You have an NSString. You want an NSNumber. Previously, you would have to rely on abusing NSPredicate to turn your string into an NSExpression that you could then evaluate. However, this has a major flaw: extending it to support functions that aren't built-in to NSExpression provided for some awkward syntax. So if you really need sin(), you have to jump through some intricate hoops to get it.
You could also have used GCMathParser. This, however, isn't extensible at all. So if you really needed a floor() function, you're out of luck.
Thus, DDMathParser. It is written to be identical to NSExpression in all the ways that matter (in fact, it actually uses NSExpression to evaluate many of its built-in functions), but with the major addition that you can define new functions as you need.
Registering new functions is easy. You just need a block, a name, and the number of arguments the function accepts. So for example:
DDMathFunction function = ^ DDExpression* (NSArray * args, NSDictionary * variables, DDMathEvaluator * evaluator) {
NSNumber * n = [[args objectAtIndex:0] evaluateWithSubstitutions:variables evaluator:evaluator];
NSNumber * result = [NSNumber numberWithDouble:[n doubleValue] * 42.0f];
return [DDExpression numberExpressionWithNumber:result];
};
[[DDMathEvaluator sharedMathEvaluator] registerFunction:function forName:@"multiplyBy42" numberOfArguments:1];
NSLog(@"%@", [[DDMathEvaluator sharedMathEvaluator] evaluateString:@"multiplyBy42(3)" withSubstitutions:nil]); //logs "126"
You can also unregister added functions. You cannot unregister built-in functions, nor can they be overridden.
Function names must begin with a letter, and can contain letters and digits. Functions are case-insensitive. (mUlTiPlYbY42 is the same as multiplyby42)
If you don't know what the value of a particular term should be when the string is constructed, that's ok; simply use a variable:
NSString * math = @"6 * $a";
Then when you figure out what the value is supposed to be, you can pass it along in the substitution dictionary:
NSLog(@"%@", [[DDMathEvaluator sharedMathEvaluator] evaluateString:math withSubstitutions:[NSDictionary dictionaryWithObject:[NSNumber numberWithInt:7] forKey:@"a"]]); //logs "42"
Variables are denoted in the source string as beginning with $ and can contain numbers or letters. They are case sensitive. ($a is not the same as $A)
By default, all binary operators are left associative. That means if you give a string, such as @"2 ** 3 ** 2", it will be parsed as @"(2 ** 3) ** 2, in order to maintain parity with NSExpression.
If you want this operator to be parsed with right associativity, you can do so like this:
DDMathParser * parser = [DDMathParser mathParserWithString:@"2 ** 3 ** 2"];
[parser setPowerAssociativity:DDMathParserAssociativityRight];
DDExpression * e = [parser parsedExpression];
All binary operators can have their associativity changed this way.
Simply copy the "DDMathParser" subfolder into your project, #import "DDMathParsing.h", and you're good to go.
DDMathParser requires blocks, so therefore is only compatible with iOS 4+ and Mac OS X 10.6+.
Copyright (c) 2010 Dave DeLong
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