问题描述
我有一个如下的数据框:
I have a data frame as follows:
planets type diameter rotation rings
Mercury Terrestrial planet 0.382 58.64 FALSE
Venus Terrestrial planet 0.949 -243.02 FALSE
Earth Terrestrial planet 1.000 1.00 FALSE
Mars Terrestrial planet 0.532 1.03 FALSE
Jupiter Gas giant 11.209 0.41 TRUE
Saturn Gas giant 9.449 0.43 TRUE
Uranus Gas giant 4.007 -0.72 TRUE
Neptune Gas giant 3.883 0.67 TRUE
我想选择最后 3 行:
I wanted to select last 3 rows:
planets_df[nrow(planets_df)-3:nrow(planets_df),]
然而，我有一些我没想到的事情:
However, I've got something I didn't expect:
planets type diameter rotation rings
Jupiter Gas giant 11.209 0.41 TRUE
Mars Terrestrial planet 0.532 1.03 FALSE
Earth Terrestrial planet 1.000 1.00 FALSE
Venus Terrestrial planet 0.949 -243.02 FALSE
Mercury Terrestrial planet 0.382 58.64 FALSE
通过试错法，我了解到
> (nrow(planets_df)-3):nrow(planets_df)
[1] 5 6 7 8
和
> nrow(planets_df)-3:nrow(planets_df)
[1] 5 4 3 2 1 0
R 如何准确评估 : 语句(参考方括号)?
How does exactly R evaluate : statement (with reference to brackets)?
推荐答案
冒号运算符将优先于算术运算.最好通过示例进行实验以将逻辑内化:
The colon operator will take precedence over the arithmetic operations. It is always best to experiment with examples to internalize the logic:
2*2:6-1
我们应该期待什么答案?有人会说4 5.想法是它会简化为2*2=4和6-1=5，因此4:5.
What answer should we expect? Some would say 4 5. The thinking is that it will simplify to 2*2=4 and 6-1=5, therefore 4:5.
2*2:6-1
[1] 3 5 7 9 11
这个答案会让任何没有考虑过操作顺序的人感到惊讶.表达式 2*2:6-1 的简化方式不同.先执行序列2:6，然后是乘法，最后是加法.我们可以把它写成 2 * (2 3 4 5 6)，也就是 4 6 8 10 12 减去 1 得到得到 3 5 7 9 11.
This answer will surprise anyone who hasn't considered the order of operations in play. The expression 2*2:6-1 is simplified differently. The sequence 2:6 is carried out first, then the multiplication, and finally the addition. We could write it out as 2 * (2 3 4 5 6), which is 4 6 8 10 12 and subtract 1 from that to get 3 5 7 9 11.
通过用括号分组，我们可以控制运算顺序，就像我们在基本算术中做的类似，以获得我们最初预期的答案.
By grouping with parantheses we can control the order of operations as we would do similarly in basic arithmetic to get the answer that we first expected.
(2*2):(6-1)
[1] 4 5
您可以将此推理应用到您的示例中，以调查 : 运算符看似奇怪的行为.
You can apply this reasoning to your example to investigate the seemingly odd behavior of the : operator.
既然你知道了密码，我们应该对 (2*2):6-1 有什么期待?
Now that you know the secret codes, what should we expect from (2*2):6-1?
这篇关于括号使向量不同.向量表达式究竟是如何计算的?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！