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# How to write boolean comparsion function in Coq

By : niu
Date : September 13 2020, 12:00 PM
will be helpful for those in need The problem is that @List.filter nat expects a function of type nat -> bool but you supplied a function of type nat -> Prop. Here, @List.filter nat is List.filter applied to the type argument nat. One difference between bool and Prop is that bool is decidable while Prop is not: there are propositions P such that neither P nor ~P are known; one can always determine whether something is true or false.
In order to resolve this situation, you need to write a function of type nat -> bool that returns true when applied to an argument greater than 7 and false otherwise. You can take advantage of the fact that the standard library defines boolean comparison functions over the natural numbers. I would also suggest reading the first volume of Software Foundations to familiarize yourself with Coq. It is more accessible and easy-going than some other prominent introductions (it was used in a program verification course at my university that presupposed little functional programming experience).
code :
``````Require Import Coq.Lists.List.
Import ListNotations.

Fixpoint filterb {A : Type} (f : A -> bool) (xs : list A) : list A :=
match xs with
| []      => []
| x :: xs => if f x then x :: filterb f xs else filterb f xs
end.

Fixpoint ltb (n m : nat) : bool :=
match n, m with
| n  , 0   => false
| 0  , S m => true
| S n, S m => ltb n m
end.

Eval compute in (filterb (fun n => ltb 7 n) [5;6;7;8;9]).
(* = [8;9] *)
``````

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## How to write boolean function in a single line in C#?

By : Tecles
Date : March 29 2020, 07:55 AM
Hope that helps , This wouldn't be a case to use ?? but this should help:
code :
``````public bool IsNewUser(int id)
{
return !DataContext.Employee.Any(e => e.id == id);
}
``````
``````public Employee GetEmployeeOrNew(int id)
{
return DataContext.Employee.Where(e => e.id == id).FirstorDefault() ?? new Employee();
}
``````

## How to write correct expression to result to Boolean? (Cannot cast from boolean to Boolean)

By : 胡祥坤
Date : March 29 2020, 07:55 AM
I think the issue was by ths following , I am experiencing difficulties with an expression that should result in showing or hiding a band in an iReport. , Are you looking for...
code :
``````showDetailGroup = Boolean.valueOf( (mainGroupInt.intValue() != 0 && subGroupInt.intValue() != 0) ||
(mainGroupInt.intValue() != 0 && showDetailGroup.booleanValue()) )
``````

## Scala: How to write a generic check function that evaluates any function that returns boolean?

By : naima
Date : March 29 2020, 07:55 AM
will help you I can't say that I really understand your motivations for wanting to do what you want to do, but I guess that's beside the point. Maybe the eval function will check something before invoking the supplied function and not invoke that other function (like a fast fail) given some certain condition. Maybe you do some post checking after invoking the function and change the result based on something else. Either way, I suppose you could accomplish something similar to what you want with code looking like this:
code :
``````def main(args: Array[String]) {
val str = "hello world"
println(eval(checkLength(str, 3)))
println(eval(intsEqual(1,1)))
}

def eval(func: => Boolean):Boolean = {
//Do whetever you want before invoking func, maybe
//not even invoke it if some other condition is present
val fres = func

//Maybe change something here before returning based on post conditions
fres
}

def checkLength(s:String, len:Int) = s.length() == len
def intsEqual(a:Int, b:Int) = a == b
``````
``````def checkMyString(str:String, len:Int) = {
eval(str.length == len)
}
``````
``````def checkMyString(str:String, len:Int) = {
def check = {
str.length == len
}
eval(check)
}
``````

## Reversing a comparsion function result given it's function pointer

By : user3371063
Date : March 29 2020, 07:55 AM
help you fix your problem Write a new function and instruct the heap to use it as the comparator. This new function needs access to the existing comparator pointer; it will use the existing pointer to call the original comparator, receive its return value, "invert" it and return the inverted result to its own caller (i.e. the heap).
For example, here is a function that returns random odd integers and is implemented in terms of one that returns totally random integers:
code :
``````int totally_random() {
return 4; // chosen by fair die roll
}

int (*pointer)();
pointer = totally_random;

int odd_random() {
int result = *pointer(); // passed in through a global but alternatives exist
return result & 1 ? result : result + 1; // if even, add one to make odd
}
``````

## Function pointer comparsion

By : Monica Dawande
Date : March 29 2020, 07:55 AM
I wish this help you Yes, two function pointers need to have compatible signatures (for the pointed function) to be compared (or assigned). This means same arity (number of arguments) and compatible types of argument of each rank and same result type.
A given implementation may have an ABI specifying different calling conventions (for functions of different signatures).