How to calculate values within arrays?

Question

I have the following algorithm to perform the calculation of a Polish reverse calculator.

function  pilhaNumeroVisor() {
  var pilha = [] ;
  var num = document.calcform.visor.value;
  if(num != "" ) {
    pilha.push(num);
    document.calcform.visor.value = "" ;
  }
  if (num == "" ) {
    resultado = pilha[0] + pilha[1];
    document.calcform.visor.value = resultado;

  } 
} 

What was to return in a input the sum result, returns NaN. Can someone help me? Thank you in advance!

Answer 1

Will always give NaN because you are declaring the array within the function; therefore, each time the function is called, the array will be empty because it is redeclared.

Another point is: if you try to add the first two indexes of the array and one of them does not exist, it will result in NaN also.

You can use the method .reduce() to add together the array elements as you add values.

See how it would look (explanatory comments in the code):

// declara a array fora da função
var pilha = [];

function  pilhaNumeroVisor() {

   // declara a variável "resultado"
   var resultado;
   var num = document.calcform.visor.value;

   if(num != "" ) {
      
      // adiciona o número na array em tipo "number"
      pilha.push(Number(num));
      document.calcform.visor.value = "" ;
   }

   if(num == "" ) {

      if(pilha.length){
         // altera a variável somando os valores
         resultado = pilha.reduce(function(a,b){
            return a + b;
         });
      }

      // o visor só vai receber valor se resultado for válido
      document.calcform.visor.value = resultado || '';
   } 
   
}

<form name="calcform">
   <input type="text" name="visor">
   <button type="button" onClick="pilhaNumeroVisor()">OK</button>
</form>

Now, you can also use this more streamlined code:

var pilha = [];

function pilhaNumeroVisor(){

   var resultado,
   vis = document.calcform.visor,
   num = vis.value;

   if(num){
      pilha.push(Number(num));
      vis.value = "";
   }else if(pilha.length){
      resultado = pilha.reduce(function(a,b){
         return a + b;
      });
   } 
   vis.value = resultado || '';
}