我用谷歌搜索了这个并得到了不完整/矛盾的观点 - 在Ruby/Rails中对一个数组做一个map和做一个实际上有什么区别collect吗？
这些文档似乎没有任何暗示,但是方法或性能可能存在差异吗？
是否有更简洁的方法将dplyr tbl的一列作为向量,从具有数据库后端的tbl(即数据帧/表不能直接是子集)？
require(dplyr)
db <- src_sqlite(tempfile(), create = TRUE)
iris2 <- copy_to(db, iris)
iris2$Species
# NULL
这太容易了,所以
collect(select(iris2, Species))[, 1]
# [1] "setosa" "setosa" "setosa" "setosa" etc.
但它似乎有点笨拙.
从这段代码我不知道两种方法之间的区别,collect和each.
a = ["L","Z","J"].collect{|x| puts x.succ} #=> M AA K
print a.class #=> Array
b = ["L","Z","J"].each{|x| puts x.succ} #=> M AA K
print b.class #=> Array
我有以下格式的数据结构:
data_hash = [
{ price: 1, count: 3 },
{ price: 2, count: 3 },
{ price: 3, count: 3 }
]
有没有一种有效的方法来获得:price像数组一样的值[1,2,3]？
我怎样才能在之后使用collect_set或collect_list使用数据帧groupby.例如:df.groupby('key').collect_set('values').我收到一个错误:AttributeError: 'GroupedData' object has no attribute 'collect_set'
是否可以使用一个电话collect来制作2个新列表？如果没有,我该如何使用partition？
什么是Linq相当于地图!还是收集!Ruby中的方法？
a = [ "a", "b", "c", "d" ]
a.collect! {|x| x + "!" }
a #=> [ "a!", "b!", "c!", "d!" ]
我可以通过使用foreach迭代集合来完成此操作,但我想知道是否有更优雅的Linq解决方案.
我有一群学生.首先,我想用标记对它们进行分组.然后我想进一步将这些集合分组到同名学生中.
Map<Integer,Map<String,List<String>>> groupping = students.stream()
.collect(Collectors.groupingBy(Student::getMarks,
Collectors.mapping(Student::getName,Collectors.toList())));
我收到一个错误说,
非静态方法不能从静态上下文中引用.
是.我非常清楚,如果没有实例,我就无法引用非静态方法.但是对于所有这些流操作,我真的很困惑.
而不是如何解决这个问题; 我真的想知道这里发生了什么.您的任何投入都表示赞赏!
因为如果我写下面的分组是完全有效的;
Map<Integer,List<Student>> m = students.stream().
collect(Collectors.groupingBy(Student::getMarks));
这是我的Student.java类(如果你需要的话)
public class Student {
private String name;
private int marks;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getMarks() {
return marks;
}
public void setMarks(int marks) {
this.marks = marks;
}
public Student(String name, int marks) {
this.name = name;
this.marks = marks;
}
@Override
public String toString() { …大多数ruby开发人员都知道如何通过执行以下操作来节省一些按键:
array.map(&:methodname)
而不是
array.map {|x| x.methodname }
有没有什么方法可以应用类似的&:methodname快捷方式来调用哈希数组中的"方法"(通过键调用的值)？
在我的情况下,它的JSON api结果以60个批量返回,作为源自JSON的哈希数组.我试图这样做,但没有成功:
array.map(&:"keyname")
但是没有成功,抛出一个NoMethodError并且说Hash没有'keyname'方法,这是合理的我猜.我想知道是否有一些神奇的子弹来模仿这个&:...红宝石家伙已经解决过的哈希的快捷方式.
这是实际的JSON响应之一,我认为仅限于5个结果
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让我们说我们必须使用这段无聊的代码:
ArrayList<Long> ids = new ArrayList<Long>();
for (MyObj obj : myList){
ids.add(obj.getId());
}
切换到Java 8后,我的IDE告诉我我可以用这个代码替换collect call它,并自动生成:
ArrayList<Long> ids = myList.stream().map(MyObj::getId).collect(Collectors.toList());
但是它给了我这个错误:
Steam中的collect(java.util.stream.Collector)无法应用于:(java.util.stream.Collector,capture,java.util.List>)
我试图铸造参数,但它给我不确定的A和R,而IDE没有给任何建议.
我很好奇你collect call在这种情况下如何使用,我找不到任何可以指导我的信息.任何人都可以光明吗？