# Documentation

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# islinear

Determine if `cfit`, `sfit`, or `fittype` object is linear

## Syntax

```flag = islinear(fun) ```

## Description

`flag = islinear(fun)` returns a `flag` of `1` if the `cfit`, `sfit`, or `fittype` object `fun` represents a linear model, and a `flag` of `0` if it does not.

### Note

`islinear` assumes that all custom models specified by the `fittype` function using the syntax `ftype = fittype('expr')` are nonlinear models. To create a linear model with `fittype` that will be recognized as linear by `islinear` (and, importantly, by the algorithms of `fit`), use the syntax `ftype = fittype({'expr1','expr2',...,'exprn'})`.

## Examples

```f = fittype('a*x+b') f = General model: f(a,b,x) = a*x+b g = fittype({'x','1'}) g = Linear model: g(a,b,x) = a*x + b h = fittype('poly1') h = Linear model Poly1: h(p1,p2,x) = p1*x + p2 islinear(f) ans = 0 islinear(g) ans = 1 islinear(h) ans = 1```