numeric integration with variable division/integration interval

[USC_gamecock]

I want to calculate the area under a certain curve (data read from a spreadsheet file) by doing a numeric integration. The problem is that the interval between two adjacent points progressively changes from the start to end. I think I can get away with it by using a for loop. But is there a simpler way to do that. Thanks!

[johnsold]

A for loop should be fine. Each pair of points from the curve defines a trapezoidal area which can be easily calculated and summed. If the two points are (Xi,Yi) and (Xi-1,Yi-1), then the area defined by those points and the points on the X-axis (Xi,0) and (Xi-1,0) is given by 0.5*(Yi-1-Yi)*(Xi-1-Xi). It is irrelevant whether delta X varies from one interval to the next.

Use shift registers to access previous values and to keep a running sum.

Lynn

[chilly_charly]

No need to use a for loop. You can calculate the area under the curve by direct calculations on the X and Y arrays. The method is not very accurate since it relies on the trapeze formula, but should work if your data points are not too distants. See the attached vi.

CC

[johnsold]

Thanks, CC.

I had not thought of the direct approach.

Lynn

[chilly_charly]

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johnsold wrote:...I had not thought of the direct approach.

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Not sure that it is really a better solution, since it uses twice the memory amount used with the FOR loop method. But it should be more rapid.

CC

[USC_gamecock]

Thanks, Lynn and CC. CC, could you give me the code in labview 7.0 or 6.1. I don't have labview 7.1. Thanks!

Kui

[Johnner]

Kui,

Here's CC's VI in version 7.0 (attached).

Regards,
John