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Problem with for loop on MATLAB

Asked by Jimmy on 21 Nov 2012

Hey, guys, one last question for today...

I need to write an algorithm that determines 50 numbers and print how many are even, odd, positive and negative.

These are the code and pseudocode I wrote, but for some reason the "for" loop doesn't work correctly. ¿What is causing this?

PSEUDOCODE

    even, odd, positive, negative
    even ← 0
    odd ← 0
    positive ← 0
    negative ← 0
    For x ← 1 to 50 then
    Read n
    If n % 2 = 0 then
    even ← even + 1
    else
    odd <- odd + 1
    end
    If n > 0 then
    positive ← positive + 1
    else
    negative ← negative + 1
    end
    disp even, odd, positive, negative

CODE:

num = input('Enter a number: ');
p = 0; %even
im = 0; %odd
pos = 0; %positives
n = 0; %negatives
for x = 1; x <= 50; x = x+1;
  if rem(num,2) == 0;
      p = p + 1;
  else
      im = im + 1;
  if num > 0
	pos = pos + 1;
  else
	n = n + 1;
  end
  end
end
 disp('the even numbers are');
 disp(p);
 disp('the odd numbers are')
 disp(im);
 disp('the positive numbers are')
 disp(pos);
 disp('the negative numbers are')
 disp(n);

For example, these are the answers I'm getting for the number 50.

Enter a number: 50
the even numbers are
   1
the odd numbers are
   0
the positive numbers are
   0
the negative numbers are
   0

And this is the correct answer for 50.

Enter a number: 50
the even numbers are
   25
the odd numbers are
   25
the positive numbers are
   50
the negative numbers are
   0

1 Comment

Jimmy on 22 Nov 2012

OK, I edited the tag.

Jimmy

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2 Answers

Answer by Walter Roberson on 22 Nov 2012
Accepted answer

In MATLAB, the line

for x = 1; x <= 50; x = x+1;

means:

  1. Start a for loop. The index variable will be named "x". The first value for "x" is 1.
  2. inside the "for" loop, every iteration, calculate whether x <= 50, creating a boolean reasult. Throw away the boolean result.
  3. next inside the "for" loop, every iteration, assign x+1 to x, so x will become 2 as the second action in the first iteration
  4. when the "end" is reached, check to see what the next value to iterate to is. As the list of values was only "1", there is no next value in the list "1", exit the loop, leaving "x" at its last value.

If this was not your intention, you might want to read the documentation for "for" at http://www.mathworks.com/help/matlab/ref/for.html

13 Comments

Jimmy on 22 Nov 2012

You're very nice for helping me, but I should apply those increments into this code? How?

Walter Roberson on 22 Nov 2012

That example shows how to process a vector, a single element at a time.

Jimmy on 22 Nov 2012

Thanks.

Walter Roberson
Answer by Muruganandham Subramanian on 22 Nov 2012

Hi,

try this code:

num = input('Enter a number: ');

p = 0; %even

im = 0; %odd

pos = 0; %positives

n = 0; %negatives

for x = 1:num;

if rem(x,2) == 0;
      p = p + 1;
else
      im = im + 1;
end
if x > 0

pos = pos + 1;

else

n = n + 1;

end
x = x+1;

end

 disp('the even numbers are');
 disp(p);
 disp('the odd numbers are')
 disp(im);
 disp('the positive numbers are')
 disp(pos);
 disp('the negative numbers are')
 disp(n);

7 Comments

Walter Roberson on 22 Nov 2012

I did say, do not use

for x = 1:num

use

for x = num
Ilham Hardy on 22 Nov 2012

Walter,

num is a single number instead of a vector of 50 numbers.

Walter Roberson on 22 Nov 2012

I saved the file as negs.m after changing the "for" line to "for x = num". I then ran:

>> negs
Enter a number: [-5 9 12 14 16 -1]
the even numbers are
     3
the odd numbers are
     3
the positive numbers are
     4
the negative numbers are
     2

Is that the incorrect answer?

I did ask above what the input was at the prompt, and you specifically said "The user must enter 50 natural numbers."

Muruganandham Subramanian

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