How to use a while loop with irregular step?

i would like to use a "while" loop ( while i<20) but with an irregular step. For example it goes this way : i=1,2,3, 5,7,9,12,15,18....it means it starts by a 1-step then by 2 then by a 3-step.

Answer 1

[Walter Roberson]

while loops do not have steps. "for" loops can have steps but also can be irregular:

for i = [1,2,3, 5,7,9,12,15,18]

[bh dhouha]

So is it impossible to do the thing with while? i can not change the counter inside the loop ? ( like if i>3 i=i+2 )

[Walter Roberson]

If you do not change the value inside the loop then you have an infinite loop, so Yes, change it inside the loop in any way you prefer.

[bh dhouha]

Just a final point :) if i'm going to use the for loop as u mentioned , and the sequence is too long [1 ,2 ,3,.....2000] how can i write this case : if i in [1,5] take 1-step. if i in [6,10] take a 2-step ....etc?

[Walter Roberson]

[1:1:5, 6:2:10, 11:3:20 ...]

[bh dhouha]

thanks a lot

Answer 2

[David Young]

You can do it with a for loop very simply:

for i = [1,2,3, 5,7,9, 12,15,18]
  % < process using i >
end

By the way, some people take the view that it is unwise to use i or j as ordinary variables because of potential confusion with the imaginary unit, so it might be better to use ii or k or something.

[bh dhouha]

The loop that i mentioned is just a simple example.But what i have in reality is much complex and needs a "while" loop not a for loop. I know the "for" loop can be used with irregular increment but the problem is does that work with the "while" loop or no ? Thanks for the answer :)

[bh dhouha]

By the way, if i'm going to use the for loop as u mentioned , and the sequence is too long [1 ,2 ,3,.....2000] how can i write this case : if i in [1,5] take 1-step. if i in [6,10] take a 2-step ....etc?