JavaScript .push to an empty array not working

Question

I'm trying to solve the Valid Parentheses, and as described in title, I can't figure how to Array.push to work in this context.

function isValidPar(par){
  var stack = [];
  var length = par.length;
  
  for (var i=0; i<length; i++) {
    var p = par[i];
    if(p === "(" || p === "{" || p === "[") {
      stack.push(p);
    } else if(p === ")") {
        if(stack.length === 0 || stack[stack.length-1] != "(") {return false}
          stack.pop();
      } else if(p === "]") {
          if(stack.length === 0 || stack[stack.length-1] != "[") {return false}
            stack.pop();
      } else if(p === "}") {
          if(stack.length === 0 || stack[stack.length-1] != "{") {return false}
            stack.pop();
    } return stack.length === 0;
  }
}

If I console.log right after stack.push(), then it shows the element I just inserted. But when I try it anywhere else, like inside else if statements or before return, it seems like the array is empty.

Haven't read through all of it, but could it be that your return statement is one block too deep? — user3297291
– user3297291, Commented Oct 4, 2017 at 12:03
Your for loop will have only one iteration because you've put a return statement at the end of it. — Titus
– Titus, Commented Oct 4, 2017 at 12:05

marvel308 · Accepted Answer · 2017-10-04 12:05:30Z

4

Here is a corrected version of your code

function isValidPar(par){
  var stack = [];
  var length = par.length;
  
  for (var i=0; i<length; i++) {
    var p = par[i];
    if(p === "(" || p === "{" || p === "[") {
      stack.push(p);
    } else if(p === ")") {
        if(stack.length === 0 || stack[stack.length-1] != "(") {return false}
          stack.pop();
      } else if(p === "]") {
          if(stack.length === 0 || stack[stack.length-1] != "[") {return false}
            stack.pop();
      } else if(p === "}") {
          if(stack.length === 0 || stack[stack.length-1] != "{") {return false}
            stack.pop();
    } 
  }
  return stack.length === 0;
}

console.log(isValidPar('(())'));

return stack.length === 0;

should be checked once the for loop is finished

answered Oct 4, 2017 at 12:05

marvel308

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1 Comment

klee8_ Over a year ago

That was an absurd mistake! Thank you so much for your help.

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