添加浮点数导致错误答案 [英] Adding Float Causes wrong answer

问题描述

我要添加两个浮点数，例如:

NSString *val1="355.00";NSString *val2="55555550.00";NSString *result=[NSString stringWithFormat:@"%.2f",[val1 floatValue]+[val2 floatValue]];

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我得到的答案是 55555908.00 而在计算器上它是55555905.00

我做错了什么?

解决方案

正如评论中已经指出的，这是一个浮点精度问题.float 精度约为 7 位十进制数字，double 精度约为 15 位十进制数字.

Foundation 框架还提供了 NSDecimalNumber 类用于执行 base-10 算术，精度为 38 位十进制数字:

NSString *val1 = @"355.00";NSString *val2 = @"55555550.00";NSDecimalNumber *dec1 = [NSDecimalNumber decimalNumberWithString:val1];NSDecimalNumber *dec2 = [NSDecimalNumber decimalNumberWithString:val2];NSDecimalNumber *decsum = [dec1 decimalNumberByAdding:dec2];NSString *result = [decsum stringValue];//-->55555905

I am adding two floats like:

NSString *val1=@"355.00";
NSString *val2=@"55555550.00";

NSString *result=[NSString stringWithFormat:@"%.2f",[val1 floatValue]+[val2 floatValue]];

The answer i am getting is 55555908.00 while on calculator it is 55555905.00

What i am doing wrong?

解决方案

As already stated in the comments, this is a floating point precision problem. A float has a precision of about 7 decimal digits, a double about 15 decimal digits.

The Foundation framework also provides the NSDecimalNumber class for doing base-10 arithmetic, with a precision of 38 decimal digits:

NSString *val1 = @"355.00";
NSString *val2 = @"55555550.00";
NSDecimalNumber *dec1 = [NSDecimalNumber decimalNumberWithString:val1];
NSDecimalNumber *dec2 = [NSDecimalNumber decimalNumberWithString:val2];
NSDecimalNumber *decsum = [dec1 decimalNumberByAdding:dec2];
NSString *result = [decsum stringValue];
// --> 55555905

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