iOS中float四舍五入探索

一、准备

测试数据

float A = 0.124000;
float B = 0.124200;
float C = 0.125000;   （重点关注）
float D = 0.125001;
float E = 0.126000;

目的：float保留两位小数显示，四舍五入。

二、几种尝试

2.1、最简单的方法

NSString * showA = [NSString stringWithFormat:@"%0.2f",X];

// 输出
A = 0.124000;  -->  A = 0.12 
B = 0.124200;  -->  B = 0.12 
C = 0.125000;  -->  C = 0.12
D = 0.125001;  -->  D = 0.13
E = 0.126000;  -->  E = 0.13

结果出现了一些偏差。也就是 float C = 0.125000 的情况。

2.2、NSDecimalNumber

/*
枚举
    NSRoundPlain,   // Round up on a tie ／／貌似取整 翻译出来是个圆 吗的垃圾百度翻译
    NSRoundDown,    // Always down == truncate  ／／只舍不入
    NSRoundUp,      // Always up    ／／ 只入不舍
    NSRoundBankers  // on a tie round so last digit is even  貌似四舍五入
*/
NSDecimalNumberHandler* roundingBehavior = [NSDecimalNumberHandler decimalNumberHandlerWithRoundingMode:NSRoundBankers
                                                                                                  scale:2
                                                                                       raiseOnExactness:NO
                                                                                        raiseOnOverflow:NO
                                                                                       raiseOnUnderflow:NO
                                                                                    raiseOnDivideByZero:YES];
NSDecimalNumber * ouncesDecimal = [[NSDecimalNumber alloc] initWithFloat:X];
NSDecimalNumber * roundedOunces = [ouncesDecimal decimalNumberByRoundingAccordingToBehavior:roundingBehavior];
NSString * showX = [NSString stringWithFormat:@"%@",roundedOunces];

// 输出
A = 0.124000;  -->  A = 0.12 
B = 0.124200;  -->  B = 0.12 
C = 0.125000;  -->  C = 0.12
D = 0.125001;  -->  D = 0.13
E = 0.126000;  -->  E = 0.13

结果同上， float C = 0.125000 的情况还不行。

2.3、roundf()

NSString * showX = [NSString stringWithFormat:@"%0.2f",roundf(X * 100)/100];

// 输出
A = 0.124000;  -->  A = 0.12 
B = 0.124200;  -->  B = 0.12 
C = 0.125000;  -->  C = 0.13
D = 0.125001;  -->  D = 0.13
E = 0.126000;  -->  E = 0.13

发现这个方法好像更靠谱点。

拓展的相关的其他函数

round  如果参数是小数  则求本身的四舍五入.
ceil   如果参数是小数  则求最小的整数但不小于本身.
floor  如果参数是小数  则求最大的整数但不大于本身.