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javascriptloops
MarcoTalin12
MarcoTalin12

Reputation: 31

How strict is the syntax of a for-loop

So I have a fairly good amount of experience in coding. I've dabbled in Basic, HTML, Javascript, C, and C++, though the ones I've been using most recently are HTML and Javascript.

I am incredibly familiar with the for-loop. I've used it many times to loop through arrays, to operate recursive functions, etc. I know what it does and how to use it, but my question is about how it works.

Premise

In most languages, the basic syntax of a for loop is such:

var upperLimit = 10;

for(var i = 0; i < upperLimit; i++) {
    /*Code to be executed*/
    console.log(i);
}

In Javascript, this will output the numbers from 0 to 9 in the console.

I know that the parentheses contains 3 parts, each separated by semicolons.

Question

So, again, my question is how strict are these definitions?

The initialization's definition doesn't allow for much. It just says that that line is executed once, it's executed before anything else in the loop, and it's scope is limited to the loop. I can't think of much else you'd want to put in that position other than an iterative variable.

But what about the other two? I've seen codes where the condition is simply a variable, and as long as it's positive (since positive numbers taken as a boolean just covert to true), the loop continues.

Then there's the increment, which seems to be the loosest of these parts. Is it really just the last thing to be executed in a code, or does it explicitly need to iterate the variable declared in the initialization? It seems to be the former for the languages I'm familiar with.

For example, I decided to make a non-standard for-loop, and I came up with this routine:

var numbers = [0,1,2,3,4,5,6,7,8,9];
        
for(var i = 0;
    numbers.length;
    console.log(numbers.pop())) {}

It runs exactly as I expected: It outputs each member of the numbers array in the console in descending order, leaving an empty numbers array afterwards, and it's done using what is basically an empty for-loop.

Ending

So are my assumptions correct? If so, are there any practical applications for using a for-loop in a format apart from the one I wrote at the top of this question (possibly closer to he second format)?

Upvotes: 2

Views: 194

Answers (3)

Eric Zeng
Eric Zeng

Reputation: 135

Before all, you give a array

var numbers = [0,1,2,3,4,5,6,7,8,9];

The codes below is a correct for loop.

for(var i = 0;
    numbers.length;
    console.log(numbers.pop())) {}

Javascript defined for like this

for ([initialization]; [condition]; [final-expression])
   statement

For you code initialization is 'var i = 0', and execute once at start of loop.

The condition is 'numbers.length', and value is 10. When a number not 0, Javascript will convert it to boolean true. So condition is true.

The final-expression is 'console.log(numbers.pop())'. When execute 'numbers.pop()', numbers.length change to 9. But it still is true.

At second time, condition will return true again. The final-expression is execute too.

Until numbers.length become 0, Javascript convert it to boolean false. The loop will end.

Upvotes: 1

vp_arth
vp_arth

Reputation: 14992

In most languages, the basic syntax of a for loop is such:

for(initialization; condition; iteration) {
    /*Code to be executed*/
}

Both three are usual expressions and you can use any valid expressions here:

for(
  var i=arr.length, othercond=true; 
  i; 
  othercond?i--:i++, console.log(i),updateothercond()
);

Upvotes: 0

Ruan Mendes
Ruan Mendes

Reputation: 92314

  • The scope of the initialized variable is not limited to the loop, it's valid for the whole function (undefined before that line). You can initialize multiple variables using a comma. for (var i=0, j=1; i < 10; i++)
  • The second part, anything that evaluates to a truthy value will cause the loop to keep going:
    • Truthy: 1, -1, true, "something", {a: 2}
    • Falsey: 0, false, null, undefined
    • You could omit this and rely on a break in your code
  • The third part just lets you update the looping variable, you could omit it and do it within the for loop body.

Here's an answer that provides a nice way to loop that is non-standard, and comes with caveats, please see the link. 

var list = [{a:1,b:2}, {a:3,b:5}, {a:8,b:2}, {a:4,b:1}, {a:0,b:8}];

for (var i=0, item; item = list[i]; i++) {
  // Look no need to do list[i] in the body of the loop
  console.log("Looping: index ", i, "item" + item);
}

Upvotes: 0

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