I have a string like this:
"productName=\"NAME\"\n"
I want to change it to this:
"productName=NAME"
What is the best way to do this in Erlang?
I like also this one, easy to read, very close to Ning proposition:
1> A="productName=\"NAME\"\n".
"productName=\"NAME\"\n"
2> lists:filter(fun ($\") -> false; ($\n) -> false; (_) -> true end,A).
"productName=NAME"
One of probably many ways:
A = "productName=\"NAME\"\n".
re:replace(A, ["[\"\n]*"], "", [global, {return, list}]).
(Another) one of probably many ways:
S0 = "productName=\"NAME\"\n".
[Char || Char <- S0, not lists:member(Char, [$\", $\n])].
Another variant, for example if you want to do this just for that particular key but leave all others alone:
A = "productName=\"NAME\"\n".
case A of
"productName=" ++ S ->
"productName=" ++ string:strip(S, both, $");
_ ->
A
end.
So as the title says - how do you convert a string into an integer?
the idea is something like this:
convert(String,Integer).
examples:
convert('1',1).
convert('33',33).
I'm using swi prolog
Use atom_number/2.
E.g:
atom_number('123', X).
X = 123.
Assuming you really meant a string and not an atom, use number_codes.
?- number_codes(11, "11").
true.
?- number_codes(11, Str).
Str = [49, 49]. % ASCII/UTF-8
?- number_codes(N, "11").
N = 11.
Perhaps use of atom_codes(?Atom, ?String) and number_chars(?Number, ?CharList) would do it.
Quite an old, but there is a predicate in SWI Prolog: number_string(N, S).
Docs
number_string(123, S).
S = "123".
For those who are still looking for it.
A simple example using Visual Prolog 10
==============================
% UNS-EPISI-LAB-IA
implement main
open core
clauses
run() :-
console::write("Valor de A? "),
A = console::readLine(),
console::write("Valor de B? "),
B = console::readLine(),
Areal = toTerm(real, A),
Breal = toTerm(real, B),
console::write("A + B = ", Areal + Breal),
_ = console::readChar().
end implement main
goal
console::runUtf8(main::run).
in Visual Prolog convert:
X=toTerm(real,H).
real/integer/unsigned...
I was wondering about which is the best (clojuresque) way to compare a character and a string in Clojure.
Obviously something like that returns false:
(= (first "clojure") "c")
because first returns a java.lang.Character and "c" is a single character string. Does exists a construct to compare directly char and string without invoking a cast? I haven't found a way different from this:
(= (str (first "clojure")) "c")
but I'm not satisfied.
Any ideas?
Bye,
Alfredo
How about the straight forward String interop?
(= (.charAt "clojure" 0) \c)
or
(.startsWith "clojure" "c")
It should be as fast as it can get and doesn't allocate a seq object (and in your second example an additional string) which is immediately thrown away again just to do a comparison.
Character literals are written \a \b \c ... in Clojure so you can simply write
(= (first "clojure") \c)
strings can be directly indexed without building a sequence from then and taking the first of that sequence.
(= (nth "clojure" 0) \c)
=> true
nth calls through to this java code:
static public Object nth(Object coll, int n){
if(coll instanceof Indexed)
return ((Indexed) coll).nth(n); <-------
return nthFrom(Util.ret1(coll, coll = null), n);
}
which efficiently reads the character directly.
first call through to this java code:
static public Object first(Object x){
if(x instanceof ISeq)
return ((ISeq) x).first();
ISeq seq = seq(x); <----- (1)
if(seq == null)
return null;
return seq.first(); <------ (2)
}
which builds a seq for the string (1) (building a seq is really fast) and then takes the first item from that seq (2). after the return the seq is garbage.
Seqs are clearly the most idomatic way of accessing anything sequential in clojure and I'm not knocking them at all. It is interesting to be aware of what you are creating when. switching out all your calls to first with calls to nth is likely to be a case of premature optimization. if you want the 100th char in the string i would suggest using an indexed access function like nth
in short: don't sweat the small stuff :)
Fundamentally (at least on the Clojure level — though see Kotarak's answer and others for alternatives to this), you're comparing two sequences: "clojure" and "c". The condition of equality is that the first element of each sequence is equal. So if you want to express this directly you can do
(apply = (map first ["clojure" "c"]))
or the other way around, where you create a lazy sequence over the equality comparison between each pair of characters, and just take the first element of it:
(first (map = "clojure" "c"))
You could use the take function from clojure.contrib.string. Or write your own function that returns the first char if that's something you need frequently.
You can just use str, as you did in your second example. There isn't really anything wrong with that. I mean, you could call first on "c" as well to make it a character, but it wont really make a difference. Is there any reason why you don't like this? It's not really adding much to your code by calling str on the character.
user=> (= (subs "clojure" 0 1) "c")
true
user=> (= (str (first "clojure") "c"))
true
These days you don't necessarily have to use Java interop:
(clojure.string/starts-with? "clojure" "c")
starts-with? is just a thin wrapper (around .startsWith).
So now if you use both Clojure and ClojureScript you won't have to remember both the Java and the JavaScript interop.
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Closed 10 years ago.
I'm personally an advocate of the ternary operator: () ? :
I do realize that it has its place, but I have come across many programmers that are completely against ever using it, and some that use it too often.
What are your feelings on it? What interesting code have you seen using it?
Use it for simple expressions only:
int a = (b > 10) ? c : d;
Don't chain or nest ternary operators as it hard to read and confusing:
int a = b > 10 ? c < 20 ? 50 : 80 : e == 2 ? 4 : 8;
Moreover, when using ternary operator, consider formatting the code in a way that improves readability:
int a = (b > 10) ? some_value
: another_value;
It makes debugging slightly more difficult since you can not place breakpoints on each of the sub expressions. I use it rarely.
I love them, especially in type-safe languages.
I don't see how this:
int count = (condition) ? 1 : 0;
is any harder than this:
int count;
if (condition)
{
count = 1;
}
else
{
count = 0;
}
I'd argue that ternary operators make everything less complex and more neat than the alternative.
Chained I'm fine with - nested, not so much.
I tend to use them more in C simply because they're an if statement that has value, so it cuts down on unnecessary repetition or variables:
x = (y < 100) ? "dog" :
(y < 150) ? "cat" :
(y < 300) ? "bar" : "baz";
rather than
if (y < 100) { x = "dog"; }
else if (y < 150) { x = "cat"; }
else if (y < 300) { x = "bar"; }
else { x = "baz"; }
In assignments like this, I find it's less to refactor, and clearer.
When I'm working in ruby on the other hand, I'm more likely to use if...else...end because it's an expression too.
x = if (y < 100) then "dog"
elif (y < 150) then "cat"
elif (y < 300) then "bar"
else "baz"
end
(Although, admittedly, for something this simple, I might just use the ternary operator anyway.)
The ternary ?: operator is merely a functional equivalent of the procedural if construct. So as long as you are not using nested ?: expressions, the arguments for/against the functional representation of any operation applies here. But nesting ternary operations can result in code that is downright confusing (exercise for the reader: try writing a parser that will handle nested ternary conditionals and you will appreciate their complexity).
But there are plenty of situations where conservative use of the ?: operator can result in code that is actually easier to read than otherwise. For example:
int compareTo(Object object) {
if((isLessThan(object) && reverseOrder) || (isGreaterThan(object) && !reverseOrder)) {
return 1;
if((isLessThan(object) && !reverseOrder) || (isGreaterThan(object) && reverseOrder)) {
return -1;
else
return 0;
}
Now compare that with this:
int compareTo(Object object) {
if(isLessThan(object))
return reverseOrder ? 1 : -1;
else(isGreaterThan(object))
return reverseOrder ? -1 : 1;
else
return 0;
}
As the code is more compact, there is less syntactic noise, and by using the ternary operator judiciously (that is only in relation with the reverseOrder property) the end result isn't particularly terse.
It's a question of style, really; the subconscious rules I tend to follow are:
Only evaluate 1 expression - so foo = (bar > baz) ? true : false, but NOT foo = (bar > baz && lotto && someArray.Contains(someValue)) ? true : false
If I'm using it for display logic, e.g. <%= (foo) ? "Yes" : "No" %>
Only really use it for assignment; never flow logic (so never (foo) ? FooIsTrue(foo) : FooIsALie(foo) ) Flow logic in ternary is itself a lie, ignore that last point.
I like it because it's concise and elegant for simple assignment operations.
Like so many opinion questions, the answer is inevitably: it depends
For something like:
return x ? "Yes" : "No";
I think that is much more concise (and quicker for me to parse) than:
if (x) {
return "Yes";
} else {
return "No";
}
Now if your conditional expression is complex, then the ternary operation is not a good choice. Something like:
x && y && z >= 10 && s.Length == 0 || !foo
is not a good candidate for the ternary operator.
As an aside, if you are a C programmer, GCC actually has an extension that allows you to exclude the if-true portion of the ternary, like this:
/* 'y' is a char * */
const char *x = y ? : "Not set";
Which will set x to y assuming y is not NULL. Good stuff.
In my mind, it only makes sense to use the ternary operator in cases where an expression is needed.
In other cases, it seems like the ternary operator decreases clarity.
I use the ternary operator wherever I can, unless it makes the code extremely hard to read, but then that's usually just an indication that my code could use a little refactoring.
It always puzzles me how some people think the ternary operator is a "hidden" feature or is somewhat mysterious. It's one of the first things I learnt when I start programming in C, and I don't think it decreases readability at all. It's a natural part of the language.
By the measure of cyclomatic complexity, the use of if statements or the ternary operator are equivalent. So by that measure, the answer is no, the complexity would be exactly the same as before.
By other measures such as readability, maintainability, and DRY (don't repeat yourself), either choice may prove better than the other.
I use it quite often in places where I'm constrained to work in a constructor - for example, the new .NET 3.5 LINQ to XML constructs - to define default values when an optional parameter is null.
Contrived example:
var e = new XElement("Something",
param == null ? new XElement("Value", "Default")
: new XElement("Value", param.ToString())
);
or (thanks asterite)
var e = new XElement("Something",
new XElement("Value",
param == null ? "Default"
: param.ToString()
)
);
No matter whether you use the ternary operator or not, making sure your code is readable is the important thing. Any construct can be made unreadable.
I agree with jmulder: it shouldn't be used in place of a if, but it has its place for return expression or inside an expression:
echo "Result: " + n + " meter" + (n != 1 ? "s" : "");
return a == null ? "null" : a;
The former is just an example, and better internationalisation and localisation support of plural should be used!
If you're using the ternary operator for a simple conditional assignment I think it's fine. I've seen it (ab)used to control program flow without even making an assignment, and I think that should be avoided. Use an if statement in these cases.
(Hack of the day)
#define IF(x) x ?
#define ELSE :
Then you can do if-then-else as expression:
int b = IF(condition1) res1
ELSE IF(condition2) res2
ELSE IF(conditions3) res3
ELSE res4;
I think the ternary operator should be used when needed. It is obviously a very subjective choice, but I find that a simple expression (specially as a return expression) is much clearer than a full test. Example in C/C++:
return (a>0)?a:0;
Compared to:
if(a>0) return a;
else return 0;
You also have the case where the solution is between the ternary operator and creating a function. For example in Python:
l = [ i if i > 0 else 0 for i in lst ]
The alternative is:
def cap(value):
if value > 0:
return value
return 0
l = [ cap(i) for i in lst ]
It is needed enough that in Python (as an example), such an idiom could be seen regularly:
l = [ ((i>0 and [i]) or [0])[0] for i in lst ]
this line uses properties of the logical operators in Python: they are lazy and returns the last value computed if it is equal to the final state.
I've seen such beasts like (it was actually much worse since it was isValidDate and checked month and day as well, but I couldn't be bothered trying to remember the whole thing):
isLeapYear =
((yyyy % 400) == 0)
? 1
: ((yyyy % 100) == 0)
? 0
: ((yyyy % 4) == 0)
? 1
: 0;
where, plainly, a series of if-statements would have been better (although this one's still better than the macro version I once saw).
I don't mind it for small things like:
reportedAge = (isFemale && (Age >= 21)) ? 21 + (Age - 21) / 3 : Age;
or even slightly tricky things like:
printf ("Deleted %d file%s\n", n, (n == 1) ? "" : "s");
I like using the operator in debug code to print error values so I don't have to look them up all the time. Usually I do this for debug prints that aren't going to remain once I'm done developing.
int result = do_something();
if( result != 0 )
{
debug_printf("Error while doing something, code %x (%s)\n", result,
result == 7 ? "ERROR_YES" :
result == 8 ? "ERROR_NO" :
result == 9 ? "ERROR_FILE_NOT_FOUND" :
"Unknown");
}
I almost never use the ternary operator, because whenever I do use it, it always makes me think a lot more than I have to later when I try to maintain it.
I like to avoid verbosity, but when it makes the code a lot easier to pick up, I will go for the verbosity.
Consider:
String name = firstName;
if (middleName != null) {
name += " " + middleName;
}
name += " " + lastName;
Now, that is a bit verbose, but I find it a lot more readable than:
String name = firstName + (middleName == null ? "" : " " + middleName)
+ " " + lastName;
Or:
String name = firstName;
name += (middleName == null ? "" : " " + middleName);
name += " " + lastName;
It just seems to compress too much information into too little space, without making it clear what's going on. Every time I saw the ternary operator used, I have always found an alternative that seemed much easier to read... then again, that is an extremely subjective opinion, so if you and your colleagues find ternary very readable, go for it.
I like them. I don't know why, but I feel very cool when I use the ternary expression.
I treat ternary operators a lot like GOTO. They have their place, but they are something which you should usually avoid to make the code easier to understand.
Well, the syntax for it is horrid. I find functional ifs very useful, and they often makes code more readable.
I would suggest making a macro to make it more readable, but I'm sure someone can come up with a horrible edge case (as there always is with C++).
I typically use it in things like this:
before:
if(isheader)
drawtext(x, y, WHITE, string);
else
drawtext(x, y, BLUE, string);
after:
drawtext(x, y, isheader == true ? WHITE : BLUE, string);
As others have pointed out they are nice for short simple conditions. I especially like them for defaults (kind of like the || and or usage in JavaScript and Python), e.g.
int repCount = pRepCountIn ? *pRepCountIn : defaultRepCount;
Another common use is to initialize a reference in C++. Since references have to be declared and initialized in the same statement you can't use an if statement.
SomeType& ref = pInput ? *pInput : somethingElse;
I like Groovy's special case of the ternary operator, called the Elvis operator: ?:
expr ?: default
This code evaluates to expr if it's not null, and default if it is. Technically it's not really a ternary operator, but it's definitely related to it and saves a lot of time/typing.
I recently saw a variation on ternary operators (well, sort of) that make the standard "() ? :" variant seem to be a paragon of clarity:
var Result = [CaseIfFalse, CaseIfTrue][(boolean expression)]
or, to give a more tangible example:
var Name = ['Jane', 'John'][Gender == 'm'];
Mind you, this is JavaScript, so things like that might not be possible in other languages (thankfully).
Only when:
$var = (simple > test ? simple_result_1 : simple_result_2);
KISS.
For simple if cases, I like to use it. Actually it's much easier to read/code for instance as parameters for functions or things like that. Also to avoid the new line I like to keep with all my if/else.
Nesting it would be a big no-no in my book.
So, resuming, for a single if/else I'll use the ternary operator. For other cases, a regular if/else if/else (or switch).
For simple tasks, like assigning a different value depending on a condition, they're great. I wouldn't use them when there are longer expressions depending on the condition though.
If you and your workmates understand what they do and they aren't created in massive groups I think they make the code less complex and easier to read because there is simply less code.
The only time I think ternary operators make code harder to understand is when you have more than three or foyr in one line. Most people don't remember that they are right based precedence and when you have a stack of them it makes reading the code a nightmare.
As so many answers have said, it depends. I find that if the ternary comparison is not visible in a quick scan down the code, then it should not be used.
As a side issue, I might also note that its very existence is actually a bit of an anomaly due to the fact that in C, comparison testing is a statement. In Icon, the if construct (like most of Icon) is actually an expression. So you can do things like:
x[if y > 5 then 5 else y] := "Y"
... which I find much more readable than a ternary comparison operator. :-)
There was a discussion recently about the possibility of adding the ?: operator to Icon, but several people correctly pointed out that there was absolutely no need because of the way if works.
Which means that if you could do that in C (or any of the other languages that have the ternary operator), then you wouldn't, in fact, need the ternary operator at all.