NSExpression

Written by Mattt Thompson on

Cocoa is the envy of other standard libraries when it comes to querying and arranging information. With NSPredicate, NSSortDescriptor, and an occasional NSFetchRequest, even the most complex data tasks can be reduced into just a few, extremely-understandable lines of code.

Now, NSHipsters are no doubt already familiar with NSPredicate (and if you aren't, be sure to tune in next week!), but if we take a closer look at NSPredicate, we see that NSPredicate is actually made up of smaller, atomic parts: two NSExpressions (a left-hand value & a right-hand value), compared with an operator (e.g. <, IN, LIKE, etc.).

Because most developers only use NSPredicate by means of +predicateWithFormat:, NSExpression is a relatively obscure class. Which is a shame, because NSExpression is quite an incredible piece of functionality in its own right.

So allow me, dear readers, to express my respect and fascination with NSExpression:

Evaluating Math

The first thing you should know about NSExpression is that it lives to reduce terms. If you think about the process of evaluating an NSPredicate, there are two terms and a comparator, so those two terms need to simplify into something that the operator can handle—very much like the process of compiling a line of code.

Which leads us to NSExpression's first trick: doing math.

NSExpression *expression = [NSExpression expressionWithFormat:@"4 + 5 - 2**3"];
id value = [expression expressionValueWithObject:nil context:nil]; // => 1

It's no Wolfram Alpha, but if your app does anything where evaluating mathematical expressions would be useful, well... there you go.

Functions

But we've only just scratched the surface with NSExpression. Not impressed by a computer doing primary-school maths? How about high school statistics, then?

NSArray *numbers = @[@1, @2, @3, @4, @4, @5, @9, @11];
NSExpression *expression = [NSExpression expressionForFunction:@"stddev:" arguments:@[[NSExpression expressionForConstantValue:numbers]]];
id value = [expression expressionValueWithObject:nil context:nil]; // => 3.21859...

NSExpression functions take a given number of sub-expression arguments. For instance, in the above example, to get the standard deviation of the collection, the array of numbers had to be wrapped with +expressionForConstantValue:. A minor inconvenience (which ultimately allows NSExpression to be incredibly flexible), but enough to trip up anyone trying things out for the first time.

If you found the Key-Value Coding Simple Collection Operators (@avg, @sum, et al.) lacking, perhaps NSExpression's built-in statistical, arithmetic, and bitwise functions will pique your interest.

A word of caution: according to this table in Apple's documentation for NSExpression, there is apparently no overlap between the availability of functions between OS X & iOS. It would appear that recent versions of iOS do, indeed, support functions like stddev:, but this is not reflected in headers or documentation. Any details in the form of a pull request would be greatly appreciated.

Statistics

  • average:
  • sum:
  • count:
  • min:
  • max:
  • median:
  • mode:
  • stddev:

Basic Arithmetic

These functions take two NSExpression objects representing numbers.

  • add:to:
  • from:subtract:
  • multiply:by:
  • divide:by:
  • modulus:by:
  • abs:

Advanced Arithmetic

  • sqrt:
  • log:
  • ln:
  • raise:toPower:
  • exp:

Bounding Functions

  • ceiling: - (the smallest integral value not less than the value in the array)
  • trunc: - (the integral value nearest to but no greater than the value in the array)

Functions Shadowing math.h Functions

So mentioned, because ceiling is easily confused with ceil(3). Whereas ceiling acts on an array of numbers, while ceil(3) takes a double (and doesn't have a corresponding built-in NSExpression function). floor: here acts the same as floor(3).

  • floor:

Random Functions

Two variations—one with and one without an argument. Taking no argument, random returns an equivalent of rand(3), while random: takes a random element from the NSExpression of an array of numbers.

  • random
  • random:

Binary Arithmetic

  • bitwiseAnd:with:
  • bitwiseOr:with:
  • bitwiseXor:with:
  • leftshift:by:
  • rightshift:by:
  • onesComplement:

Date Functions

  • now

String Functions

  • lowercase:
  • uppercase:

No-op

  • noindex:

Custom Functions

In addition to these built-in functions, it's possible to invoke custom functions in an NSExpression. This article by Dave DeLong describes the process.

First, define the corresponding method in a category:

@interface NSNumber (Factorial)
- (NSNumber *)factorial;
@end

@implementation NSNumber (Factorial)
- (NSNumber *)factorial {
    return @(tgamma([self doubleValue] + 1));
}
@end

Then, use the function thusly (the FUNCTION() macro in +expressionWithFormat: is shorthand for the process of building out with -expressionForFunction:, et al.):

NSExpression *expression = [NSExpression expressionWithFormat:@"FUNCTION(4.2, 'factorial')"];
id value = [expression expressionValueWithObject:nil context:nil]; // 32.578...

The advantage here, over calling -factorial directly is the ability to invoke the function in an NSPredicate query. For example, a location:withinRadius: method might be defined to easily query managed objects nearby a user's current location.

As Dave mentions in his article, the use cases are rather marginal, but it's certainly an interesting trick to have in your repertoire.


Next week, we'll build on what we just learned about NSExpression to further explore NSPredicate, and everything it has hidden up its sleeves. Stay tuned!

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NSPredicate is a Foundation class that specifies how data should be fetched or filtered. Its query language, which is like a cross between a SQL WHERE clause and a regular expression, provides an expressive, natural language interface to define logical conditions on which a collection is searched.