NSExpression: simple examples

The example from last time (here) introduced the NSExpression class. That one was rather complex in code, though what it does is just a simple filtering of values.

Here is another example which looks less forbidding: it combines two expressions in a single predicate. These can then be used to construct a comparison predicate that grabs the object for key='value' from an array, and checks it against the number 10, like this:

As before the code block is inside a standard main:

#import <Foundation/Foundation.h> int main (int argc, const char * argv[]) { NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init]; code here.. [pool drain]; return 0; }

and compiled like this:

> gcc -o test test.m -framework Foundation

code:

NSExpression *lhs = [NSExpression expressionForKeyPath:@"value"]; NSNumber *ten = [NSNumber numberWithInt:10]; NSExpression *rhs = [NSExpression expressionForConstantValue:ten]; NSPredicate *p = [NSComparisonPredicate predicateWithLeftExpression:lhs rightExpression:rhs modifier:NSDirectPredicateModifier type:NSGreaterThanOrEqualToPredicateOperatorType options:0]; NSArray *A = [NSArray arrayWithObjects: [NSMutableDictionary dictionaryWithObject: [NSNumber numberWithInt:3] forKey:@"value"], [NSMutableDictionary dictionaryWithObject: [NSNumber numberWithInt:15] forKey:@"value"], nil]; NSArray *fA = [A filteredArrayUsingPredicate:p]; for (id obj in fA) { NSLog(@"%@", [obj description]); }

> ./test 2011-03-13 10:58:53.885 test[218:903] { value = 15; }

Here is a second example, taken from the docs, of how to construct an expression that uses a built-in function (they call this a function expression):

NSArray *A = [NSArray arrayWithObjects: [NSNumber numberWithInt:3], [NSNumber numberWithInt:6], nil]; NSExpression *eA = [NSExpression expressionForConstantValue:A]; NSArray *args = [NSArray arrayWithObject:eA]; NSExpression *e = [NSExpression expressionForFunction:@"average:" arguments:args]; id result = [e expressionValueWithObject:nil context:nil]; NSLog(@"%@ %@", [result description], [result class]); float f = [result floatValue]; printf("result = %3.2f\n", f);

2011-03-13 11:01:00.337 test[236:903] 4.5 NSCFNumber result = 4.50

There are lots of built-in functions available (here).

The third example is based on the first part (the simple part) of this post from Dave DeLong. It defines a category on NSNumber

@interface NSNumber (FactorialExpression) - (NSNumber *) factorial; @end @implementation NSNumber (FactorialExpression) - (NSNumber *) factorial { double baseValue = [self doubleValue]; double result = tgamma(baseValue+1); return [NSNumber numberWithDouble:result]; } @end

and the code block is:

NSNumber *n = [NSNumber numberWithDouble:4.2]; NSLog(@"%@ %@", n, [n factorial]); NSLog(@"%p %d", n, [n respondsToSelector:@selector(factorial)]); NSExpression *f = [NSExpression expressionForConstantValue:n]; NSExpression *e = [NSExpression expressionForFunction:f selectorName:@"factorial" arguments:nil]; NSLog(@"operand %@ %@", [e operand], [[e operand] class]); NSLog(@"operand %@", [e function]); id result = [e expressionValueWithObject:nil context:nil]; NSLog(@"%@ %@", [result description], [result class]);

> ./test 2011-03-13 11:02:46.798 test[251:903] 4.2 32.57809605033135 2011-03-13 11:02:46.800 test[251:903] 0x100108d20 1 2011-03-13 11:02:46.801 test[251:903] operand 4.2 NSConstantValueExpression 2011-03-13 11:02:46.801 test[251:903] operand factorial 2011-03-13 11:02:46.802 test[251:903] 32.57809605033135 NSCFNumber

And now, the way to solve my initial question seems clear:

Define a category on NSString that does what I want.
Wrap the call up in an NSExpression and then an NSPredicate. Next time, if I succeed.