Logic problem in Javascript

Question

I am asking to print only the name and notes. However it is duplicating at the time of printing. Ie... If I typed John, 1,2,3,4; Peter,3,4,5,6. At the time of printing, he prints twice the same thing.

//criar sala de aula com 4 alunos que possuem 4 notas e depois calcular a média de cada aluno.
//Sem objeto

var sala = [];
var aluno = [];

for (var i = 0; i < 4; i++) {
  for (var j = 0; j < 5; j++) {
    if (j == 0) {
      aluno.push(prompt("Digite o nome"));
    } else {
      aluno.push(prompt("Digite a nota"));
    }

  }
  sala.push(aluno);
}
console.info(sala.length);
for (var i = 0; i < sala.length; i++) {
  for (var j = 0; j < sala[i].length; j++) {
    if (j == 0) {
      console.log(sala[i][j]);
    } else {
      //console.log(parseFloat(sala[i][j]));
      console.log(sala[i][j]);
    }
  }
}

Answer 1

In the statement, he asks to create one room, and four students, and these with four notes. Only, by creating your variables, you create one student only:

var sala = [];
var aluno = []; // <- isso deveria representar apenas 1 aluno

This in itself does not represent a problem, you could keep more students in this same array, but then you wouldn’t need the array sala. See where the problem is:

for (var i = 0; i < 4; i++) {
    for (var j = 0; j < 5; j++) {
        if (j == 0) {
            aluno.push(prompt("Digite o nome")); // aluno recebe 4 nomes
        } else {
            aluno.push(prompt("Digite a nota")); // e 16 notas
        }
    }
    sala.push(aluno); // o mesmo aluno é passado para sala, 4 vezes
}

That is to say:
sala[0], sala[1], sala[2] and sala[3] will have reference to exactly the same object. For a "visual" example of what is occurring, take a look at this fiddle.

This is because in javascript, arrays are objects, and therefore are passed as reference for functions and not as copy of value as is the case of primitives.

The @joserogerio84 solution works, because it’s not just "zeroing" the array with every iteration, it’s creating a new.

See an example working below:

/*
*  criar sala de aula com 4 alunos que possuem 4 notas
*  e depois calcular a média de cada aluno.
*  Sem objeto
*/

    let sala = [];
    let aluno = [];
    let t = document.getElementById('table').getElementsByTagName('tbody')[0];

    for (let i = 0; i < 4; i++) {
        for (let j = 0; j < 5; j++) {
            if (j == 0) {
                aluno.push(prompt("Digite o nome"));
            } else {
                aluno.push(prompt("Digite a nota"));
            }
        }
        sala.push(aluno); // mandamos a referencia desse aluno para sala
        aluno = []; // isso cria um novo array com o mesmo nome
    }

    for (let i = 0; i < sala.length; i++) {
        let linha = t.insertRow(t.rows.length);
        for (let j = 0; j < sala[i].length; j++) {
          saida(linha, j, sala[i][j]);   
        }
    }
    
function saida(linha, numero, texto) {
    linha.insertCell(numero).appendChild(document.createTextNode(texto));
}



 

table, th, td {
    border: 1px solid black;
    border-collapse: collapse;
}

th, td {
    padding: 10px;
}

<table id="table">
  <th>Nome</th>
  <th>Nota</th>
  <th>Nota</th>
  <th>Nota</th>
  <th>Nota</th>  
</table>

Answer 2

It was only necessary to reset the array of students at the start of the:

//criar sala de aula com 4 alunos que possuem 4 notas e depois calcular a média de cada aluno.
//Sem objeto

var sala = [];

for (var i = 0; i < 4; i++) {
  var aluno = [];

  for (var j = 0; j < 5; j++) {
    if (j == 0) {
      aluno.push(prompt("Digite o nome"));
    } else {
      aluno.push(prompt("Digite a nota"));
    }

  }
  sala.push(aluno);
}
console.info(sala.length);
for (var i = 0; i < sala.length; i++) {
  for (var j = 0; j < sala[i].length; j++) {
    if (j == 0) {
      console.log(sala[i][j]);
    } else {
      //console.log(parseFloat(sala[i][j]));
      console.log(sala[i][j]);
    }
  }
}