'For loop' for beginners

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I'm having a hard time figuring out the for loop command. I want to create a .m file that will prompt a user to enter an array and then be able to identify positive and negative numbers or numbers equal to zero. But I'm not sure how to create the for loop command for this.
I know how to get the prompts in the program file but that's about all I'm getting to. I've been reading through a Matlab book and searching online but with no success. It's hard to teach yourself this stuff!
An example of what I would like to do is:
input('Enter the number of elements in a vector: ') v=input('Enter the array: ')
Now all I would like to do is set this up. I know my commands would be something like:
v<0 and v>0 % to get my negative and positive numbers
and v=0: to get what ever is equal to zero.
But how would I set this up using a for loop?

Accepted Answer

per isakson
per isakson on 11 Jul 2012
Edited: per isakson on 11 Jul 2012
No need for a loop:
positive_numbers = v(v>0);
negative_numbers = v(v<0);
zero_numbers = v(v==0);
.
See "Logical indexing"
.
--- Example based on Comment 1 and 2 ---
In your code in the comment you overwrite the variables with identical results a number of times. I meant "No loop needed".
I misunderstood your question. I thought you wanted separate the positive values from the negative values. DOUBLE in the example below communicates the intent. Matlab doesn't need it.
Run this code as is (without a loop)
v = [ 1, 5, -3, 0 ];
positive_numbers = v(v>0);
negative_numbers = v(v<0);
zero_numbers = v(v==0);
numbers_of_positives = sum( double( v>0 ) );
numbers_of_negatives = sum( double( v<0 ) );
numbers_of_zeros = sum( double( v==0 ) );
This certainly doesn't serve as an exercise with loops, but it returns a result.
  5 Comments
Aaron
Aaron on 11 Jul 2012
Awesome. Thank you.

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More Answers (2)

Luffy
Luffy on 11 Jul 2012
Edited: Luffy on 11 Jul 2012
sprintf('Enter elements of vector');
% after entering v,run below code
for i = 1:length(v)
if v(i) > 0
sprintf('v(%i)>0',i)
elseif v(i) <0
sprintf('v(%i)<0',i)
else
sprintf('v(%i)=0',i)
end
end

quaza mouinuddin mohammad
sprintf('Enter elements of vector');
% after entering v,run below code
for i = 1:length(v)
if v(i) > 0
sprintf('v(%i)>0',i)
elseif v(i) <0
sprintf('v(%i)<0',i)
else
sprintf('v(%i)=0',i)
end
end

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