {\n      let left = s()\n      s = right => ({ operation: t.operation, left, right })\n    }\n    else {\n      throw Error(`unexpected token: ${JSON.stringify(t)}`)\n    }\n  }\n  return s()\n}\n<\/code><\/pre>\n
console.log(parse(tokenize(input)))\n<\/code><\/pre>\n
{\n  operation: \"x\",\n  left: {\n    operation: \"+\",\n    left: {\n      operation: \"x\",\n      left: {\n        operation: \"+\",\n        left: { number: 73 },\n        right: { number: 6 }\n      },\n      right: { number: 8 }\n    },\n    right: { number: 54 }\n  },\n  right: { number: 2 }\n}\n<\/code><\/pre>\n
Finally eval<\/code> which evaluates the syntax tree as a program –<\/p>\n
function eval (e) {\n  if (e?.number)\n    return e.number\n  else if (e?.operation)\n    return evalOp(e)\n  else\n    throw Error(`unexpected expression: ${JSON.stringify(e)}`)\n}\n<\/code><\/pre>\n
We simplify eval<\/code> by writing evalOp<\/code> separately. This technique is known as mutual recursion and is super effective for traversing tree-like structures –<\/p>\n
function evalOp (e) {\n  switch (e?.operation) {\n    case \"+\": return eval(e.left) + eval(e.right)\n    case \"x\": return eval(e.left) * eval(e.right)\n    default: throw Error(`unexpected operation: ${e.operation}`)\n  }\n}\n<\/code><\/pre>\n
Bring tokenize<\/code>, parse<\/code>, and eval<\/code> together to compute the result –<\/p>\n
const input = \n  [ '7', '3', '+', '6', 'x', '8', '+', '5', '4', 'x', '2' ]\n\nconsole.log(eval(parse(tokenize(input))))\n<\/code><\/pre>\n
1372\n<\/code><\/pre>\n
This is correct when evaluated using pocket calculator order of operations –<\/p>\n
73 \n... + 6 = 79\n... * 8 = 632\n... + 54 = 686\n... * 2 = 1372\n<\/code><\/pre>\n
If you wanted to evaluate using a different order of operations, ie PEMDAS, you would have to rewrite parse<\/code> to interpret the stream of tokens as a different program.<\/p>\n
Expand the snippet below to verify the result in your browser –<\/p>\n<\/p>\n
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const input = \n  [ '7', '3', '+', '6', 'x', '8', '+', '5', '4', 'x', '2' ]\n\nfunction* tokenize (es) {\n  let r = 0, n\n  for (const e of es) {\n    switch (e) {\n      case \"+\":\n      case \"x\":\n        yield { number: r }\n        yield { operation: e }\n        r = 0\n        break\n      default:\n        n = Number.parseInt(e, 10)\n        if (Number.isNaN(n))\n          throw Error(`unexpected input: ${e}`)\n        else\n          r = r * 10 + n\n        break\n    }\n  }\n  if (r > 0) yield { number: r }\n}\n\nfunction parse (ts) {\n  let s = e => e\n  for (const t of ts) {\n    if (t?.number) {\n      let r = s(t)\n      s = _ => r\n    }\n    else if (t?.operation) {\n      let left = s()\n      s = right => ({ operation: t.operation, left, right })\n    }\n    else {\n      throw Error(`unexpected token: ${JSON.stringify(t)}`)\n    }\n  }\n  return s()\n}\n\nfunction eval (e) {\n  if (e?.number)\n    return e.number\n  else if (e?.operation)\n    return evalOp(e)\n  else\n    throw Error(`unexpected expression: ${JSON.stringify(e)}`)\n}\n\nfunction evalOp (e) {\n  switch (e?.operation) {\n    case \"+\": return eval(e.left) + eval(e.right)\n    case \"x\": return eval(e.left) * eval(e.right)\n    default: throw Error(`unexpected operation: ${e.operation}`)\n  }\n}\n\nconsole.log(eval(parse(tokenize(input))))<\/code><\/pre>\n<\/div>\n<\/div><\/div>\n
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            <\/span> <\/p><\/div>\n<\/div>\n
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solved Is there an alternative to slice in Javascript? [closed] <\/p>\n","protected":false},"excerpt":{"rendered":"
[ad_1] You could write tokenize which consumes your text input and produces a stream of tokens – const input = [ ‘7’, ‘3’, ‘+’, ‘6’, ‘x’, ‘8’, ‘+’, ‘5’, ‘4’, ‘x’, ‘2’ ] function* tokenize (es) { let r = 0, n for (const e of es) { switch (e) { case “+”: case “x”: … Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[320],"tags":[361,511,333,391,1558],"class_list":["post-20142","post","type-post","status-publish","format-standard","hentry","category-solved","tag-arrays","tag-iteration","tag-javascript","tag-loops","tag-slice"],"yoast_head":"\n[Solved] Is there an alternative to slice in Javascript? 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