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iosobjective-cjsonstring-parsing

JSON string becomes different value when converting to float

I'm parsing some weather JSON from a local file. 

 "temp_f":40.1,
 "temp_c":4.5,

The problem is that when I parse it into an c value 

NSLog(@"%f", [weatherDict[@"temp_f"]floatValue]);

//Logs: 40.099998

If I log it as a string value it returns 40.1. 

NSLog(@"%@", [weatherDict objectForKey:@"temp_f"]);
//Logs: 40.1

The problem seems to be when calling floatValue

Any ideas why this could be happening?

Solution

  • You are asking "why".

    The complete answer is found in the most famous article "What every computer scientist should know about floating-point arithmetic": http://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html

    It is a must read unless you want to be forever guessing why strange things happen with your numbers.

    BTW. Use double, not float, unless you can give me a good reason why float would be better for you.