why doesn’t this work? it’s a simple console log inside a loop [closed]

var line = "Stands so high   ";

let position = -1;
for (; line[position] == " ";) {
  position--;
  console.log(position);
}

the output gives nothing. why is it this way? I want it to console.log the value position. I have used both node and the chrome developer tools console. Doesn’t work on either one.

Answer

As much as your question is not clear enough, I hope I can shed light to the use of for…loop.

for…loop is used for iteration. To traverse(pass-through) a sequence(such as string) or collections(such as arrays), which are subscriptable(i.e use index numbers). To be able to use for…loop syntax, you need to provide 3-parts in the for…loop declaration part. initialization, test_expression and step. Although, their use IS NOT RESTRICTED to within the pair of parenthesis. That is why your piece of code can be used just needs a little restructuring. In fact, they can be omitted entirely to form an infinite loop. See syntax below:

for (initialization ; test_expression ; step/incrementation_or_decrementation){ ... }


for (; ; ){ ... } // Omitting the 3-parts turns this for...loop into an infinite loop.

Before the body of the loop (denoted by … above) can be executed, your test_expression MUST evaluate to true. Your initialization and step are ensuring that at a point, that test_expression is evaluated to false, so that your for…loop does not enter into an infinite loop.

Let’s look closer at our Code[Follow my comment]:

var line="Stands so high   ";
let position=-1;  // This line serves your **initialization** part below.
for(;line[position]==" ";) {  // Your **test_expression** IS NOT evaluating to **true**
    position--;   // This line serves your **step** part above.
    console.log(position);
}

NOTE: In Javascript, it’s not possible to provide a negative index like you did in your for…loop declaration. This is 100% possible with Python but not JS. That’s why your test_expression IS NOT evaluating to true.

line[position]  // will always evaluates to **undefined** because position is **-1** for the 1st expected iteration.

…and undefined IS NOT EQUALS TO ” “.

SOLUTION 1: Consider restructuring your code as follows if you want to log position in the iteration up to the first occurrence of white-space:

var line="Stands so high   ";

let position=-1; // Your **initialization** suggest you're traversing from letter **S** to the right side.
for(;line[position]!=" ";) { // Consider changing **==** to **!=**
    position++; // Consider **step to increment** i.e move to next character to test it.
    console.log(position);
}

…if this is your intention, you can simply achieve this by quickly locating your first white-space.

var line="Stands so high   ";
console.log( line.indexOf(" ") ); // This will log 6, which is the position of first white-space.

SOLUTION 2: If your intention is to count the number of words in your variable line, use the following:

var line="Stands so high   ";
console.log( line.trim().replace("  ", " ").split(" ").length ); // This will output **3**, meaning 3-words.

…here, I have chained several methods to each other to feed the output of one method to the input of another. line.trim() removes the leading and trailing white-spaces(i.e spaces before and after the real sentence) .replace(” “, ” “) replace all occurrence of double white-spaces with single white-space. .split(” “) breaks the sentence into an array of words using the single white-space as delimiter. since white-spaces are used as separator of words. .length counts the total words in the array.

Feel free to ask me any question.