How can I fix my problem with (If loop) ?

Hi everybody.

I have the following code to find the integral of (sin(x)) using monte-carlo simulation:

num = 10;100;1000;10000;1000000;

i10 = 0; i100 = 0 ; i1000 = 0; i10000 = 0; i1000000 = 0; 
x = [0:(3.1416./100):3.1416];
y = [0:0.01:1];

for num = 10
  figure (1);
  hold on
  axis([-0.2 3.6 -0.2 1.2]);
  x = [0:(3.1416/100):3.1416];
  y = [0:0.01:1];
  xp = 0 + (3.1416 - 0).*rand (num,1);
  yp = 0 + (1 - 0).*rand (num,1);
  plot (x, sin(x))
  rectangle ('position', [0,0,pi,1])
  xlabel ('X'); ylabel ('sin (x)'); title ('sin (x) Vs. X'); 
  legend ('a = 0, b = pi, No. of points: 10')
  if (xp >= 0 & xp <= 3.1416) & (yp >= 0 & yp <= sin(xp))  
    i10 = i10 + 1
    plot (xp, yp, '*')
  else 
    plot (xp, yp, 'r+')
  end
end

The problem id that when MATLAB run the program it produce a figure that ignore the (if loop) with the condition of: (xp >= 0 & xp <= 3.1416) & (yp >= 0 & yp <= sin(xp))

I am trying to fix this problem since four days but not able to do so. Please advise me, I am strongly upset.

The strange thing is that MATLAB does not show any error and it does perform the (for loop) and the (else) statement but ignoring the (if loop) statements.

Thanks a lot in advance

[Stephen23]

It really helps everyone if you edit your question and format your code properly. Two ways you can do this:

1. Look just above the text box, and you will find some useful tools for adding text formatting.
2. Put two space characters at the beginning of each code line.

Make sure that you provide exactly the code that you are using, with indentation and everything :)

[Jan]

I've edited the code.

Answer 1

[Jan]

What do you expect as result of this line:

num = 10;100;1000;10000;1000000;

It sets num to 10 and ignores the rest.

This is a loop over one value only, so it is not a loop in the usual ssense:

|for num = 10|

It is equivalent to:

num = 10;

After ths lines

xp = 0 + (3.1416 - 0).*rand (num,1);
yp = 0 + (1 - 0).*rand (num,1);

xp and yp are vectors. In the if condition:

if (xp >= 0 & xp <= 3.1416) & (yp >= 0 & yp <= sin(xp))

the argument is a vector also, such that Matlab performs applies an all() command internally. In consequence this condition is not true, when any element of yp is > sin(xp) .

[oaam]

Thanks a lot Jan Simon.

usin an all() with if loop make difference. However, in my code I'm trying to make the points randomly generated under sin(x) as stars (*) and any other points as plus sign (+) using the else statement.

When using all(), it will not execute (if loop) unless all the points generated are below sin(x) which is not useful as I am generating big number of points (up to 10^6 points). I tried using any() but the similar problem occurs as (if loop) will be executed even for the points above sin(x).