Interpretation of loop of repetition

Question

I’m having trouble understanding the following code that is presented as a book exercise.

$a = true;
$b = -2;
$c = 7;

for ( ; $b < $c || $a; $b++){
    if ($c + $b * 2 > 20)
        $a = false;
        echo $b." ";
}

The result is: -2 -1 0 1 2 3 4 5 6 7

Because the first parameter of for is not past ? I need a complete explanation because I cannot assimilate the code with the result.

Answer 1

It’s a very confusing code and I don’t know if it’s good to try to understand if you still lack basic knowledge of how syntax works.

The for has no parameters, it has 3 statements, or statements to be made: the initialization of a variable, its stop condition, and the step, usually an increment. If he expects 3 have to put the 3, even if one of them goes blank, can not remove the ; because that would be 2.

If you do not need to initialize some variable there is no reason to put something, then you leave it blank. Rarely does this make sense. If initialize the variable will only exist inside the for, but as in this case has nothing else can initialize $b right there.

$c does not vary so I see no sense to keep this variable. Nothing useful is done with $a, then just take her out too.

The whole code has a readability problem, so it gets better and produces the same result:

for ($b = -2; $b < 7; $b++) echo $b . " ";

I put in the Github for future reference.

Answer 2

The narrative would be, b begins execution with -2, and while b is less than c, which in this case is worth 7, or a is true (true), the code from within the repeat loop runs, testing the following condition: if b that is worth -2 * 2 + c that is worth 7 is greater than 20, resets the state from a to false (false), and shows on the screen the value of b concatenated to an empty space. For each loop in the loop, b starts to assert itself + 1 by changing its value, the test is redone until the condition of the loop is not satisfied, either by the test of b < c or a being true. At least one of the two conditions must be true for the loop to happen.

$a = true; // Valor inicial de desta variável.
$b = -2; // Valor inicial de desta variável.
$c = 7; // Valor inicial de desta variável.

// Para cada vez que a repetição contatar que b realmente é menir que c ou a for verdadeiro, executa o código identado após arepetição e acrescenta mais 1 unidade ao valor de b.
for ( ; $b < $c || $a; $b++){
    // Todas as vezes que as condições acima forem satisfeitas, é realizada uma multiplicação, depois uma adição, e por fim, testado se seu valor é mair do que 20.
    if ($c + $b * 2 > 20)
        // Caso as condições acima sejam satisfeitas, o estado de a é modificado para false, e é mostrado na tela o valor de b.
        $a = false;
        echo $b." ";

    // Por fim o loop adiciona uma unidade ao valor de b, conforme citado anteriormente, e o loop volta ao inicio para um novo teste.
}

Answer 3

After the proper explanations and operation of the FOR loop, both in javascript and PHP (to emphasize the similarity of the two), see at the end of this reply, your code commented and running passo a passo in the ideone.

The for statement creates a loop consisting of three optional expressions, within parentheses and separated by semicolons, followed by a statement or a sequence of statements executed in sequence.

for ([inicialização]; [condição]; [expressão final])

initialization

An expression (including assignment expressions) or variable statements. Usually used to start the variable counter. This expression can optionally declare new variables with the var keyword. These variables are not local in the loop, i.e., they are in the same scope as the for loop is. The result of this expression is discarded.

condition

An expression to be evaluated before each loop iteration. If this expression is evaluated to true, statement will be executed. This condition test is optional. If omitted, the condition will always be evaluated as true. If the expression is evaluated as false, the execution will go to the first expression after the loop construction is. final expression An expression that will be evaluated at the end of each loop iteration. This occurs before the next condition assessment. Usually used to update or increment the counter variable.

statement

A statement that is executed as long as the condition is true. To perform multiple conditions within the loop, use a block instruction ({...}) to group these conditions. To not execute statements within the loop, use an empty statement (;).

Examples of use

The declaration starts by declaring the variable i and initializing it as 0. It checks if i is less than nine, executes the two subsequent instructions and increments 1 variable i after each passage through the loop.

for (var i = 0; i < 9; i++) {
   console.log(i);
}

All three expressions in the for loop condition are optional.

For example, in the boot block, it is not necessary to initialize variables:

var i = 0;
for (; i < 9; i++) {
    console.log(i);
}

As with the boot block, the condition is also optional. If you are omitting this expression, you should make sure to break the loop in the body so as not to create an infinite loop.

for (var i = 0;; i++) {
   console.log(i);
   if (i > 3) break;
}

You can also omit all three blocks. Again, make sure to use a break instruction at the end of the loop and also modify (increment) a variable, so the break condition is true at some point.

var i = 0;

for (;;) {
  if (i > 3) break;
  console.log(i);
  i++;
}

SOURCE

With PHP It’s very similar, see

Examples of use

The declaration starts by declaring the variable i and initializing it as 0. It checks if i is less than nine, executes the two subsequent instructions and increments 1 variable i after each passage through the loop.

In this first example we have our common FOR, as we all learn:

 <?php

    for($i = 0; $i < 5; $i++)
    {
            echo $i;
            echo '<br />';
    }

In this second example, we take the first parameter from FOR, and leave the $i variable defined outside the loop:

 <?php

    $i = 0;

    for(; $i < 5; $i++)
    {
            echo $i;
            echo '<br />';
    }

Now, let’s take the third parameter, and increment the variable within the loop.

<?php

    $i = 0;

    for(; $i < 5;)
    {
            echo $i;
            echo '<br />';
            $i++;
    }

Finally, we will remove the second parameter.

<?php

    $i = 0;

    for(; ; )
    {
            echo $i;
            echo '<br />';

            if($i == 5)
            {
                    break;
            }

            $i++;
    }

Want to see to believe? São Tomé test on ideone

Note that in all of these examples above, we escape the Infinite Loop with some condition or mechanism that we put inside the loop. Be careful. This last example if nothing is done inside the loop, easily generates an infinite loop.

Your commented code

$a = true;
$b = -2;
$c = 7;
$condicao="";


//realiza uma iteração FOR toda vez que $b for menor que $c ou $a for true
//retiramos o primeiro parâmetro do FOR, a variável $b já foi definida fora do laço
for ( ; $b < $c || $a; $b++){

        if ($c + $b * 2 > 20)
        // quando a condição acima for verdadeira, $a se torna false
        $a = false;
        //e a condição FOR já não será mais verdadeira, isto é, zefini, ou seja, "c'est fini"
        echo $b."  ";

}

Follow the step by step of your code on IDEONE

Answer 4

As stated in the comments: "The variable $b is already stated"

The traditional format of the loop for is:

for (
    $i=0; //Declaração de variável
    $i < 10; //Condição para que o loop continue
    $i++ //Ação a cada interação, no caso incremento
) {
}

In his:

//Declaração das variáveis
$a = true;
$b = -2;
$c = 7;

for (
    ; //Não há uma declaração porque ela já foi feita antes
    $b < $c || $a; //Duas condição para continuar o loop, que $b seja menor que $c e $a seja veridadeiro
    $b++ //Incrementa o valor de $b para continuar as iterações
) {
    if ($c + $b * 2 > 20)
        $a = false;
    echo $b." ";
}

It is important to add the first semicolon (;) because without it, it’s like missing a piece of code, PHP doesn’t divide those parts on its own

This code could also be written like this:

for (
    $a = true, $b = -2, $c = 7; //Muda as declarações de lugar
    $b < $c || $a; //Duas condição para continuar o loop, que $b seja menor que $c e $a seja veridadeiro
    $b++ //Incrementa o valor de $b para continuar as iterações
) {
    if ($c + $b * 2 > 20)
        $a = false;
    echo $b." ";
}

Or with another kind of loop:

$a = true;
$b = -2;
$c = 7;

while ($b < $c || $a){
    if ($c + $b * 2 > 20)
        $a = false;
    echo $b++." ";
    //O incremento poderia ser numa linha separada mas para reduzir deixei assim
}

Observing:

• The line breaks within the for is only to increase readability
• Watch your step, man echo has a tab further, giving the impression that it will only be executed if the condition if be true, which is not the case. Without the keys ({ }) only the next command of if will be "inside of it"