基于值数组将 NSString 转换为 NSInteger 的最佳方法是什么？

Question

I want to convert characters into integers based on predetermined values, for example:

a = 0 
b = 1
c = 2
d = 3

etc...

Right now I'm doing it with an If/Else If, I just want to know if there is a faster/better way I should be doing it because the list of conversions may get quite long.

Here's what I'm using now:

-(NSInteger)ConvertToInt:(NSString *)thestring {
    NSInteger theint;
    if([thestring isEqualToString:@"a"] == YES){
        theint = 0;
    } else if ([thestring isEqualToString:@"b"] == YES){
        theint = 1;
    } //etc...

    return theint;
}

This works fine, but as I said, if it makes more sense can I create an array with all the key/values then just run through that to return the integers?

Please provide examples as I'm a beginner with Objective C/iOS. I come from Web languages.

Thanks!

EDIT: Thanks for the help everyone. I used taskinoors answer but I replaced the NSDictionary which was giving error messages with this:

NSDictionary *dict;
dict = [NSDictionary dictionaryWithObjectsAndKeys:
        [NSNumber numberWithInt:0], @"a",
        [NSNumber numberWithInt:1], @"b",
        [NSNumber numberWithInt:2], @"c", nil];

Answer 1

unichar ch = [thestring characterAtIndex:0];
theint = ch - 'a';

Note that, 'a' with a single quote is character a , not string "a" .

If the values are not regular like your example then you can store all predefined values into a dictionary. For example:

"a" = 5;
"b" = 1;
"c" = 102;

NSArray *values = [NSArray arrayWithObjects:[NSNumber numberWithInt:5],
    [NSNumber numberWithInt:1], [NSNumber numberWithInt:102], nil];
NSArray *keys = [NSArray arrayWithObjects:@"a", @"b", @"c", nil];
NSDictionary *dic = [NSDictionary dictionaryWithObjects:values forKeys:keys];

theint = [[dic valueForKey:thestring] intValue];

Answer 2

If you wanted to keep some flexibility in what strings map to what integers, and your integers run from 0 to n-1 where you have n unique items in the array, you could do something like this:

-(NSInteger)ConvertToInt:(NSString *)thestring {
    NSArray *arr = [NSArray arrayWithObjects:@"a", @"b", @"c", @"d", nil];
    NSInteger theint = [arr indexOfObject:thestring];
    return theint;
}

Now this will build the array each time, which would be very inefficient, the optimal way would be to build the array once in your class, and then just use a reference to that array with the indexOfObject method call.