I am trying to make an if statement:
if(product = 'a', 'red', if(city = 'south', 'red', 'green')
This returns green only. I want it to return red and green
What am I doing wrong?
It's hard to say whether there's an issue with your data, the expression, or your logic without seeing your data, but here is a screenshot that explains a few different scenarios for your If() expression:
Make sure you are using the If() function correctly -- if it's not returning what you're expecting it to based on your intended logic, maybe consider adding in some AND and OR operators as needed to apply the correct logic to the expression. You can also see some more examples of using the If() function on this Qlik Help page.
Feel free to edit your question with an example of your data if you have further questions!
Related
I want to do this:
var arr = [0,1,0,1,0,2,1,0,0]
switch arr {
case [1,_,_,_,_,_,_,_]: print("Yay")
default: print("Nay")
}
But I get this error: "'_' can only appear in a pattern or on the left side of an assignment"
I searched around on this error message and "wildcards swift array switch statements" and it seems like it is not possible to use wildcards in this context, but there may be some sort of workaround using 'operator overloading' with something involving ~=. However, this is way over my current knowledge, I haven't found a case example exactly like mine, and there seems to be a fair amount of implied concern that operator overloading is bad for some reason. Also, the examples I've found that is sort of like my case look very convoluted. I'm a self-taught amateur just trying to hack together a basic app to make my work life easier, so apologies for any stupid questions but I'd like to better understand the following:
Why can't I use wildcards in this context? It seems like they work with tuples but not arrays for some reason, and I don't get why one would work but not the other.
Is operating overloading actually 'bad' and is there a relatively straightforward way to implement this functionality?
If not- can someone recommend an alternate approach? I chose this approach because I have two arrays, one for a series of questions, one to record responses to each question. I want to implement pretty complicated logic where the configuration of responses to prior questions will determine which question is presented next (example below). I wasted a bunch of time drowning in convoluted 'if' statements before realizing I needed a cleaner approach and this seemed to be it. However, without wildcards or something similar, it would not be practical to explicitly define every case. Could someone recommend a different approach in this situation? Obviously, this is a streamlined example and there would be many more defined cases. Thanks in advance.
var responseArr = [0,2,1,2,1,0,0] // 0 = unasked, 1 = asked answered no, 2 = asked answered yes
switch responseArr {
case: [0,_,_,2,1,0,_]: nextQuestion = questionArr[0] // If 'yes' to question 4, no to question 5, and questions 1 and 6 were not asked yet, ask question 1
case: [1,_,_,2,1,0,_]: nextQuestion = questionArr[5] // If 'yes' to question 4, no to question 5, no to question 1, and question 6 was not asked yet, ask question 6
...
default: print("error")
}
It seems like they work with tuples but not arrays for some reason
The reason is that a tuple is a multi-value type, each value is evaluated separately, on the other hand an array is one value.
A valid syntax is
switch arr {
case arr where arr[0] == 0 && arr[5] == 2 : print("Yay")
default: print("Nay")
}
The tuple syntax would be something like
switch (arr[0], arr[1], arr[2], arr[3], arr[4], arr[5], arr[6], arr[7], arr[8]) {
case (0,_,_,2,1,0,_,_,_): print("Yay")
default: print("Nay")
}
This is heavily implied by Joakim's and vadian's responses, but for the benefit of other beginners that might come across this- a very simple conversion of the array to a tuple (as below) seems to work perfectly for what I was trying to accomplish in the first place, and only really requires addition of one intermediary variable and replacing some brackets with parentheses:
var arr2 = [0,1,0,1,0,2,1,0,0]
let arr2AsTuple = (arr2[0],arr2[1],arr2[2],arr2[3],arr2[4],arr2[5],arr2[6],arr2[7],arr2[8])
switch arr2AsTuple {
case (1,_,_,_,_,_,_,_,_): print("Yay")
default: print("Nay")
}
Let's say I maybe want to import a component based on some condition, let's say a boolean variable. I've tried this, but it gives me an error message. For instance, consider the following code:
model myNewModel
parameter Boolean use_Something;
myLibrary.myComponent component[if use_Something then 1 else 0];
// In other words (pseudo):
// if use_Something then 'Import The Component' else 'Do Nothing At All';
end myNewModel;
This is, intuitively, a safe statement, and as long as the boolean variable is true, it'll work as intended. For some units, for instance the fluidports of the Modelica Standard Library, it also works with the [0] size. But as soon as I turn the variable to false, I encounter errors regarding the fact that many components are not compatible with "zero size". I've had this problem, for instance, with the MassFlowSources in the Modelica Standard Library. Is there a smooth/elegant way to work around this? Thanks in advance!
You can use conditional components in Modelica.
model myNewModel
parameter Boolean use_Something;
myLibrary.myComponent component if use_Something;
end myNewModel;
This component may then only be used in connect-statements. If the condition is false, these connections are ignored by the tool.
At my company we've recently been working on a modification to an existing program that used the following logic to perform some database field assignments:
db-buffer1.field1 = if db-buffer1.field1 <> db-buffer2.field2
then db-buffer2.field2
else db-buffer1.field1
I'm not 100% sure of the original programmer's intention here. Can anyone help me understand the value in comparing whether or not the field values are different before deciding whether to assign the new field?
If the comparison is 'false' and we end up assigning db-buffer1.field1 = db-buffer1.field1 do we avoid a write operation on the database?
Also note that this example is part of a larger ASSIGN statement that contains several fields running similar assignment/comparison logic. Does that have any effect on the value of this additional code? (i.e. Do all comparisons in the ASSIGN statement have to succeed in order to avoid a DB write?)
An "IF function" on the right hand side of an expression is a lot like a "ternary operator" in C or Javascript (the "?:" construct).
It only impacts the portion of the ASSIGN that it is part of. When I write code using those I always encase it in parens to make it clear. Like so:
assign
a = ( if x = y then b else c )
z = 2
.
Whether or not a WRITE happens depends a lot on the rest of the code.
Judging only by this snippet you are going to write SOMETHING (at least logically). db-buffer.field1 is going to get a value assigned to it regardless. The IF logic on the right-hand side is just picking if it will be field1 or field2. In the case that boils down to field1 = field1 you might hope that some lower layer will optimize the write out of existence. I don't see your Progress version posted but if it is v9 or better then it may very well be optimized away. (Prior to v9 it would not have been.)
If you really want to "avoid the write" at the application level you should code it like this:
if field1 <> field2 then
assign
field1 = field2
.
This form does not use the IF function, it is just a normal IF ... THEN statement. It is much clearer and it does not depend on optimizations at a lower level. Of course the snippet shown is said to be part of a larger ASSIGN -- so it may or may not be sensible to break it out and write it as I've shown. But it would be worth thinking about.
It appears that the programmer is checking if field1 is not equal to field2 go ahead and update it.
They might as well simply use the following since either way the field is being assigned:
assign db-buffer1.field1 = db-buffer2.field2.
When using the ASSIGN statement (recommended always), from a logical perspective you have to keep in mind that each line is assigned separately.
Tom has a great answer here that gives more information regarding efficiency/history of the ASSIGN statement.
I would be very interested in seeing the rest of the assign statement, but I would also get rid of the if logic in this case as it adds no value, just and extra instruction. As Terry stated above
ASSIGN db-buffer1.field1 = db-buffer2.field2.
Is there an idiomatic way in Matlab to bind the value of an expression to the nth return value of another expression?
For example, say I want an array of indices corresponding to the maximum value of a number of vectors stored in a cell array. I can do that by
function I = max_index(varargin)
[~,I]=max(varargin{:});
cellfun(#max_index, my_data);
But this requires one to define a function (max_index) specific for each case one wants to select a particular return value in an expression. I can of course define a generic function that does what I want:
function y = nth_return(n,fun,varargin)
[vals{1:n}] = fun(varargin{:});
y = vals{n};
And call it like:
cellfun(#(x) nth_return(2,#max,x), my_data)
Adding such functions, however, makes code snippets less portable and harder to understand. Is there an idiomatic to achieve the same result without having to rely on the custom nth_return function?
This is as far as I know not possible in another way as with the solutions you mention. So just use the syntax:
[~,I]=max(var);
Or indeed create an extra function. But I would also suggest against this. Just write the extra line of code, in case you want to use the output in another function. I found two earlier questions on stackoverflow, which adress the same topic, and seem to confirm that this is not possible.
Skipping outputs with anonymous function in MATLAB
How to elegantly ignore some return values of a MATLAB function?
The reason why the ~ operator was added to MATLAB some versions ago was to prevent you from saving variables you do not need. If there would be a syntax like the one you are searching for, this would not have been necessary.
Sometimes I'm writing ugly if-else statements in C# 3.5; I'm aware of some different approaches to simplifying that with table-driven development, class hierarchy, anonimous methods and some more.
The problem is that alternatives are still less wide-spread than writing traditional ugly if-else statements because there is no convention for that.
What depth of nested if-else is normal for C# 3.5? What methods do you expect to see instead of nested if-else the first? the second?
if i have ten input parameters with 3 states in each, i should map functions to combination of each state of each parameter (really less, because not all the states are valid, but sometimes still a lot). I can express these states as a hashtable key and a handler (lambda) which will be called if key matches.
It is still mix of table-driven, data-driven dev. ideas and pattern matching.
what i'm looking for is extending for C# such approaches as this for scripting (C# 3.5 is rather like scripting)
http://blogs.msdn.com/ericlippert/archive/2004/02/24/79292.aspx
Good question. "Conditional Complexity" is a code smell. Polymorphism is your friend.
Conditional logic is innocent in its infancy, when it’s simple to understand and contained within a
few lines of code. Unfortunately, it rarely ages well. You implement several new features and
suddenly your conditional logic becomes complicated and expansive. [Joshua Kerevsky: Refactoring to Patterns]
One of the simplest things you can do to avoid nested if blocks is to learn to use Guard Clauses.
double getPayAmount() {
if (_isDead) return deadAmount();
if (_isSeparated) return separatedAmount();
if (_isRetired) return retiredAmount();
return normalPayAmount();
};
The other thing I have found simplifies things pretty well, and which makes your code self-documenting, is Consolidating conditionals.
double disabilityAmount() {
if (isNotEligableForDisability()) return 0;
// compute the disability amount
Other valuable refactoring techniques associated with conditional expressions include Decompose Conditional, Replace Conditional with Visitor, Specification Pattern, and Reverse Conditional.
There are very old "formalisms" for trying to encapsulate extremely complex expressions that evaluate many possibly independent variables, for example, "decision tables" :
http://en.wikipedia.org/wiki/Decision_table
But, I'll join in the choir here to second the ideas mentioned of judicious use of the ternary operator if possible, identifying the most unlikely conditions which if met allow you to terminate the rest of the evaluation by excluding them first, and add ... the reverse of that ... trying to factor out the most probable conditions and states that can allow you to proceed without testing of the "fringe" cases.
The suggestion by Miriam (above) is fascinating, even elegant, as "conceptual art;" and I am actually going to try it out, trying to "bracket" my suspicion that it will lead to code that is harder to maintain.
My pragmatic side says there is no "one size fits all" answer here in the absence of a pretty specific code example, and complete description of the conditions and their interactions.
I'm a fan of "flag setting" : meaning anytime my application goes into some less common "mode" or "state" I set a boolean flag (which might even be static for the class) : for me that simplifies writing complex if/then else evaluations later on.
best, Bill
Simple. Take the body of the if and make a method out of it.
This works because most if statements are of the form:
if (condition):
action()
In other cases, more specifically :
if (condition1):
if (condition2):
action()
simplify to:
if (condition1 && condition2):
action()
I'm a big fan of the ternary operator which get's overlooked by a lot of people. It's great for assigning values to variables based on conditions. like this
foobarString = (foo == bar) ? "foo equals bar" : "foo does not equal bar";
Try this article for more info.
It wont solve all your problems, but it is very economical.
I know that this is not the answer you are looking for, but without context your questions is very hard to answer. The problem is that the way to refactor such a thing really depends on your code, what it is doing, and what you are trying to accomplish. If you had said that you were checking the type of an object in these conditionals we could throw out an answer like 'use polymorphism', but sometimes you actually do just need some if statements, and sometimes those statements can be refactored into something more simple. Without a code sample it is hard to say which category you are in.
I was told years ago by an instructor that 3 is a magic number. And as he applied it it-else statements he suggested that if I needed more that 3 if's then I should probably use a case statement instead.
switch (testValue)
{
case = 1:
// do something
break;
case = 2:
// do something else
break;
case = 3:
// do something more
break;
case = 4
// do what?
break;
default:
throw new Exception("I didn't do anything");
}
If you're nesting if statements more than 3 deep then you should probably take that as a sign that there is a better way. Probably like Avirdlg suggested, separating the nested if statements into 1 or more methods. If you feel you are absolutely stuck with multiple if-else statements then I would wrap all the if-else statements into a single method so it didn't ugly up other code.
If the entire purpose is to assign a different value to some variable based upon the state of various conditionals, I use a ternery operator.
If the If Else clauses are performing separate chunks of functionality. and the conditions are complex, simplify by creating temporary boolean variables to hold the true/false value of the complex boolean expressions. These variables should be suitably named to represent the business sense of what the complex expression is calculating. Then use the boolean variables in the If else synatx instead of the complex boolean expressions.
One thing I find myself doing at times is inverting the condition followed by return; several such tests in a row can help reduce nesting of if and else.
Not a C# answer, but you probably would like pattern matching. With pattern matching, you can take several inputs, and do simultaneous matches on all of them. For example (F#):
let x=
match cond1, cond2, name with
| _, _, "Bob" -> 9000 // Bob gets 9000, regardless of cond1 or 2
| false, false, _ -> 0
| true, false, _ -> 1
| false, true, _ -> 2
| true, true, "" -> 0 // Both conds but no name gets 0
| true, true, _ -> 3 // Cond1&2 give 3
You can express any combination to create a match (this just scratches the surface). However, C# doesn't support this, and I doubt it will any time soon. Meanwhile, there are some attempts to try this in C#, such as here: http://codebetter.com/blogs/matthew.podwysocki/archive/2008/09/16/functional-c-pattern-matching.aspx. Google can turn up many more; perhaps one will suit you.
try to use patterns like strategy or command
In simple cases you should be able to get around with basic functional decomposition. For more complex scenarios I used Specification Pattern with great success.