Problem with for loop on MATLAB

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

[Jimmy]

OK, I edited the tag.

Answer 1

[Walter Roberson]

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

[Jimmy]

Thanks, but my code doesn't work, I need someone to tell me what I'm doing wrong.

[Walter Roberson]

Your "for" loop is wrong. Have you changed it yet?

[Jimmy]

No, it should be like this?

for x =1:50;

Need a little help here, please.

[Walter Roberson]

Yes, it should be.

The semi-colon is not strictly needed, but is advised if you have other statements on the same line.

[Jimmy]

I just fixed "for", but there's still a error, because I always get this answer with any number:

_
the even numbers are
 0
the odd numbers are
 50
the positive numbers are
 50
the negative numbers are
 0

Please, help me to fix the code, this is my last exercise...

[Walter Roberson]

Your code only inputs one number, and then repeats the same code with the same number 50 times.

When the user is prompted to enter a number, is the user expected to enter a single number or 50 numbers?

[Jimmy]

The user must enter 50 natural numbers. The algorithm must show how many of them are even, odd, negative and positive.

[Walter Roberson]

Your loop tests "num" in each step. "num" is a vector of numbers, so

if rem(num,2) == 0;

is calculating a vector of remainders, comparing those remainders to 0 and creating a boolean array, and applying "if" to that entire boolean array.

http://www.mathworks.com/help/matlab/ref/if.html

An evaluated expression is true when the result is nonempty and contains all nonzero elements (logical or real numeric). Otherwise, the expression is false.

Please re-read the documentation for "for", and in particular take a closer look at the examples. http://www.mathworks.com/help/matlab/ref/for.html

[Jimmy]

So, I should have used mod instead of rem?

[Walter Roberson]

mod() applied to a vector of numbers is going to calculate a vector of remainders, exactly the same. Whether you use mod() or rem() is not relevant to your problem.

Go back to the documentation for "for" and read the example that starts with "Step by increments"

[Jimmy]

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

[Walter Roberson]

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

[Jimmy]

Thanks.

Answer 2

[Muruganandham Subramanian]

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);

[Jimmy]

Oh, awesome, I was so blind...

Thank you very much!

[Walter Roberson]

No, the code should be

for x = num

and remove the "x = x + 1" at the end.

[Muruganandham Subramanian]

No,It won't work ever..

[Walter Roberson]

What will not work, Muruganandham ? Remember, "num" is to be entered as a vector by the user at the input() prompt.

[Walter Roberson]

I did say, do not use

for x = 1:num

use

for x = num

[Ilham Hardy]

Walter,

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

[Walter Roberson]

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."