A simple expression evaluator category on NSString which can handle adding, subtracting, multiplication and division operations with parentheses precedence.

NSString+Evaluate.h

// erkanyildiz
// 20150105-1132
//
// NSString+Evaluate.h

#import <Foundation/Foundation.h>

extern NSString* const kEYStringEvaluateError;

@interface NSString (Evaluate)
-(double)evaluate;
@end

NSString+Evaluate.m

// erkanyildiz
// 20150105-1132
//
// NSString+Evaluate.m

#import "NSString+Evaluate.h"

@implementation NSString (Evaluate)

NSString* const kEYStringEvaluateError = @"kEYStringEvaluateError";
char* c;
NSInteger parenthesisCount;

-(double)evaluate
{
    c = (char*)[self UTF8String];
    return [self start];
}


-(double)error:(NSString*)description
{
    NSString *errorMessage = [@"Expression error: " stringByAppendingString:description];
    [NSNotificationCenter.defaultCenter postNotificationName:kEYStringEvaluateError object:errorMessage];
    return NAN;
}



#pragma mark -

-(double)start
{
    parenthesisCount = 0;
    
    double result = [self summands];
    
    if(parenthesisCount != 0 || *c == ')')
        return [self error:@"Unbalanced parentheses"];
    
    if (*c != '\0')
        return [self error:@"Invalid character or missing operator"];
    
    return result;
}


-(double)summands
{
    double r1 = [self factors];
    
    while (1)
    {
        [self skipWhiteSpace];
    
        char operator = *c;
        if(operator != '+' && operator != '-')
            return r1;

        c++;
        double r2 = [self factors];
        if(operator == '+')
            r1 += r2;
        else
            r1 -= r2;
    }
}


-(double)factors
{
    double r1 = [self atomics];
    
    while (1)
    {
        [self skipWhiteSpace];
    
        char operator = *c;
        if(operator != '*' && operator != '/')
            return r1;

        c++;
        double r2 = [self atomics];
        if(operator == '*')
            r1 *= r2;
        else if (r2 != 0)
            r1 /= r2;
        else
            return [self error:@"Division by zero"];
    }
}


-(double)atomics
{
    [self skipWhiteSpace];
    
    if(*c == '(')
    {
        parenthesisCount++;
        c++;
    
        double result = [self summands];
        if (*c != ')')
            return [self error:@"Unbalanced parentheses within atomics"];
    
        c++;
        parenthesisCount--;
        return result;
    }
    
    char* end;
    double result = strtod(c, &end);
    if(end == c)
        return [self error:@"Invalid character or missing operator within atomics"];
    
    c = end;
    return result;
}


-(void)skipWhiteSpace
{
    while (*c == ' ' || *c == '\n' || *c == '\t')
        c++;
}


/*
#pragma mark -
+(void)testExpressions
{
    NSArray* testExpressions = @[
                                    @{@"exp":@"3+4",                @"res":@7},
                                    @{@"exp":@"100-20",             @"res":@80},
                                    @{@"exp":@"5*6",                @"res":@30},
                                    @{@"exp":@"100/3",              @"res":@33.33333333},
                                    @{@"exp":@"1.4*3.67",           @"res":@(5.138)},
                                    @{@"exp":@".2*5.0",             @"res":@(1)},
                                    @{@"exp":@"-.2*5.0",            @"res":@(-1)},
                                    @{@"exp":@"-4*-3.0",            @"res":@(12)},
                                    @{@"exp":@"40*4-10/2+6",        @"res":@(161)},
                                    @{@"exp":@"(-5)",               @"res":@(-5)},
                                    @{@"exp":@"48/4-(5*(9-3))",     @"res":@(-18)},
                                    @{@"exp":@"4*(5+7*(10/2))*9",   @"res":@1440},
                                    @{@"exp":@"4+3.5*8",            @"res":@32},
                                    @{@"exp":@"5-6*8+9-7",          @"res":@(-41)},
                                    @{@"exp":@"1-5*(-6)-6",         @"res":@25},
                                    @{@"exp":@"1/2/2/2",            @"res":@(0.125)},
                                    @{@"exp":@"3     + 5",          @"res":@(8)},
                                    @{@"exp":@"-4*(2+1)",           @"res":@(-12)},
                                    @{@"exp":@"4*(5+7*10/2))*9",    @"res":@(NAN)},
                                    @{@"exp":@"8(9*(5+1))",         @"res":@(NAN)},
                                    @{@"exp":@"7/0",                @"res":@(NAN)},
                                    @{@"exp":@"50/(5-5)",           @"res":@(NAN)},
                                    @{@"exp":@"7//2",               @"res":@(NAN)},
                                    @{@"exp":@"zxcv1*5",            @"res":@(NAN)},
                                ];
    
    [testExpressions enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop)
    {
         NSLog(@"Test %lu\n"
               "Expression: %@\n"
               "Expected Result: %g\n"
               "Calculated Result: %f\n\n",
               idx+1,
               obj[@"exp"],
               [obj[@"res"] doubleValue],
               [obj[@"exp"] evaluate] );
    }];
}
*/

@end