How to check that element of a list of lists matches a condition?

Question

I have a list of lists:

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

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

Answer 1

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

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

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

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

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

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

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

---

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

Here are the equivalents for df:

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