Referring one data frame to perform analysis on another data frame

I have one data frame, lets call it titles:

titles <- data.frame(V1=c("A", "B", "A", "B", "A", "A"))

I have another data frame called, stuff:

stuff <- data.frame(V1=c(1,2,3,4,5,6), V2=c(10,20,30,40,50,60), V3=c(1,3,5,7,9,11), V4=c(2,4,6,8,10,12), V5=c(9,8,7,6,5,4))

I would like to iterate through, so that in each column, I multiply all the rows that correspond with A and multiply all values in the rows that correspond to B

so basically…

for V1 in “stuff”, I would multiply (1*3*5*6) because they correspond to A

read more

Why does memory NX need hardware support in Linux kernel?

In Linux kernel, the page table entry (PTE) contains R/W bits but no eXecute bit because there is no NX bit in early x86 architecture.

I don’t understand why X bit in PTE needs hardware support.

As I see, we can define a bit to struct PTE as X bit in kernel source code. Kernel will check this bit in PTE when accessing page table.

Any explanation will be appreciated. Thanks!

Why does memory NX need hardware support in Linux kernel?

navigate to previous view controller with default back button

I want to switch from one view to another and after that by the help of navigation controller i want jump my pervious view controller with by default back button . how i do this? 

Problem is :-
On the login screen i add a button register here.I click and jump to next after that again i want to switch login page with using by default back button.

navigate to previous view controller with default back button

Lambda Expression use case and connection with Functional Interface

I am currently learning about Lambda expressions in Java. As per my understanding it is a block of code that we can pass around so that it can be executed later. But I cannot think of examples when we want to execute the code later.

What are various scenarios where this is useful? Also what is the connection between Lambda Expression and Functional interface?

Lambda Expression use case and connection with Functional Interface