Simple question for using 'for loop'

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I want to use 'for loop' to generate a =[2 2^3 2^5 2^7 2^9].
this is my first solution.
for n = 1:2:10 %This is my homework request.I can't change this.
a(n) = 2^n
Z = find(a == 0)
a(Z) = []
And this one is my second solution.
for n = 1:2:10
a(n) = 2^n
if a(n) == 0 %I want to use logical operation to find out that variable in 'a' is equal zero.And asign these varaible =[].
a(n) = []
Why second solution is wrong?
How do I fix it?

Accepted Answer

Walter Roberson
Walter Roberson on 5 Sep 2021
The first time you store into a(1), the second time you store into a(3), then a(5), then a(7), then a(9)
You always store a non-zero value because 2^n is only zero when n is more negative than -1074 (in double precision numbers.) So testing a(n) will never be 0 in your code.
But... you are not storing anything specific into a(2) or a(4) or a(6) or a(8) so those would be 0. They would not be located testing a(n) only, but they would be found with the find()
Hint: you are not required to store at a(n) . You could take n and use it to compute a relative index
  • n = 1 -> index 1
  • n = 3 -> index 2
  • n = 5 -> index 3
  • n = 7 -> index 4
Notice what happens if you add 1 to n and then take half of the result.
  1 Comment
Han Shiang Chuang
Han Shiang Chuang on 8 Sep 2021
Thanks for the help! It's clear to me now.

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More Answers (2)

KSSV on 5 Sep 2021
a = zeros([],1) ;
c = 0 ;
for n = 1:2:10 %This is my homework request.I can't change this.
c = c+1 ;
a(c) = 2^n ;
2 8 32 128 512

Sreedhar Arumugam
Sreedhar Arumugam on 7 Sep 2021
Edited: Sreedhar Arumugam on 8 Sep 2021
Your code always iterates over the odd indices from 1 to 10.
Line 2 of your code -> a(n) = 2^n stores the values in a(1), a(3), a(5) and so on.
The logic that you are using in the second solution will not work, as the code will never run into a 0. The code only iterates over odd indices, and since the 0s are only stored in even indices, they will not be removed from the final array.
In your first solution, the code produced the correct output as find(a==0) returns all the indices that contain 0. So it did not matter that you were never visiting these indices directly through your loop.
The following code would be a fix to your second solution :
b = 2:2:10-1 % This array contains all the even indices till 10 (excluding 10 itself)
for n = 1:2:10 % This is my homework request.I can't change this.
a(n) = 2^n
a(b) = []




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