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# Checking for errors¶

## Concatenating comparison operators¶

This is a brutal and very common error. Imagine the following equation: `a < b < c`, now let's set `a = 1`, `b = 3` and `c = 2` so it will roughly translate to `1 < 3 < 2`. Those of you not familiarized with operator precedence evaluation might think that this will result in `false` but it will actually return `true`. Why? Well:

```a < b < c -> ((a < b) < c)
```

So the example will become:

```1 < 3 < 2 -> ((1 < 3) < 2) -> ((1) < 2) -> 1
```

So the morality of this tale is: if unsure, DON'T CONCATENATE OPERATORS

# Variable naming¶

Avoid variables and functions with the same name as build-in functions, constant or operators (or even custom functions). Even though this is possible and permitted by this version of `exp4j` we intent to create a more strict version in the future.

Why is this bad? well... if we have a built-in function called `sin(x)` and create a second function called `sin(x, y)` and use a variable called `sin` you might end up with expressions like: `sin(sin(sin, sin(sin)) + sin)` this may sound dumb, but unfortunately is very common.

# Performance¶

## Multiple calls to `Expression#evaluate()`¶

This is a common mistake, values returned from `Expression#evaluate()` are not cached, so every call will reevaluate the entire expression. There are two options:

1. Save the value of `Expression#evaluate()` instead of calling it multiple times
2. Implement the result cache! Checkout #752 for some details

## Simplifier enabled for single evaluation¶

The simplifier needs to do a full evaluation of the code in order to make the simplifications, so it should not be enabled for single evaluation, since this will create a 100% loss in performance (the expression needs to be evaluated twice).

## Write simplifiable expressions¶

Even though you can use the `Expression#toString()` method to checkout the number of tokens, the rule of thumb is to sort the expression:

For instance the expression `2 + 3x - 1` is not simplifiable, because the `Simplifier` does not alter the order of the operators (Operators can be overridden and there's no way of handling those scenarios).
If you change `2 + 3x - 1` to `2 - 1 + 3x` it will be simplified to `1 + 3x`.

Updated by Federico Vera almost 3 years ago · 7 revisions

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