Use conditional statement to choose between different for loops in Javascript

Question

Depending on a certain condition, I want one of two different for loops to execute. So in the example below, I want console.log to return descending numbers if a > b, and ascending numbers if b > a. I was in no way surprised that this didn't work, but it gives you the idea of what I'm after.

for (if (a > b) {(i = 2; i > 0; i--)} else {(i = 0; i < 2; i++)} {
console.log(i);
};

Obviously I could work around this by having if-else select between two complete and separate loops each with their own console.log(i) code block, but in the case I'm dealing with, that will involve duplicating a huge amount of code.

what code would be duplicated a bunch? The body of the loop? If so, extract it to a separate function. — Nicholas Tower
– Nicholas Tower, Commented Oct 8, 2018 at 16:14
what you want to achieve? post some sample input and expected output. I think you're doing over engineering with simple task i.e getting ascending and descending numbers with if and for — A l w a y s S u n n y
– A l w a y s S u n n y, Commented Oct 8, 2018 at 16:15

Jonas Wilms · Accepted Answer · 2018-10-08 16:19:56Z

4

 for(let i = a > b ? 2 : 0 ; a > b ? i > 0 : i < 2; a > b ? i-- : i++)
   console.log(i);

you have to move the condition into each expression. Or move the body in its own function:

 function body(i) {
   console.log(i);
 }

 if (a > b) {
   for(i = 2; i > 0; i--) body(i);
 } else {
   for(i = 0; i < 2; i++) body(i);
}

Or the functional way:

 const maybe = (condition, op) => arg => condition ? op(arg) : arg;
 const range = (start, end) => Array.from({ length: end - start }, (_, i) => start + i);

maybe(a > b, it => it.reverse())(range(0, 2)).forEach(console.log);

edited Oct 8, 2018 at 16:19

answered Oct 8, 2018 at 16:16

Jonas Wilms

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Comments

CertainPerformance · Accepted Answer · 2018-10-08 16:17:22Z

1

You could put the body of the loop into a function, and on every iteration of both loops, call that function. Also note that for loop } end brackets should not have semicolons at the end:

const a = 4;
const b = 5;

if (a > b) {
  for (let i = 2; i > 0; i--) {
    loop(i);
  }
} else {
  for (i = 0; i < 2; i++) {
    loop(i);
  }
}
function loop(i) {
  console.log(i);
}

answered Oct 8, 2018 at 16:17

CertainPerformance

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Comments

Lenny86 · Accepted Answer · 2018-10-08 16:33:28Z

1

jsfiddle

function choose(a, b) {
  if (a > b) {
    for (let i = 2; i >= 0; i--) {
      console.log(i);
    }
  } else {
    for (let i = 0; i <= 2; i++) {
      console.log(i);
    }
  }
}

choose(2, 4);

answered Oct 8, 2018 at 16:33

Lenny86

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Comments

Tom Balcaen · Accepted Answer · 2018-10-08 16:16:32Z

0

I would turn it around like this: First do if statement then for loop.

if(a > b){
  for(let i = 2; i > 0; i--) console.log(i)
} else {
  for(let i = 0; i < 2; i++) console.log(i)
}

answered Oct 8, 2018 at 16:16

Tom Balcaen

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Comments

Willem van der Veen · Accepted Answer · 2018-10-08 16:18:50Z

0

Just try wrapping it in a function like this:

function loop(a, b) {
  if (a > b) {
    for (i = 2; i > 0; i--) {
      console.log(i);
    }
  } else {
    for (i = 0; i < 2; i++) {
      console.log(i);
    }
  }

}


loop(5,4);
loop(4,5);

answered Oct 8, 2018 at 16:18

Willem van der Veen

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