# Math Tips

- How can I calculate the local standard deviation or variance in an image?
- How can I determine if a point is inside a polygon?
- What's up with the IDL FFT function? I don't get the same values as my FORTRAN FFT function.
- How do I obtain either a binary or hexadecimal representation of a number?
- I want a more flexible curve fitting program. Can you help?
- How can I calculate the logarithm base 2 of a vector in IDL?
- How do you check for NANs in an array?
- How do I test for NANs in my programs?
- How do I calculate the center of mass of a 2D or 3D array?
- What is the most efficient way to rotate a volume?
- Help! The sky is falling with the way this number is represented in IDL!
- I'd like to fit a polygon about a set of points. How do I do that?
- How do I add two vectors that have NANs in them?
- Do these floating underflow
errors
*mean*anything!? - Is there a fast way to get the sum of the diagonals of an array?
- How do I get a Gaussian distributon of numbers with a specified mean and sigma?
- Whoa! Did the behavior of ATAN change recently?
- I'm having some trouble getting random points on the surface of a sphere. Can you help?
- Histogram: On the Razor's Edge. A fable of interest to all computer users.
- Are the Sigma and Chi-Square parameters in CURVEFIT screwy? Or am I just losing my mind?
- Yikes! My double precision value is not very precise.
- How can I interpolate a value on a regular grid?
- I'm having trouble calculating derivatives in IDL. Can you help?
- Is there a easy way to interpolate bad data points in a vector?
- Is there a better way to represent missing data then using large numbers?
- How is a floating point
number
*really*stored in IDL? - Do you know any interesting facts about exponential operations in IDL?
- When does one plus one equal one in IDL?
- Is it possible to interpolate without changing the mean value of the original data?
- What is the meaning of the value returned by the “Chi Square” keyword?
- When is zero less than a negative number?

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Last Updated: 13 March 2011