Why do I see
proc simple(a, b: int) : int =
result = a + b
so often in nim code when it seems as if
proc simple(a, b: int) : int =
a + b
would suffice? Is there any semantic difference between those two that I'm missing?
The only reference to implicitly returning the last statement I found on nim-lang wasn't in the manual but in the tut where it states that
[...] a proc's body can consist of a single expression whose value is
then returned implicitly.
Which seems misleading: it seems as it works for every 'last expression' (unless result was already set, then the result of the statement has to be discarded)
In the coding conventions (https://nim-lang.org/docs/nep1.html#introduction-coding-conventions) they recommend to use return only if it's needed in the flow.
The book Nim in Action says 'it's not idiomatic to use the return keyword as the last statement of the proc', but it's not explicit about result = a + b vs a + b. From the snippets around the book, the convention seems to be:
Prefer a + b.
Use result = a + b only if you are modifying result, as in result.add(b).
Use return a only to do an early exit from the proc.
The book also list this gotcha that won't compile:
proc resultVar: string =
result = "The result"
"This cause an error"
The reason behind code like result = a + b or return a is that people can't get all the idiomatics, specially when they are beginners like me. I still see for i in range(len(variable)) in Python code, which is not only non-pythonic but ugly and underperformant.
One of the more exotic features is the implicit result variable: every procedure in Nim with a non-void return type has an implicit result variable that represents the value that will be returned [Wikipedia].
Related
I need to evaluate a dynamic logical expression and I know that in ABAP it is not possible.
I found the class cl_java_script and with this class I could achieve my requeriment. I've try something like this:
result = cl_java_script=>create( )->evaluate( `( 1 + 2 + 3 ) == 6 ;` ).
After the method evaluate execution result = true as espected. But my happiness is over when I look into the class documentation that says This class is obsolete.
My question is, there is another way to achieve this?
Using any turing complete language to parse a "dynamic logical expression" is a terrible idea, as an attacker might be able to run any program inside your expression, i.e. while(true) { } will crash your variant using cl_java_script. Also although I don't know the details of cl_java_script, I assume it launches a separate JS runtime in a separate thread somewhere, this does not seem to be the most efficient choice to calculate such a small dynamic expression.
Instead you could implement your own small parser. This has the advantage that you can limit what it supports to the bare minimum whilst being able to extend it to everything you need in your usecase. Here's a small example using reverse polish notation which is able to correctly evaluate the expression you've shown (using RPN simplifies parsing a lot, though for sure one can also build a full fledged expression parser):
REPORT z_expr_parser.
TYPES:
BEGIN OF multi_value,
integer TYPE REF TO i,
boolean TYPE REF TO bool,
END OF multi_value.
CLASS lcl_rpn_parser DEFINITION.
PUBLIC SECTION.
METHODS:
constructor
IMPORTING
text TYPE string,
parse
RETURNING VALUE(result) TYPE multi_value.
PRIVATE SECTION.
DATA:
tokens TYPE STANDARD TABLE OF string,
stack TYPE STANDARD TABLE OF multi_value.
METHODS pop_int
RETURNING VALUE(result) TYPE i.
METHODS pop_bool
RETURNING VALUE(result) TYPE abap_bool.
ENDCLASS.
CLASS lcl_rpn_parser IMPLEMENTATION.
METHOD constructor.
" a most simple lexer:
SPLIT text AT ' ' INTO TABLE tokens.
ASSERT lines( tokens ) > 0.
ENDMETHOD.
METHOD pop_int.
DATA(peek) = stack[ lines( stack ) ].
ASSERT peek-integer IS BOUND.
result = peek-integer->*.
DELETE stack INDEX lines( stack ).
ENDMETHOD.
METHOD pop_bool.
DATA(peek) = stack[ lines( stack ) ].
ASSERT peek-boolean IS BOUND.
result = peek-boolean->*.
DELETE stack INDEX lines( stack ).
ENDMETHOD.
METHOD parse.
LOOP AT tokens INTO DATA(token).
IF token = '='.
DATA(comparison) = xsdbool( pop_int( ) = pop_int( ) ).
APPEND VALUE #( boolean = NEW #( comparison ) ) TO stack.
ELSEIF token = '+'.
DATA(addition) = pop_int( ) + pop_int( ).
APPEND VALUE #( integer = NEW #( addition ) ) TO stack.
ELSE.
" assumption: token is integer
DATA value TYPE i.
value = token.
APPEND VALUE #( integer = NEW #( value ) ) TO stack.
ENDIF.
ENDLOOP.
ASSERT lines( stack ) = 1.
result = stack[ 1 ].
ENDMETHOD.
ENDCLASS.
START-OF-SELECTION.
" 1 + 2 + 3 = 6 in RPN:
DATA(program) = |1 2 3 + + 6 =|.
DATA(parser) = NEW lcl_rpn_parser( program ).
DATA(result) = parser->parse( ).
ASSERT result-boolean IS BOUND.
ASSERT result-boolean->* = abap_true.
SAPs BRF is an option, but potentially massive overkill in your scenario.
Here is a blog on calling BRF from abap.
And here is how Rules/Expressions can be defined dynamically.
BUT, if you know enough about the source problem to generate
1 + 2 + 3 = 6
Then it is hard to imagine why a simple custom parser cant be used.
Just how complex should the expressions be ?
Id probably write my own parser before investing in calling BRF.
Since some/many BSPs use server side JAVAscript and not ABAP as the scripting language, i cant see SAP removing the Kernel routine anytime soon.
SYSTEM-CALL JAVA SCRIPT EVALUATE.
SO maybe consider Just calling the cl_java_script anyway until it is an issue.
Then worry about a parser if and when it is really no longer a valid call.
But definitely some movement in the obsolete space here.
SAP is pushing/forcing you to cloud with the SDK, to execute such things.
https://sap.github.io/cloud-sdk/docs/js/overview-cloud-sdk-for-javascript
I don't understand why the recursive function always gives me zero result, even if I put values inside the array.
it seems that size (a) == 0
recursive function binarySearch_R (a, value) result (bsresult)
real, intent(in) :: a(6), value
integer :: bsresult, mid
mid = size(a)/2 + 1
if (size(a) == 0) then
bsresult = 0 ! not found
else if (a(mid) > value) then
bsresult= binarySearch_R(a(:mid-1), value)
else if (a(mid) < value) then
bsresult = binarySearch_R(a(mid+1:), value)
if (bsresult /= 0) then
bsresult = mid + bsresult
end if
else
bsresult = mid ! SUCCESS!!
end if
end function binarySearch_R
program hji
read*, a
read*, value
print*, binarySearch_R
end program hji
Chapter 1: The dangers of implicit typing
The first thing I strongly recommend you do is to include the line
implicit none
after the program line. This will suppress implicit typing, and the resulting errors will give you some useful insight into what is happening.
If you did that, you'd get an error message:
$ gfortran -o binsearch binsearch.f90
binsearch.f90:23:12:
read*, a
1
Error: Symbol ‘a’ at (1) has no IMPLICIT type
binsearch.f90:27:25:
print*,binarySearch_R
1
Error: Symbol ‘binarysearch_r’ at (1) has no IMPLICIT type
binsearch.f90:24:16:
read*, value
1
Error: Symbol ‘value’ at (1) has no IMPLICIT type
It doesn't matter that a, value, and binarySearch_R were defined in the function. As the function is not part of the program block, the program doesn't know what these are.
With implicit typing active, it simply assumed that all three are simple real variables. (The type depends on the first letter of the variable name, i through n are integer, everything else is real)
Because this implicit typing can so easily hide coding errors, it's strongly, strongly suggested to always switch it off.
Which also means that we have to declare the variables a and value in the program:
program hji
implicit none
real :: a(6), value
...
end program hji
Chapter 2: How to introduce a function to the program?
So how does the program get access to the function? There are four ways:
The best way: Use a module
module mod_binsearch
implicit none
contains
recursive function binarySearch_R (a, value) result (bsresult)
...
end function binarySearch_R
end module mod_binsearch
program hji
use mod_binsearch
implicit none
real :: a(6), value
...
end program hji
Note that the use statement has to be before the implicit none.
This method leaves the function separate, but callable.
It automatically checks that the parameters (that's something we'll be coming to in a bit) are correct.
Have the function contained in the program.
Between the final line of code of the program and the end program statement, add the keyword contains, followed by the function code (everything from recursive function ... to end function ...).
This is the quick-and-dirty method. You have to be careful with this method as the function will automatically have access to the program's variables unless there's a new variable with that name declared inside the function.
The convoluted way: Interfaces
Create an interface block in the declaration section of your program's source code,
and repeat the interface information in there.
This still allows the compiler to check whether the function is invoked correctly, but it's up to you to ensure that this interface block is correct and matches the actual implementation.
The really, really ugly way: Declare it like a variable, invoke it like a function.
Please don't do that.
Chapter 3: Calling a function
When you call a function, you have to use the parentheses and give it all the parameters that it expects. In your case, you need to type
print *, binarySearch_r(a, value)
Chapter 4: Dynamic arrays as dummy parameters
In the successive recursive calls to the function, the array gets smaller and smaller.
But the dummy parameter is always the same size (6). Not only will this interfere with your algorithm, but this can also lead to dangerously undefined memory access.
Fortunately, specially for intent(in) dummy parameters, you can use dynamic arrays:
recursive function binarySearch_R(a, value)
real, intent(in) :: a(:), value
The single colon tells the compiler to expect a one-dimensional array, but not the length of it. Since you're already using size(a), it should automatically work.
Too long for a comment, but not an answer (and to any Fortran experts reading this, yes, there are one or two places where I gloss over some details because I think they are unimportant at this stage) ...
The way the code is written does not allow the compiler to help you. As far as the compiler is concerned there is no connection between the function and the program. As far as the program is concerned a is, because you haven't told the compiler otherwise, assumed to be a real scalar value. The a in the program is not the same thing as the a in the function - there is no connection between the function and the program.
The same is true for value.
The same is true for binarysearch_r - and if you don't believe this delete the function definition from the source code and recompile the program.
So, what must you do to fix the code ?
First step: modify your source code so that it looks like this:
program hji
... program code goes here ...
contains
recursive function binarySearch_R (a, value) result (bsresult)
... function code goes here ...
end function binarySearch_R
end program hji
This first step allows the compiler to see the connection between the program and the function.
Second step: insert the line implicit none immediately after the line program hji. This second step allows the compiler to spot any errors you make with the types (real or integer, etc) and ranks (scalar, array, etc) of the variables you declare.
Third step: recompile and start dealing with the errors the compiler identifies. One of them will be that you do not pass the arguments to the function so the line
print*, binarySearch_R
in the program will have to change to
print*, binarySearch_R(a, value)
I' m having a problem parsing the lat and long cords from TinyGPS++ to a Double or a string. The code that i'm using is:
String latt = ((gps.location.lat(),6));
String lngg = ((gps.location.lng(),6));
Serial.println(latt);
Serial.println(lngg);
The output that i'm getting is:
0.06
Does somebody know what i'm doing wrong? Does it have something to do with rounding? (Math.Round) function in Arduino.
Thanks!
There are two problems:
1. This does not compile:
String latt = ((gps.location.lat(),6));
The error I get is
Wouter.ino:4: warning: left-hand operand of comma has no effect
Wouter:4: error: invalid conversion from 'int' to 'const char*'
Wouter:4: error: initializing argument 1 of 'String::String(const char*)'
There is nothing in the definition of the String class that would allow this statement. I was unable to reproduce printing values of 0.06 (in your question) or 0.006 (in a later comment). Please edit your post to have the exact code that compiles, runs and prints those values.
2. You are unintentionally using the comma operator.
There are two places a comma can be used: to separate arguments to a function call, and to separate multiple expressions which evaluate to the last expression.
You're not calling a function here, so it is the latter use. What does that mean? Here's an example:
int x = (1+y, 2*y, 3+(int)sin(y), 4);
The variable x will be assigned the value of the last expression, 4. There are very few reasons that anyone would actually use the comma operator in this way. It is much more understandable to write:
int x;
1+y; // Just a calculation, result never used
2*y; // Just a calculation, result never used
3 + (int) sin(y); // Just a calculation, result never used
x = 4; // A (trivial) calculation, result stored in 'x'
The compiler will usually optimize out the first 3 statements and only generate code for the last one1. I usually see the comma operator in #define macros that are trying to avoid multiple statements.
For your code, the compiler sees this
((gps.location.lat(),6))
And evaluates it as a call to gps.location.lat(), which returns a double value. The compiler throws this value away, and even warns you that it "has no effect."
Next, it sees a 6, which is the actual value of this expression. The parentheses get popped, leaving the 6 value to be assigned to the left-hand side of the statement, String latt =.
If you look at the declaration of String, it does not define how to take an int like 6 and either construct a new String, or assign it 6. The compiler sees that String can be constructed from const char *, so it tells you that it can't convert a numeric 6 to a const char *.
Unlike a compiler, I think I can understand what you intended:
double latt = gps.location.lat();
double lngg = gps.location.lon();
Serial.println( latt, 6 );
Serial.println( lngg, 6 );
The 6 is intended as an argument to Serial.println. And those arguments are correctly separated by a comma.
As a further bonus, it does not use the String class, which will undoubtedly cause headaches later. Really, don't use String. Instead, hold on to numeric values, like ints and floats, and convert them to text at the last possible moment (e.g, with println).
I have often wished for a compiler that would do what I mean, not what I say. :D
1 Depending on y's type, evaluating the expression 2*y may have side effects that cannot be optimized away. The streaming operator << is a good example of a mathematical operator (left shift) with side effects that cannot be optimized away.
And in your code, calling gps.location.lat() may have modified something internal to the gps or location classes, so the compiler may not have optimized the function call away.
In all cases, the result of the call is not assigned because only the last expression value (the 6) is used for assignment.
I was wondering. Are there languages that use only pass-by-reference as their eval strategy?
I don't know what an "eval strategy" is, but Perl subroutine calls are pass-by-reference only.
sub change {
$_[0] = 10;
}
$x = 5;
change($x);
print $x; # prints "10"
change(0); # raises "Modification of a read-only value attempted" error
VB (pre .net), VBA & VBS default to ByRef although it can be overriden when calling/defining the sub or function.
FORTRAN does; well, preceding such concepts as pass-by-reference, one should probably say that it uses pass-by-address; a FORTRAN function like:
INTEGER FUNCTION MULTIPLY_TWO_INTS(A, B)
INTEGER A, B
MULTIPLY_BY_TWO_INTS = A * B
RETURN
will have a C-style prototype of:
extern int MULTIPLY_TWO_INTS(int *A, int *B);
and you could call it via something like:
int result, a = 1, b = 100;
result = MULTIPLY_TWO_INTS(&a, &b);
Another example are languages that do not know function arguments as such but use stacks. An example would be Forth and its derivatives, where a function can change the variable space (stack) in whichever way it wants, modifying existing elements as well as adding/removing elements. "prototype comments" in Forth usually look something like
(argument list -- return value list)
and that means the function takes/processes a certain, not necessarily constant, number of arguments and returns, again, not necessarily a constant, number of elements. I.e. you can have a function that takes a number N as argument and returns N elements - preallocating an array, if you so like.
How about Brainfuck?
If I use the inline function in MATLAB I can create a single function name that could respond differently depending on previous choices:
if (someCondition)
p = inline('a - b','a','b');
else
p = inline('a + b','a','b');
end
c = p(1,2);
d = p(3,4);
But the inline functions I'm creating are becoming quite epic, so I'd like to change them to other types of functions (i.e. m-files, subfunctions, or nested functions).
Let's say I have m-files like Mercator.m, KavrayskiyVII.m, etc. (all taking a value for phi and lambda), and I'd like to assign the chosen function to p in the same way as I have above so that I can call it many times (with variable sized matrices and things that make using eval either impossible or a total mess).
I have a variable, type, that will be one of the names of the functions required (e.g. 'Mercator', 'KavrayskiyVII', etc.). I figure I need to make p into a pointer to the function named inside the type variable. Any ideas how I can do this?
Option #1:
Use the str2func function (assumes the string in type is the same as the name of the function):
p = str2func(type); % Create function handle using function name
c = p(phi, lambda); % Invoke function handle
NOTE: The documentation mentions these limitations:
Function handles created using str2func do not have access to variables outside of their local workspace or to nested functions. If your function handle contains these variables or functions, MATLAB® throws an error when you invoke the handle.
Option #2:
Use a SWITCH statement and function handles:
switch type
case 'Mercator'
p = #Mercator;
case 'KavrayskiyVII'
p = #KavrayskiyVII;
... % Add other cases as needed
end
c = p(phi, lambda); % Invoke function handle
Option #3:
Use EVAL and function handles (suggested by Andrew Janke):
p = eval(['#' type]); % Concatenate string name with '#' and evaluate
c = p(phi, lambda); % Invoke function handle
As Andrew points out, this avoids the limitations of str2func and the extra maintenance associated with a switch statement.