如何检查列表列表中的元素是否符合条件? [英] How to check that element of a list of lists matches a condition?

问题描述

我有一个列表列表:

  pairs <- list(
    list(Name="A",Value=11), 
    list(Name="B",Value=17), 
    list(Name="C",Value=23)
  )

如何检查 pairs 列表中是否包含Name =="A"的元素?而且我也想获得该元素.

How can I check that pairs list contains an element with Name=="A"? And I'd also like to get that element.

推荐答案

如果您只想知道任何列表组件是否具有 Name =='A':

If you just want to know whether any list component has Name=='A':

any(sapply(pairs,function(x) x$Name=='A'));
## [1] TRUE

如果要具有 Name =='A'的列表组件的数量:

If you want the number of list components that have Name=='A':

sum(sapply(pairs,function(x) x$Name=='A'));
## [1] 1

如果要具有 Name =='A'的列表组件的 Value :

If you want the Value of the list component(s) that have Name=='A':

unlist(lapply(pairs,function(x) if (x$Name=='A') x$Value));
## [1] 11

如果要具有 Name =='A'的组件的子列表:

If you want the sublist of components that have Name=='A':

pairs[sapply(pairs,function(x) x$Name=='A')];
## [[1]]
## [[1]]$Name
## [1] "A"
##
## [[1]]$Value
## [1] 11

如果您想要第一个具有 Name =='A'的内部列表(如果确定只有一个匹配项，则可以删除 [1] ):

If you want the first inner list that has Name=='A' (can drop the [1] if you're certain there will only be one match):

pairs[[which(sapply(pairs,function(x) x$Name=='A'))[1]]];
## $Name
## [1] "A"
##
## $Value
## [1] 11

---

或者，由于您的数据看起来很规则，因此您可以转换为data.frame，这将简化所有这些操作:

---

Alternatively, since your data appears to be regular, you can convert to a data.frame, which will simplify all these operations:

df <- do.call(rbind,lapply(pairs,as.data.frame));
df;
##   Name Value
## 1    A    11
## 2    B    17
## 3    C    23

以下是 df 的等效项:

any(df$Name=='A');
## [1] TRUE
sum(df$Name=='A');
## [1] 1
df$Value[df$Name=='A'];
## [1] 11
subset(df,Name=='A');
##   Name Value
## 1    A    11
subset(df,Name=='A')[1,];
##   Name Value
## 1    A    11

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