Ising Model C++ Metropolis Algorithm

Quote:

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Originally Posted by 2kaud

Code:

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if (delta_E <= 0 || ((double)rand() / (double)(RAND_MAX)) < (double)exp((double)-delta_E))

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the if condition is a logical or. If either or both of the seperate conditons (left and right of the ||) is true then the if condition is true.
The first condition is easy. For the second, (double) means cast (convert) the result of what follows to be of type double. As rand() and RAND_MAX are both integers, they are first cast to double so that the result is of type double because in c/c++ an integer divided by an integer gives an integer (3/4 = 0). So the expression to the left of < gives a number of type double in the range 0 to 1 (for the probability required). The expression to the right of the < is what you stated was needed. The (double) casts are needed again to make sure that the types are as required.

See http://msdn.microsoft.com/en-us/libr...=vs.60%29.aspx

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This would mean that the site (x, y) would be flipped, without doubt. If delta_E < 0, flip. If delta_E > 0, it means that the exponential is between 0 and 1, so flip. I need the flipping process to be such:

1. If delta_E < 0 flip.
2. Otherwise, flip with probability exp (-delta_E). If w = 0.7 = 70%, this means that if there are 10 sites with the same value of w, only 7 out of 10 times will site be flipped.

Quote:

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This would mean that the site (x, y) would be flipped, without doubt

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No

If delta_E <= 0 then flip
if delta_E > 0 then flip if exp(-delta_E) > random_number_0_to_1

When evaluating logical or conditions, the left hand expression is evaluated first. If this is true then the right hand expression is not evaluated. The right hand expression is only evaluated if the left hand expression is evaluated to false. if delta_E is <= 0 then the left condition is true and the right is not evaluated. The right expression is only evaluated if the left is false.

See http://msdn.microsoft.com/en-us/library/z68fx2f1.aspx

Quote:

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Originally Posted by 2kaud

No

If delta_E <= 0 then flip
if delta_E > 0 then flip if exp(-delta_E) > random_number_0_to_1

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Ah I see, I missed out the > random_number_0_to_1. Ok this makes sense. If exp(-delta_E) is between 0 and 1, the chances of it being bigger than a number between 0 and 1 is also 0 and 1. This will work!