According to the documentation, pg_attribute.attgenerated is typed as char and has a value of "a zero byte" if the column is not generated, and there is at least one other possible value, with potentially more in the future.
I want to query for all non-generated columns. Since I would prefer to not be tripped up by additions in future versions, the query predicate needs to be WHERE attgenerated = ZERO BYTE rather than an inequality, but I have no idea how to represent that value correctly in SQL.
What's the correct way to write this? In most programming languages you'd say '\0', and you can use escape sequences by prepending an e to the string literal, but if I say e'\0' it errors out with "invalid byte sequence for encoding "UTF8": 0x00". So I'm not quite sure what the right way to do this is.
It's simply an empty string:
WHERE attgenerated = ''
Related
I'm trying to parse string to decimal in vb.net which could contain dot or comma, for ex. '5000.00', '5000,00' (actually for Belgium and Niederlands).
Code for decimal with dot:
Decimal.TryParse(amountStr, amountVal)
Code for decimal with comma:
Decimal.TryParse(amountStr, NumberStyles.AllowDecimalPoint, CultureInfo.CreateSpecificCulture("nl-BE"), amountVal)
Is it possible to combine these into one code without replacing comma in string?
Is it possible to combine these into one code without replacing comma in string?
String-replacement is the "usual" solution to this problem. A slightly more elegant alternative would be to check if the string contains a . or a , and then provide the "correct" CultureInfo to TryParse:
Dim isBelgianFormat As Boolean = amountStr.Contains(",")
Dim ci As CultureInfo = If(isBelgianFormat,
CultureInfo.GetCultureInfo("nl-BE"),
CultureInfo.InvariantCulture)
...Decimal.TryParse(amountStr, NumberStyles.AllowDecimalPoint, ci, amountVal)...
This will also allow you to "fine-tune" your guessing logic by replacing the first line with a more complicated algorithm. (For example, this "simple" solution will fail if your users use thousands separators, i.e., if you want to correctly "guess" the value of both 500.000,00 and 500,000.00.)
That having been said, you can make your code more complicated to cover these cases as well, but how do you want to treat, for example, 500.000 or 500,000? Is it half a million or 500?
Thus, I urge you to reconsider your requirements. Especially when parsing monetary values, failing with a helpful error message is often preferable to guessing what the user might have meant.
I am using VBA in Ms Access environment, to handle long string (memo field storing HTML originally).
After positioning by Instr(), I put the position into Mid(vStr,vStartPos,vEndPos-vStartPos+1) to extract the string, but the output doesn't match. I have already carefully checked this in immediate windows, as well as NotePad++. What I can say is Instr() and NotePad++ have given the same counting result, while Mid() is different. Mid()'s result are former than Instr()'s in some cases, and latter in other cases. I don't know the reason, and can just believe Mid() use different mechanism or have defeative (surprised!) in handling long string mixed with single-byte and bi-byte chars (but this is common in the world, and meet no problem before), and possibly some special characters.
I believe I need to custom-make a Mid() function. Any idea how to do it effectively and efficiently?
Thanks all for your reply. After I created a custom Mid() by RegEx and find that the problem has no change, I have found out the silly mistake I made. The Instr() and Mid() have no problem, but the string has been carelessly modified between them. So this case should be closed now.
I need to store an alphanumeric string in an integer column on one of my models.
I have tried:
#result.each do |i|
hex_id = []
i["id"].split(//).each{|c| hex_id.push(c.hex)}
hex_id = hex_id.join
...
Model.create(:origin_id => hex_id)
...
end
When I run this in the console using puts hex_id in place of the create line, it returns the correct values, however the above code results in the origin_id being set to "2147483647" for every instance. An example string input is "t6gnk3pp86gg4sboh5oin5vr40" so that doesn't make any sense to me.
Can anyone tell me what is going wrong here or suggest a better way to store a string like the aforementioned example as a unique integer?
Thanks.
Answering by request form OP
It seems that the hex_id.join operation does not concatenate strings in this case but instead sums or performs binary complement of the hex values. The issue could also be that hex_id is an array of hex-es rather than a string, or char array. Nevertheless, what seems to happen is reaching the maximum positive value for the integer type 2147483647. Still, I was unable to find any documented effects on array.join applied on a hex array, it appears it is not concatenation of the elements.
On the other hand, the desired result 060003008600401100500050040 is too large to be recorded as an integer either. A better approach would be to keep it as a string, or use different algorithm for producing a number form the original string. Perhaps aggregating the hex values by an arithmetic operation will do better than join ?
I'd like to create a regular expression such that when I compare the a string against an array of strings, matches are returned with the regex ignoring certain characters.
Here's one example. Consider the following array of names:
{
"Andy O'Brien",
"Bob O'Brian",
"Jim OBrien",
"Larry Oberlin"
}
If a user enters "ob", I'd like the app to apply a regex predicate to the array and all of the names in the above array would match (e.g. the ' is ignored).
I know I can run the match twice, first against each name and second against each name with the ignored chars stripped from the string. I'd rather this by done by a single regex so I don't need two passes.
Is this possible? This is for an iOS app and I'm using NSPredicate.
EDIT: clarification on use
From the initial answers I realized I wasn't clear. The example above is a specific one. I need a general solution where the array of names is a large array with diverse names and the string I am matching against is entered by the user. So I can't hard code the regex like [o]'?[b].
Also, I know how to do case-insensitive searches so don't need the answer to focus on that. Just need a solution to ignore the chars I don't want to match against.
Since you have discarded all the answers showing the ways it can be done, you are left with the answer:
NO, this cannot be done. Regex does not have an option to 'ignore' characters. Your only options are to modify the regex to match them, or to do a pass on your source text to get rid of the characters you want to ignore and then match against that. (Of course, then you may have the problem of correlating your 'cleaned' text with the actual source text.)
If I understand correctly, you want a way to match the characters "ob" 1) regardless of capitalization, and 2) regardless of whether there is an apostrophe in between them. That should be easy enough.
1) Use a case-insensitivity modifier, or use a regexp that specifies that the capital and lowercase version of the letter are both acceptable: [Oo][Bb]
2) Use the ? modifier to indicate that a character may be present either one or zero times. o'?b will match both "o'b" and "ob". If you want to include other characters that may or may not be present, you can group them with the apostrophe. For example, o['-~]?b will match "ob", "o'b", "o-b", and "o~b".
So the complete answer would be [Oo]'?[Bb].
Update: The OP asked for a solution that would cause the given character to be ignored in an arbitrary search string. You can do this by inserting '? after every character of the search string. For example, if you were given the search string oleary, you'd transform it into o'?l'?e'?a'?r'?y'?. Foolproof, though probably not optimal for performance. Note that this would match "o'leary" but also "o'lea'r'y'" if that's a concern.
In this particular case, just throw the set of characters into the middle of the regex as optional. This works specifically because you have only two characters in your match string, otherwise the regex might get a bit verbose. For example, match case-insensitive against:
o[']*b
You can add more characters to that character class in the middle to ignore them. Note that the * matches any number of characters (so O'''Brien will match) - for a single instance, change to ?:
o[']?b
You can make particular characters optional with a question mark, which means that it will match whether they're there or not, e.g:
/o\'?b/
Would match all of the above, add .+ to either side to match all other characters, and a space to denote the start of the surname:
/.+? o\'?b.+/
And use the case-insensitivity modifier to make it match regardless of capitalisation.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
String vs StringBuilder
I just revisited some of the books that I used to pick up VB.NET. I am not sure I've got this in my head, understand how/what StringBuilder is.
What is the guidance for using? Is it best to use it if you are are concatenating 2 strings or 50?
Or when the the total string length is greater than 128 characters?
Or will you see a performance benefit whenever you use it to add strings together?
In which case is it better to use a StringBuilder instance to build a SQL statement than string.format("Select * from x where y = {0}",1)?
It's always struck me that declaring another variable and including a name space is not beneficial for small string concatenations, but I am not sure now.
Sorry, lot of documentation tells you what to use, just not what's best.
I've got an article on this very topic. In summary (copied from the bottom of the page):
Definitely use StringBuilder when you're concatenating in a non-trivial loop - especially if you don't know for sure (at compile time) how many iterations you'll make through the loop. For example, reading a file a character at a time, building up a string as you go using the += operator is potentially performance suicide.
Definitely use the concatenation operator when you can (readably) specify everything which needs to be concatenated in one statement. (If you have an array of things to concatenate, consider calling String.Concat explicitly - or String.Join if you need a delimiter.)
Don't be afraid to break literals up into several concatenated bits - the result will be the same. You can aid readability by breaking a long literal into several lines, for instance, with no harm to performance.
If you need the intermediate results of the concatenation for something other than feeding the next iteration of concatenation, StringBuilder isn't going to help you. For instance, if you build up a full name from a first name and a last name, and then add a third piece of information (the nickname, maybe) to the end, you'll only benefit from using StringBuilder if you don't need the (first name + last name) string for other purpose (as we do in the example which creates a Person object).
If you just have a few concatenations to do, and you really want to do them in separate statements, it doesn't really matter which way you go. Which way is more efficient will depend on the number of concatenations the sizes of string involved, and what order they're concatenated in. If you really believe that piece of code to be a performance bottleneck, profile or benchmark it both ways.
Here is my rule of thumb:
StringBuilder is best used when the exact number of concatenations is unknown at compile time.
Coding Horror has a good article concerning this question, The Sad Tragedy of Micro-Optimization Theater.
Personally I use StringBuilder when I have more than just one or two strings to concatenate. I'm not sure if there's a real performance hit to be gained, but I've always read and been told that doing a regular concatenation with multiple strings creates an extra copy of the string each time you do it, while using StringBuilder keeps one copy until you call the final ToString() method on it.
Someone's figured out experimentally that the critical number is 6. More than 6 concatenations in a row and you should use a StringBuilder. Can't remember where I found this.
However, note that if you just write this in a line:
"qwert" + "yuiop" + "asdf" + "gh" + "jkl;" + "zxcv" + "bnm" + ",."
That gets converted into one function call (I don't know how to write it in VB.net)
String.Concat("qwert", "yuiop", "asdf", "gh", "jkl;", "zxcv", "bnm", ",.");
So if you're doing all concatenations on one line, then don't bother with StringBuilder because String.Concat effectively will do all the concatenations in one go. It's only if you're doing them in a loop or successively concatenating.
My rule - when you're adding to a string in a For or Foreach loop, use the StringBuilder.