# ExpFit

Advanced Analysis Library Only

AnalysisLibErrType ExpFit (double arrayX[], double arrayY[], int numberOfElements, double outputArray[], double *amplitude, double *decay, double *meanSquaredError);

## Purpose

| Note This function is obsolete. National Instruments recommends that you use ExpFitEx instead.

Finds the coefficient values that best represent the exponential fit of the data points (X, Y) using the least squares method. ExpFit obtains the element of the output array using the following formula:

Zi = aedXi

where | a = amplitude
| d = decay
| i = array index

ExpFit obtains the mean squared error using the following formula:

where n is the number of sample points.

ExpFit can perform this operation in place; that is, the input and output arrays can be the same.

### Example

/* Generate an exponential pattern and find the best exponential fit. */
double x[200], y[200], z[200];
double first, last, a, b, amp, decay, mse;
n;
n = 200;
first = 0.0;
last = 1.99E2;
Ramp (n, first, last, x); /* x[i] = i */
a = 3.5;
b = -2.75;
for (i=0; i<n; i++)

y[i] = a * exp(b*x[i]);

/* Find the best exponential fit in z. */
ExpFit (x, y, n, z, &amp, &decay, &mse);

| Note The y values must be all positive or all negative to perform an exponential fit.

## Parameters

Input
Name | Type | Description
arrayX | double-precision array | An array whose elements contain the X coordinates of the (X,Y) data sets.
arrayY | double-precision array | An array whose elements contain the Y coordinates of the (X,Y) data sets.
numberOfElements | integer | Number of sample points used in the exponential fit operation.
Output
Name | Type | Description
outputArray | double-precision array | Best exponential fit.
amplitude | double-precision | Value that best describes the amplitude of the exponential curve.
decay | double-precision | Decay value that best describes the exponential curve.
meanSquaredError | double-precision | The mean squared error generated by the difference between the fitted curve and the raw data.

## Return Value

Name | Type | Description
status | AnalysisLibErrType | A value that specifies the type of error that occurred. Refer to analysis.h for definitions of these constants.