When manually generating a JSON object or array, it's often easier to leave a trailing comma on the last item in the object or array. For example, code to output from an array of strings might look like (in a C++ like pseudocode):
s.append("[");
for (i = 0; i < 5; ++i) {
s.appendF("\"%d\",", i);
}
s.append("]");
giving you a string like
[0,1,2,3,4,5,]
Is this allowed?
Unfortunately the JSON specification does not allow a trailing comma. There are a few browsers that will allow it, but generally you need to worry about all browsers.
In general I try turn the problem around, and add the comma before the actual value, so you end up with code that looks like this:
s.append("[");
for (i = 0; i < 5; ++i) {
if (i) s.append(","); // add the comma only if this isn't the first entry
s.appendF("\"%d\"", i);
}
s.append("]");
That extra one line of code in your for loop is hardly expensive...
Another alternative I've used when output a structure to JSON from a dictionary of some form is to always append a comma after each entry (as you are doing above) and then add a dummy entry at the end that has not trailing comma (but that is just lazy ;->).
Doesn't work well with an array unfortunately.
No. The JSON spec, as maintained at http://json.org, does not allow trailing commas. From what I've seen, some parsers may silently allow them when reading a JSON string, while others will throw errors. For interoperability, you shouldn't include it.
The code above could be restructured, either to remove the trailing comma when adding the array terminator or to add the comma before items, skipping that for the first one.
Simple, cheap, easy to read, and always works regardless of the specs.
$delimiter = '';
for .... {
print $delimiter.$whatever
$delimiter = ',';
}
The redundant assignment to $delim is a very small price to pay.
Also works just as well if there is no explicit loop but separate code fragments.
Trailing commas are allowed in JavaScript, but don't work in IE. Douglas Crockford's versionless JSON spec didn't allow them, and because it was versionless this wasn't supposed to change. The ES5 JSON spec allowed them as an extension, but Crockford's RFC 4627 didn't, and ES5 reverted to disallowing them. Firefox followed suit. Internet Explorer is why we can't have nice things.
As it's been already said, JSON spec (based on ECMAScript 3) doesn't allow trailing comma. ES >= 5 allows it, so you can actually use that notation in pure JS. It's been argued about, and some parsers did support it (http://bolinfest.com/essays/json.html, http://whereswalden.com/2010/09/08/spidermonkey-json-change-trailing-commas-no-longer-accepted/), but it's the spec fact (as shown on http://json.org/) that it shouldn't work in JSON. That thing said...
... I'm wondering why no-one pointed out that you can actually split the loop at 0th iteration and use leading comma instead of trailing one to get rid of the comparison code smell and any actual performance overhead in the loop, resulting in a code that's actually shorter, simpler and faster (due to no branching/conditionals in the loop) than other solutions proposed.
E.g. (in a C-style pseudocode similar to OP's proposed code):
s.append("[");
// MAX == 5 here. if it's constant, you can inline it below and get rid of the comparison
if ( MAX > 0 ) {
s.appendF("\"%d\"", 0); // 0-th iteration
for( int i = 1; i < MAX; ++i ) {
s.appendF(",\"%d\"", i); // i-th iteration
}
}
s.append("]");
PHP coders may want to check out implode(). This takes an array joins it up using a string.
From the docs...
$array = array('lastname', 'email', 'phone');
echo implode(",", $array); // lastname,email,phone
Interestingly, both C & C++ (and I think C#, but I'm not sure) specifically allow the trailing comma -- for exactly the reason given: It make programmaticly generating lists much easier. Not sure why JavaScript didn't follow their lead.
Rather than engage in a debating club, I would adhere to the principle of Defensive Programming by combining both simple techniques in order to simplify interfacing with others:
As a developer of an app that receives json data, I'd be relaxed and allow the trailing comma.
When developing an app that writes json, I'd be strict and use one of the clever techniques of the other answers to only add commas between items and avoid the trailing comma.
There are bigger problems to be solved...
Use JSON5. Don't use JSON.
Objects and arrays can have trailing commas
Object keys can be unquoted if they're valid identifiers
Strings can be single-quoted
Strings can be split across multiple lines
Numbers can be hexadecimal (base 16)
Numbers can begin or end with a (leading or trailing) decimal point.
Numbers can include Infinity and -Infinity.
Numbers can begin with an explicit plus (+) sign.
Both inline (single-line) and block (multi-line) comments are allowed.
http://json5.org/
https://github.com/aseemk/json5
No. The "railroad diagrams" in https://json.org are an exact translation of the spec and make it clear a , always comes before a value, never directly before ]:
or }:
There is a possible way to avoid a if-branch in the loop.
s.append("[ "); // there is a space after the left bracket
for (i = 0; i < 5; ++i) {
s.appendF("\"%d\",", i); // always add comma
}
s.back() = ']'; // modify last comma (or the space) to right bracket
According to the Class JSONArray specification:
An extra , (comma) may appear just before the closing bracket.
The null value will be inserted when there is , (comma) elision.
So, as I understand it, it should be allowed to write:
[0,1,2,3,4,5,]
But it could happen that some parsers will return the 7 as item count (like IE8 as Daniel Earwicker pointed out) instead of the expected 6.
Edited:
I found this JSON Validator that validates a JSON string against RFC 4627 (The application/json media type for JavaScript Object Notation) and against the JavaScript language specification. Actually here an array with a trailing comma is considered valid just for JavaScript and not for the RFC 4627 specification.
However, in the RFC 4627 specification is stated that:
2.3. Arrays
An array structure is represented as square brackets surrounding zero
or more values (or elements). Elements are separated by commas.
array = begin-array [ value *( value-separator value ) ] end-array
To me this is again an interpretation problem. If you write that Elements are separated by commas (without stating something about special cases, like the last element), it could be understood in both ways.
P.S. RFC 4627 isn't a standard (as explicitly stated), and is already obsolited by RFC 7159 (which is a proposed standard) RFC 7159
It is not recommended, but you can still do something like this to parse it.
jsonStr = '[0,1,2,3,4,5,]';
let data;
eval('data = ' + jsonStr);
console.log(data)
With Relaxed JSON, you can have trailing commas, or just leave the commas out. They are optional.
There is no reason at all commas need to be present to parse a JSON-like document.
Take a look at the Relaxed JSON spec and you will see how 'noisy' the original JSON spec is. Way too many commas and quotes...
http://www.relaxedjson.org
You can also try out your example using this online RJSON parser and see it get parsed correctly.
http://www.relaxedjson.org/docs/converter.html?source=%5B0%2C1%2C2%2C3%2C4%2C5%2C%5D
As stated it is not allowed. But in JavaScript this is:
var a = Array()
for(let i=1; i<=5; i++) {
a.push(i)
}
var s = "[" + a.join(",") + "]"
(works fine in Firefox, Chrome, Edge, IE11, and without the let in IE9, 8, 7, 5)
From my past experience, I found that different browsers deal with trailing commas in JSON differently.
Both Firefox and Chrome handles it just fine. But IE (All versions) seems to break. I mean really break and stop reading the rest of the script.
Keeping that in mind, and also the fact that it's always nice to write compliant code, I suggest spending the extra effort of making sure that there's no trailing comma.
:)
I keep a current count and compare it to a total count. If the current count is less than the total count, I display the comma.
May not work if you don't have a total count prior to executing the JSON generation.
Then again, if your using PHP 5.2.0 or better, you can just format your response using the JSON API built in.
Since a for-loop is used to iterate over an array, or similar iterable data structure, we can use the length of the array as shown,
awk -v header="FirstName,LastName,DOB" '
BEGIN {
FS = ",";
print("[");
columns = split(header, column_names, ",");
}
{ print(" {");
for (i = 1; i < columns; i++) {
printf(" \"%s\":\"%s\",\n", column_names[i], $(i));
}
printf(" \"%s\":\"%s\"\n", column_names[i], $(i));
print(" }");
}
END { print("]"); } ' datafile.txt
With datafile.txt containing,
Angela,Baker,2010-05-23
Betty,Crockett,1990-12-07
David,Done,2003-10-31
String l = "[" + List<int>.generate(5, (i) => i + 1).join(",") + "]";
Using a trailing comma is not allowed for json. A solution I like, which you could do if you're not writing for an external recipient but for your own project, is to just strip (or replace by whitespace) the trailing comma on the receiving end before feeding it to the json parser. I do this for the trailing comma in the outermost json object. The convenient thing is then if you add an object at the end, you don't have to add a comma to the now second last object. This also makes for cleaner diffs if your config file is in a version control system, since it will only show the lines of the stuff you actually added.
char* str = readFile("myConfig.json");
char* chr = strrchr(str, '}') - 1;
int i = 0;
while( chr[i] == ' ' || chr[i] == '\n' ){
i--;
}
if( chr[i] == ',' ) chr[i] = ' ';
JsonParser parser;
parser.parse(str);
I usually loop over the array and attach a comma after every entry in the string. After the loop I delete the last comma again.
Maybe not the best way, but less expensive than checking every time if it's the last object in the loop I guess.
The problem here is that I do not understand well the difference between statements and blocks in control flow.
Looking the ternary operator I can use it to assign a variable. But this is an operator, so it is like applying a function--isn't it?
> my $variable = True ?? 34 !! 42;
34
since in the raku documentation says:
if documentation
if
To conditionally run a block of code, use an if followed by a
condition. The condition, an expression, will be evaluated immediately
after the statement before the if finishes. The block attached to the
condition will only be evaluated if the condition means True when
coerced to Bool. Unlike some languages the condition does not have to
be parenthesized, instead the { and } around the block are mandatory:
do documentation
do
The simplest way to run a block where it cannot be a stand-alone statement is by writing do before it:
so this should work in both cases:
> my $variable = do {34};
34
> my $variable = if True {34;} else {43;}
===SORRY!===
Word 'if' interpreted as a listop; please use 'do if' to introduce the statement control word
------> my $variable = if⏏ True {34;} else {43;}
Unexpected block in infix position (two terms in a row)
------> my $variable = if True⏏ {34;} else {43;}
as said in the error I need to add the do:
> my $variable = do if True {34;} else {43;}
34
So the if really does not run the block...or what is the real problem here?
TL;DR: The actual difference is between statement and expression, not statement and block. do is a statement prefix that creates expressions.
if actually creates a statement (anything that is run in Raku is), however, what it's not is an expression. do is a statement prefix, and what it does is turn statements into expressions.
However, if is not really a first-class function that you can assign to a variable or handle around. Whenever you find pieces of syntax such as that one (or for, for instance), you need to prefix them with do to "expressionify" them.
say &say.^name;# OUTPUT: «Sub» say &do.^name; # OUTPUT: «===SORRY!=== Error while compiling <tmp>Undeclared routine:...
say &if.^name; # OUTPUT: «===SORRY!=== Error while compiling <tmp>Undeclared routine: if used at line 1»
So if, by itself, does not create a block, it does not create an expression, it simply creates a statement. You need to precede it with do if you want it to actually turn it into a expression. It does run the block that's behind it, however.
Let's go back to the original question, statements and blocks. Blocks are objects, first-class citizens. You can use them, apply them, pass them around.
my &ifs = { if $_ {34} else {43}};
ifs(True).say; # OUTPUT: «34»
Statements are throwaway blocks you simply run. In some cases, they are also expressions: they yield a result which, then, you can assign.
my &ifs = { if $_ {34} else {43}};
my $result = ifs(True).say; # OUTPUT: «34»
say $result; # OUTPUT: «True»
The ifs(True).say statement prints to output, it also produces a result that can be assigned. All three lines are also statements, and as a matter of fact, expressions too.
Some control structures, however, do not create expressions.
Some others do; for creates a expression; while does not.
if is an example of this. They don't produce a result. You use them for the side effects: running a statement (if true) or another (if not). You might turn them into a block, as above, and pass them around. Or you can just precede them with do and make them produce a throwaway result, which you can then use.
So it will very much depend on your actual use case. You can surround the if statement with curly braces and create a block; or you can simply use the result creating an expression. Or you might want to use it just for the side effects, doing nothing.
Is there any reason that this syntax shouldn't work in Qlikview load script??
Let v_myNumber = year(today());
Let v_myString = '2017-08';
If left($(v_myString),4) = text($(v_myNumber)) Then
'do something
Else
'do something else
End If;
I've tried both ways where I convert variable string to number and evaluate against the number variable directly and this way. They won't evaluate to equivalence when they should..
Left function is expecting a string as is getting something else as a parameter. As you are currently doing, the function will be called as Left(2017-08, 4) which is unhandle by QlikView.
If you use Left('$(v_myString)',4), it will evaluate as Left('2017-08', 4) as work as expected. Just adding quotes around the variable it should work.
Although QlikView calls them variables, they should really be seen as "stuff to replaced (at sometimes evaluated) at runtime", which is slightly different from a standard "variable" behaviour.
Dollar sign expansion is a big subject, but in short:
if you are setting a variable - no need for $().
if you are using a variable - you can use $(). depends on its context.
if you are using a variable that needs to be evaluated - you have to use $().
for example in a load script: let var1 = 'if(a=1,1,2)' - here later on the script you will probably want to use this variable as $(var1) so it will be evaluated on the fly...
I hope its a little more clear now. variable can be used in many ways at even can take parameters!
for example:
var2 = $1*$2
and then you can use like this: $(var2(2,3)) which will yield 6
For further exploration of this, I would suggest reading this
I'm trying to simplify my code by using #define statements. This is because it contains a lot of repetitive "chunks" of code that cannot be repeated using the obvious alternative, functions, because in these chunks, variables need to be declared like you'd do in a #define statement, e.g. #define dostuff(name) int name##Variable;.
Code
#define createBody(name,type,xpos,ypos,userData,width,height) b2BodyDef name##BodyDef;\
name##BodyDef.type = type==#"dynamic"?b2_dynamicBody:b2_staticBody;\
name##BodyDef.position.Set(xpos,ypos);\
name##BodyDef.userData = userData;\
name=world->CreateBody(&name##BodyDef);\
b2PolygonShape name##shape;\
name##shape.SetAsBox(width/ptm_ratio/2,height/ptm_ratio/2);
... and applying that in the following:
createBody(block, #"dynamic", winSize.width*5/6/ptm_ratio, winSize.height*1/6/ptm_ratio, ((__bridge void*)blockspr), blockspr.contentSize.width, blockspr.contentSize.height)
// error appears there: ^
Now my point is that everything's working great, no errors, except a single one that's freaking me out:
Expected unqualified-id
which points at the first bracket in ((__bridge ..., as indicated. (That argument gets passed via the userData argument to createBody.)
I know this code is nowhere near simple, but since everything else is working, I believe that an answer must exist.
This is my first question on SO, so if there's anything unclear or insufficient, please let me know!
I'm trying to simplify my code by using #define statements.
This sounds an alarm in my mind.
Break this down into functions. You said you can't. I say you can.
Notice that your macro here:
createBody(name,type,xpos,ypos,userData,width,height);
It has exactly the same syntax as a C function. So you've already created a function, you only declared it as a macro. There's no reason why you couldn't rewrite it as a function (C or Objective-C doesn't matter). You do not need to give each body its own name, instead you could store them in a dictionary (careful though because Box2D takes ownership of the bodies).