连接字典的字符串：语法错误

Question

The following code to conditionally concatenate strings for a dictionary seems to work up to the point where I try to place the concatenated result in the dictionary. Can anyone see the error?

 NSDictionary *jsonDictionary;
        NSString* dictString = @"@\"first\":first,@\"last"
                NSString *dictString2=dictString;
                if (date.length>0&&![date isKindOfClass:[NSNull class]]) {
//only include this key value pair if the value is not missing 
                    dictString2 = [NSString stringWithFormat:@"%@%s", dictString, "@\"date\":date"];

                }

                jsonDictionary = @{dictString2}; //syntax error. Says expected colon but that does not fix anything

Answer 1

The syntax for creating an NSDictionary using object literals is:

dictionary = @{key:value}

(and optionally, it can contain multiple key/value pairs separated by commas, but never mind that right now.)

Where "key" and "value" are both NSObject s.

Your line that is throwing the error only contains 1 thing. The contents of a the string in dictString2 has nothing to do with it.

It looks to me like you are trying to build a JSON string manually. Don't do that. Use NSJSONSerialization . That class has a method dataWithJSONObject that takes an NSObject as input and returns NSData containing the JSON string. That's how you should be creating JSON output.

Answer 2

Creating an NSDictionary with values that may be null:

NSDictionary *dict = @{
                       @"key" : value ?: [NSNull null],
                      };

When serializing a dictionary, NSNull s are translated to null in the JSON.

If you want to exclude such keys completely, instead of having them with a null value, you'll have to do more work. The simplest is to use an NSMutableDictionary and test each value before adding it.