问题
I have already implemented a custom calculator where I am using following code to evaluate the arithmetic expression something like 5+3*5-3.
- (NSNumber *)evaluateArithmeticStringExpression:(NSString *)expression {
NSNumber *calculatedResult = nil;
@try {
NSPredicate * parsed = [NSPredicate predicateWithFormat:[expression stringByAppendingString:@" = 0"]];
NSExpression * left = [(NSComparisonPredicate *)parsed leftExpression];
calculatedResult = [left expressionValueWithObject:nil context:nil];
}
@catch (NSException *exception) {
NSLog(@"Input is not an expression...!");
}
@finally {
return calculatedResult;
}
}
But when I am having integers with division operation I am getting only integer as a result. Let's say for 5/2 I am getting 2 as a result. It's right for the shake of programming because of the integer division.
But I need floating point result.
How can I get it rather scanning the expression string and replace the integer divisor as a floating point. In our example 5/2.0 or 5.0/2.
回答1:
I found it by myself.
- (NSNumber *)evaluateArithmeticStringExpression:(NSString *)expression {
NSNumber *calculatedResult = nil;
@try {
NSPredicate * parsed = [NSPredicate predicateWithFormat:[NSString stringWithFormat:@"1.0 * %@ = 0", expression]];
NSExpression * left = [(NSComparisonPredicate *)parsed leftExpression];
calculatedResult = [left expressionValueWithObject:nil context:nil];
}
@catch (NSException *exception) {
NSLog(@"Input is not an expression...!");
}
@finally {
return calculatedResult;
}
}
it's simply start the expression with the operand "1.0 * " and everything will be floating point calculation onwards.
NSPredicate * parsed = [NSPredicate predicateWithFormat:[NSString stringWithFormat:@"1.0 * %@ = 0", expression]];
NB: Thanks @Martin R but, my question wasn't about integer division, it's totally about NSExpression. I have clearly excluded with my last sentence.
@Zaph, Exception handling is used here for a strong reasons. It is a place where my method is accepting user input wehre user can enter something like w*g and - expressionValueWithObject: context: will throw an exception and I have to avoid the abnormal termination of my App. If the user entered a valid expression then he/she will get an answer in the form of NSNumber otherwise a nil NSNumber object.
来源：https://stackoverflow.com/questions/23892130/how-to-get-floating-point-number-from-the-nsexpressions-expressionvaluewithobje