objective-c - 比较浮点值

Question

我尝试比较浮点值，但我做不到，我使用以下代码：

float a = [[array objectAtIndex:(i+1)] floatValue];
float b = [[array objectAtIndex:(i)] floatValue];

a = [[NSString stringWithFormat:@"%.2f",a] floatValue];
b = [[NSString stringWithFormat:@"%.2f",b] floatValue];
step = [[NSString stringWithFormat:@"%.2f",step] floatValue];

float newStep = a-b;

if (newStep != step) {
      NSLog(@"NewStep: %f Step: %f",newStep,step);
}

这是输出：

2013-04-28 19:07:57.396 Pro[24265:c07] NewStep: 0.220000 Step: 0.220000
2013-04-28 19:07:57.397 Pro[24265:c07] NewStep: 0.220000 Step: 0.220000
2013-04-28 19:07:57.397 Pro[24265:c07] NewStep: 0.220000 Step: 0.220000
2013-04-28 19:07:57.397 Pro[24265:c07] NewStep: 2.420000 Step: 0.220000
2013-04-28 19:07:57.397 Pro[24265:c07] NewStep: 0.220000 Step: 0.220000
2013-04-28 19:07:57.397 Pro[24265:c07] NewStep: 0.220000 Step: 0.220000

显然不起作用，错误在哪里？代码似乎是正确的！

Answer 1

永远不要比较浮点变量是否相等！由于精度有限，浮点数只是一个近似值。利用

if (fabs(a-b)<0.0001) {...

或类似的东西，如果你想检查平等。