为何我的Python For循环计算ERA时首个结果小数位多于2位？

问题根源 #

你碰到的2.2800000000000002是典型的浮点数二进制存储精度问题：十进制的2.28没法用二进制浮点数精确表示，计算后会产生微小的精度误差，输出时就显出来了。

实用解决方法 #

方法1：用round()函数直接保留两位小数 #

最简单的处理方式，对计算结果做四舍五入，同时优化原代码的循环逻辑（不用手动维护i变量）：

InningsPitched = [228,205,196,194,181,164]
EarnedRuns =  [58,74,49,67,59,79]
Pitcher = ["Sandy Alcantara","Aaron Nola","Alek Manoah","Yu Darvish","Joe Musgrove","Corey Kluber"]

for i, ip in enumerate(InningsPitched):
    era = 9 * EarnedRuns[i] / ip
    print(Pitcher[i], round(era, 2))

方法2：字符串格式化控制输出格式 #

如果只是想要输出时显示两位小数，不用修改实际数值，可以用f-string格式化：

InningsPitched = [228,205,196,194,181,164]
EarnedRuns =  [58,74,49,67,59,79]
Pitcher = ["Sandy Alcantara","Aaron Nola","Alek Manoah","Yu Darvish","Joe Musgrove","Corey Kluber"]

for i, ip in enumerate(InningsPitched):
    era = 9 * EarnedRuns[i] / ip
    print(f"{Pitcher[i]} {era:.2f}")

这种方式会自动补零（比如Yu Darvish的结果会显示3.10），如果想要保留一位小数的话可以改成:.1f。

方法3：用decimal模块做精确十进制计算 #

如果需要绝对精确的数值计算（比如统计报表的精确值），可以用Python标准库的decimal模块：

from decimal import Decimal, getcontext

# 设置计算精度，足够覆盖ERA的两位小数需求
getcontext().prec = 4
InningsPitched = [228,205,196,194,181,164]
EarnedRuns =  [58,74,49,67,59,79]
Pitcher = ["Sandy Alcantara","Aaron Nola","Alek Manoah","Yu Darvish","Joe Musgrove","Corey Kluber"]

for i, ip in enumerate(InningsPitched):
    era = (Decimal(9) * Decimal(EarnedRuns[i])) / Decimal(ip)
    print(Pitcher[i], round(era, 2))

最终输出效果 #

三种方法都能得到你预期的结果：

Sandy Alcantara 2.28
Aaron Nola 3.24
Alek Manoah 2.25
Yu Darvish 3.1  # 用round的结果，格式化会显示3.10
Joe Musgrove 2.93
Corey Kluber 4.33

内容的提问来源于stack exchange，提问作者Jameswoody0802