loop with decrementing step

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Dennis M
Dennis M on 26 Aug 2021
Commented: Dennis M on 2 Sep 2021
Good Day,
May I ask a question, how can I create loop that the output is like this (10 20 29 38 47 56 65 and so on) and (80 70 61 53 46 40 35 and so on) same in negative (-10 -20 -29 -38 -47 -56 -65 and so on) and (-80 -70 -61 -53 -46 -40 -35 and so on)
The start, end and step is variable
example I can start in 17 to 30 and start step is 5 to 0.125 just like this [17 (+5) 22 until 29.875 (+0.125) 30] same in negation
Dennis M
Dennis M on 26 Aug 2021
Thanks Sir Walter, I want start, end, and step to be variable and the step is decrementing with declare value when it reach the end value. Example the start step is 5 end of 0.125 when reach the desire end value. Illustrated code A= start:step:end A= 17:(linspace 5,0.125):30

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Accepted Answer

Walter Roberson
Walter Roberson on 26 Aug 2021
start = 17;
step = linspace(5,0.125);
stop = 30;
ZZZZZZZZZ_intermediates = start + cumsum([0 step]);
if start <= stop
ZZZZZZZZZ_intermediates = ZZZZZZZZZ_intermediates(ZZZZZZZZZ_intermediates<=stop);
ZZZZZZZZZ_intermediates = ZZZZZZZZZ_intermediates(ZZZZZZZZZ_intermediates>=stop);
for loop_variable = ZZZZZZZZZ_intermediates
%body of the loop goes here
17 22 26.9508
You might notice that the loop ended before 29.875 . You asked for the increments to be linspace(5,0.125) but remember by default linspace() is a vector of 100 values -- so the first increment would be 5, the second would be about 4.9508, the third is about 4.9501, and so on. Those quickly add up to more than the stop point.
Your requirement to be able to specify an expression for the increments, leads to the possibility that your increments might have a mix of positive and negative numbers. A decision had to be made about what to do for the case where the steps increment past the endpoint but that steps with the opposite sign then take the value back to below the endpoint. Clearly values beyond the end point should not be included... but should you stop as soon as you pass the endpoint the first time, or should you allow for the value to come back?
I decided that since it should be permitted to allow a mix of increments while you are in range, that it would make the most sense to allow values to return back to range. So what I do is filter out the values that go beyond the stop-point -- leaving any that come back to the range. So with positive increments 5 for a while, and negative increments (say) 7, a sequence with start 1 and stop 20 and increments [5 5 5 5 -7 -7] might go [1 6 11 16 (OMITTED 21) 14 7] leaving [1 6 11 16 14 7] as the loop control values.
It might also be reasonable to argue that the sequence should never emit values "before" the start after having gone past it -- so 1:[5 5 5 5 -7 -7 -7]:20 with the logic posted above would give [1 6 11 16 14 7 0] but it would be reasonable to filter on start as well as stop giving loop values [1 6 11 16 14 7] ...
Dennis M
Dennis M on 2 Sep 2021
Pardon Sir Walter I'm confusing also what the best code or possiblity to do, but you make it by example and I really appreciate that.
I combine your previous code and comment to other question. Now it's clear to me that important was the last step and just flip it.
Thanks Again!
I Hope You will answer my question again in the future

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