Objective C中的浮点值除法错误
Posted
技术标签:
【中文标题】Objective C中的浮点值除法错误【英文标题】：Float values dividing error in Objective C 【发布时间】：2012-03-01 09:11:06 【问题描述】：当我运行这段代码时：
float angle = startAngle;
int i = 0;
for (float f = self.minNumber; f <= self.maxNumber; f += minorTickIncrement)
points[i++] = CGPointMake(centerX + cos(angle) * (radius - tickInset), centerY + sin(angle) * (radius - tickInset));
CGFloat myTickLength;
NSLog(@"f : %f",f);
NSLog(@"tickIncrement : %f",tickIncrement);
NSLog(@"f / tickIncrement : %f",f / tickIncrement);
NSLog(@"(int)(f / tickIncrement) : %d",(int)(f / tickIncrement));
NSLog(@"(f / tickIncrement - (int)(f / tickIncrement)) : %f",(f / tickIncrement - (int)(f / tickIncrement)));
NSLog(@"fabs((f / tickIncrement - (int)(f / tickIncrement))) : %f",fabs((f / tickIncrement - (int)(f / tickIncrement))));
float checkValue = (f / tickIncrement - (int)(f / tickIncrement));
if (fabs(checkValue) < 0.05) // if is major tick
myTickLength = self.tickLength;
NSString *string = [[NSString alloc] initWithFormat:@"%0.1f", f];
float textWidth = textHeight * [string length] / 2;
CGContextShowTextAtPoint(ctx, centerX + cos(angle) * (radius - textInset) - textWidth / 2.0, centerY + sin(angle) * (radius - textInset) + textHeight / 4.0, [string UTF8String], [string length]);
[string release];
else
myTickLength = self.minorTickLength;
points[i++] = CGPointMake(centerX + cos(angle) * (radius - myTickLength - tickInset), centerY + sin(angle) * (radius - myTickLength - tickInset));
angle += minorTickAngleIncrement;
显示以下输出：
f : 0.100000
tickIncrement : 0.100000
f / tickIncrement : 1.000000
(int)(f / tickIncrement) : 1
(f / tickIncrement - (int)(f / tickIncrement)) : 0.000000
fabs((f / tickIncrement - (int)(f / tickIncrement))) : 0.000000
f : 0.120000
tickIncrement : 0.100000
f / tickIncrement : 1.200000
(int)(f / tickIncrement) : 1
(f / tickIncrement - (int)(f / tickIncrement)) : 0.200000
fabs((f / tickIncrement - (int)(f / tickIncrement))) : 0.200000
f : 0.140000
tickIncrement : 0.100000
f / tickIncrement : 1.400000
(int)(f / tickIncrement) : 1
(f / tickIncrement - (int)(f / tickIncrement)) : 0.400000
fabs((f / tickIncrement - (int)(f / tickIncrement))) : 0.400000
f : 0.160000
tickIncrement : 0.100000
f / tickIncrement : 1.600000
(int)(f / tickIncrement) : 1
(f / tickIncrement - (int)(f / tickIncrement)) : 0.600000
fabs((f / tickIncrement - (int)(f / tickIncrement))) : 0.600000
f : 0.180000
tickIncrement : 0.100000
f / tickIncrement : 1.800000
(int)(f / tickIncrement) : 1
(f / tickIncrement - (int)(f / tickIncrement)) : 0.800000
fabs((f / tickIncrement - (int)(f / tickIncrement))) : 0.800000
f : 0.200000
tickIncrement : 0.100000
f / tickIncrement : 2.000000
**(int)(f / tickIncrement) : 1**
(f / tickIncrement - (int)(f / tickIncrement)) : 1.000000
fabs((f / tickIncrement - (int)(f / tickIncrement))) : 1.000000
...................
在哪里，您可以看到，在输出的最后一个块中，(int)(f / tickIncrement) : 1 必须返回 2，而是返回 1。
为什么会这样？
【问题讨论】：
【参考方案1】：您很可能会遇到浮点表示问题。
尝试使用双精度，但要知道它不会解决根本问题，您看到的 2.0 实际上是 1.99999 ...，它只是被格式化为显示 2.0
阅读“What Every Computer Scientist Should Know About Floating-Point Arithmetic”
同时，尝试简单地使用：
(int)(f / tickIncrement + tickIncrement / 5.0)
【讨论】：
以上是关于Objective C中的浮点值除法错误的主要内容，如果未能解决你的问题，请参考以下文章