while not TBLOrder.Eof do
begin
TBLOrder.Locate('OrderID', Total, []);
TBLOrder.Delete;
end;
This just deletes every single row in my Access Database, which is really annoying.
I'm trying to get the program to delete the selected row (which is Total).
From what I understand, It should locate the selected row, which is equal to Total. e.g. If Total = 3 it should find the row where OrderID = 3 and then delete that row.
Any help is appreciated.
Try this instead (Max's routine requires you to loop through the entire dataset, which is fine unless it's got many rows in it):
while (TblOrder.Locate('OrderID', Total, [])) do
TblOrder.Delete;
TDataSet.Locate returns a Boolean; if it's True, the found record is made the active record and you can then delete it. If it returns False (meaning the record is not found), the call to Delete is never made.
BTW, the problem with your original code is that you test for Eof, but never check to see if the Locate finds the record; you just delete whatever record you're on, and then test for Eof again. If you're not at Eof, you call Locate, ignore whether or not it found the record, and delete whatever row you're on. This then repeats over and over again until there are no more records, at which point Eof returns true and you break the loop.
If there is just one row that contains an ORDERID equal to 3, you don't need the WHILE loop.
If you expect more than one row with an ORDERID equal to 3, do this:
TBLOrder.first; // you could also do the locate here if it's a big table
while not TBLOrder.Eof do
begin
if TBLOrder.FieldByName('OrderID').AsInteger = 3 then
TBLOrder.delete
else
TBLOrder.next;
end;
Otherwise, you could also use SQL.
Related
I solved my problem but I need to know why this problem raised to me ?!
I write a project that load file to listBox then delete the strings one by one,
but when I delete listBox strings this exception raised to me!
list index out of bounds (5) !
I type this for loop to read list box and delete strings:
for i := 0 to ListBox3.Count -1 do
begin
ShowMessage(ListBox3.Items[i]);
ListBox3.items.Delete(i);
end;
and my problem solved by do a little change in for-loop statement
for i := ListBox3.Items.Count - 1 downto 0 do
begin
ShowMessage(ListBox3.Items[i]);
ListBox3.items.Delete(i);
end;
Why the first statement raised an exception, and the second one work fine ?
By deleting items moving forward, you're cutting the branch off that you're standing on. :-) The upper bounds of the loop is only evaluated once, before the loop begins, and if you delete items there are now fewer in the list than there were when the bound was calculated.
Loop limit is evaluated (for example, List.Count - 1 = 5). Valid indexes into it are [0..4]
The loop starts, and you retrieve List[0] and delete it. List Count = 4,
bounds is still 5
The index is incremented, you retrieve and delete List[1]. List Count = 3, bounds is still 5
The index is incremented, you retrieve and delete List[2]. List Count = 2, bounds is still 5.
The index is incremented, you retrieve List[3] - Oops! There are only 2 items in the list, now at indexes [0..1] - List index out of bounds(3).
By iterating backwards, even though the bounds is still only calculated at the beginning, you're removing the items from the end and decrementing the count at the same time.
Bounds is 5, and you retrieve List[4] and delete it. Count is now 4, bounds is still 5
Index is decremented, and you retrieve List[3] and delete it. Count is now 3, bounds is still 5
Index is decremented, and you retrieve List[2] and delete it. Count is now 2, bounds is still 5.
Index is decremented, and you retrieve and delete List[1]. Count is now 1, bounds is still 5.
Index is decremented, and you retrieve and delete List[0]. List is now empty, but we've reached the terminating condition of the loop (downto 0) and the loop exits safely.
Each time you delete an item from the list, the list contains one less item. However, the for statement copies the list count at the beginning, and is not updated upon each iteration. Therefore, by the time you get halfway through the list, the counter i becomes larger than the current (new) list count, even though the list no longer contains the original number of items.
As an alternative, you could also do a loop like this:
while ListBox3.Items.Count > 0 do begin
ShowMessage(ListBox3.Items[0]);
ListBox3.items.Delete(0);
end;
There may be situation when you DO NOT delete certain items. Then the generic approach would be
i := 0;
while i < ListBox3.Items.Count do
begin
ShowMessage(ListBox3.Items[i]);
if <wantToDelete> then // some condition there
ListBox3.Items.Delete(i)
else
Inc(i);
end;
I'm relatively new to delphi and I would like to know how to delete an entry in a dbGrid without using a dbNavigator but a button. The number of the entry that should be deleted must be entered in a spinedit, not be clicked on in the dbGrid. Thanks for any help.
First it's nice to position in the first record of the DataSet, then it will delete from the first to the N record.
DBGrid1.DataSource.DataSet.First;
Now you create the Loop (Don't forget to create the variable {var I : integer})
For I:=0 to SpinEdit1.Value-1 Do
Before start deleting records, you will need to verify if there is any record on DataSet.
You can do something like this:
if DBGrid1.DataSource.DataSet.RecordCount > 0 then
And finally you can delete the record
DBGrid1.DataSource.DataSet.Delete;
The final code would be like this:
DBGrid1.DataSource.DataSet.First; //Set on the first Record of the DataSet
For I:=0 to SpinEdit1.Value-1 Do //Do loop
if DBGrid1.DataSource.DataSet.RecordCount > 0 then //Check if have records
DBGrid1.DataSource.DataSet.Delete; //Delete
I have a table called Artist which currently contains four records and a TSQLQuery that contains the following statement:
SELECT name
FROM Artist
The table Artist also contains the following records:
id name
1 Artist 1
2 Artist 2
3 Artist 3
4 Artist 4
Current method of record retrieval:
Query1.Open;
for i := 0 to qArtist.FieldCount -1 do
with cbArtist.ListBox.ListItems[i] do
Text := qArtist.Fields[i].AsString;
Previous method of record retrieval:
Data bind Query1 to ComboBox1.
With the "previous" method, ComboBox1 would display all the expected records from the Artist table. However, when I try to use "current" method Query1 is only selecting the very first record from the Artist table despite there being three other existing values. I have tried the "current" method across other queries and they also returned only the first value of the table.
The reason I am going for this new approach is because I feel that I am very limited in what I can do if I continue to the "previous" / data bind method, but that is besides the point.
So how can I fix this problem? i.e. the problem of the query only selecting the very first record from the table.
You must use the Eof and Next methods to iterate over the records.
Query1.Open;
while not Query1.eof do
begin
cbArtist.Items.Add(Query1.FieldByName('Artist').AsString);
Query1.Next;
end;
You code show an interaction over fields, if you need iterate all record then you must use a code like:
Query1.Open;
Query1.first;
while not Query1.eof do
begin
with cbArtist.ListBox.ListItems[i] do
Text := qArtist.Fields[1].AsString; //put here field you want to bind on ListBox.
Query1.next;
end;
I don't think you are navigating your query's dataset correctly. The FieldCount and Fields[i] access the field metadata (columns going across), not the rows. I beleive in Delphi you use While not Eof begin... end.
Navigating Datasets
I would consider altering the data binding fields to suit your needs. Delphi's Databinding is very powerful. Manually iterating the dataset just to populate a control will just be extra code where bugs can hide. Utilize the built-in capabilities of the tools and it will be easier to understand and maintain.
Delphi does not seem to like multi-field indexes.
How do I physically sort a a table so that I wind up with a table that has the rows in the desired order?
Example:
mytable.dbf
Field Field-Name Field-Type Size
0 Payer Character 35
1 Payee Character 35
2 PayDate Date
3 Amount Currency
I need to produce a table sorted alphabetically by "Payee"+"Payer"
When I tried using an index of "Payee+Payer", I got an error:
"Field Index out of range"
The index field names need to be separated by semicolons, not plus symbols. Try that and it should work.
Ok, let's try to put some order.
First, isn't advisable to physically sort a table. In fact the most RDBMS even don't provide you this feature. Usually, one, in order to not force a full table scan (it is called sometimes natural scan) creates indexes on the table fields on which he thinks that the table will be sorted / searched.
As you see, the first step in order to sort a table is usually index creation. This is a separate step, it is done once, usually at, let's say, "design time". After this, the DB engine will take care to automatically update the indexes.
The index creation is done by you (the developer) using (usually) not Delphi (or any other development tool) but the admin tool of your RDBMS (the same tool which you used when you created your table).
If your 'DB engine' is, in fact, a Delphi memory dataset (TClientDataSet) then you will go to IndexDefs property, open it, add a new index and set the properties there accordingly. The interesting property in our discussion is Fields. Set it to Payee;Payer. Set also the Name to eg. "idxPayee". If you use other TDataSet descendant, consult the docs of your DB engine or ask another question here on SO.com providing the details.
Now, to use the index. (IOW, to sort the table, as you say). In your program (either at design time either at run time) set in your 'Table' the IndexName to "idxPayee" or any other valid name you gave or set IndexFieldNames to Payee;Payer.
Note once again that the above is an example based on TClientDataSet. What you must retain from the above (if you don't use it) is that you must have an already created index in order to use it.
Also, to answer at your question, yes, there are some 'table' types (TDataSet descendants in Delphi terminology) which support sorting, either via a Sort method (or the like) either via a SortFields property.
But nowadays usually when one works with a SQL backend, the preferred solution is to create the indexes using the corresponding admin tool and then issue (using Delphi) an SELECT * FROM myTable ORDER BY Field1.
HTH
If you're still using BDE you can use the BDE API to physically sort the DBF table:
uses
DbiProcs, DbiTypes, DBIErrs;
procedure SortTable(Table: TTable; const FieldNums: array of Word; CaseInsensitive: Boolean = False; Descending: Boolean = False);
var
DBHandle: hDBIDb;
RecordCount: Integer;
Order: SORTOrder;
begin
if Length(FieldNums) = 0 then
Exit;
Table.Open;
RecordCount := Table.RecordCount;
if RecordCount = 0 then
Exit;
DBHandle := Table.DBHandle;
Table.Close;
if Descending then
Order := sortDESCEND
else
Order := sortASCEND;
Check(DbiSortTable(DBHandle, PAnsiChar(Table.TableName), nil, nil, nil, nil, nil,
Length(FieldNums), #FieldNums[0], #CaseInsensitive, #Order, nil, False, nil, RecordCount));
end;
for example, in your case:
SortTable(Table1, [2, 1]); // sort by Payee, Payer
Cannot check, but try IndexFieldNames = "Payee, Payer".
Sure indexes by these 2 fields should exist.
You can create an index on your table using the TTable.AddIndex method in one call. That will sort your data when you read it, that is if you use the new index by setting the TTable.IndexName property to the new index. Here's an example:
xTable.AddIndex('NewIndex','Field1;Field2',[ixCaseInsensitive]);
xTable.IndexName := 'NewIndex';
// Read the table from top to bottom
xTable.First;
while not xTable.EOF do begin
..
xTable.Next;
end;
What would be the best way, in delphi, to create and store data which will often be searched on and modified?
Basically, I would like to write a function that searches an existing database for telephone numbers and keeps track of how many times each telephone number has been used, the first date used, and the latest date used. The database that is being searched is basically a log of orders placed, containing the telephone number that was used to place the order. It's not an SQL database or anything that can easily be queried for such things (it's an old btrieve database), so I need to create a way of gaining this information (to eventually output to a text file).
I am thinking of creating a record containing the phone number, the two dates, and the number of times used, and then adding a record to a dynamic array for each telephone number. I would then search the array, entry by entry, for each record in the database, to see if the phone number for the current record is already in the array. Then updating or creating a record as necessary.
This seems like it would work, but as there are tens of thousands of entries in the database, it may not be the best way, and a rather slow and inefficient way of doing things. Is there a better way, given the limited actions I can perform on the database?
Someone suggested that rather than using an array, use a MySQL table to keep track of the numbers, and then query each number for every database record. This seems like even more overhead though!
Thanks a lot for your time.
I would register the aggregates in a totally disconnected TClientDataset(cds), and updating the values as you get them from the looping. If the Btrieve could be sorted by telephone number, much better. Then use the data on the cds to generate the report.
(If you go this way, I suggest get Midas SpeedFix from the Andreas Hausladen' blog, along with the other finest stuff you can find there).
Ok, here is a double pass old-school method that works well and should scale well (I used this approach against a multi-million record database once, it took time but gave accurate results).
Download and install Turbo Power
SysTools -- the sort engine
works very well for this process.
create a sort, with a fixed record
of phone number, you will be using
this to sort.
Loop thru your records, at each order, add the
phone number to the sort.
Once the first iteration is done, start
popping the phone numbers from the
sort, increment a counter if the
phone number is the same as the last
one read, otherwise report the
number and clear your counter.
This process can also be done with any SQL Database, but my experience has been that the sort method is faster than managing a temporary table and generates the same results.
EDIT -- You stated that this is a BTrieve database, why not just create a key on the phone number, sort on that key, then apply step 4 over this table (next instead of pop). Either way you will need to touch every record in your database to get counts, the index/sort just makes your decision process easier.
For example, lets say that you have two tables, one the customer table is where the results will be stored, and the other the orders table. Sort both by the same phone number. Then start a cursor at the top of both lists and then apply the following psuedocode:
Count := 0;
While (CustomerTable <> eof) and (OrderTable <> eof) do
begin
comp = comparetext( customer.phone, order.phone );
while (comp = 0) and (not orderTable eof) do
begin
inc( Count );
order.next;
comp = comparetext( customer.phone, order.phone );
end;
if comp < 0 then
begin
Customer.TotalCount = count;
save customer;
count := 0;
Customer.next;
end
else if (Comp > 0) and (not OrderTable EOF) then
begin
Order.Next; // order no customer
end;
end;
// handle case where end of orders reached
if (OrdersTable EOF) and (not CustomersTable EOF) then
begin
Customer.TotalCount = count;
save customer;
end;
This code has the benefit of walking both lists once. There are no lookups necessary since both lists are sorted the same, they can be walked top to bottom taking action only when necessary. The only requirement is that both lists have something in common (in this example phone number) and both lists can be sorted.
I did not handle the case where there is an order and no customer. My assumption was that orders do not exist without customers and would be skipped for counting.
Sorry, couldn't edit my post (wasn't registered at the time). The data will be thrown away once all the records in the database have been iterated through. The function won't be called often. It's basically going to be used as a way of determining how often people have ordered over a period of time from records we already have, so really it's just needed to produce a one off list.
The data will be persistent for the duration of the creation of the list. That is, all telephone numbers will need to be present to be searched on until the very last database record is read.
If you were going to keep it in memory and don't want anything fancy, you'd be better off using a TStringList so you can use the Find function. Find uses Hoare's selection or Quick-select, an O(n) locator. For instance, define a type:
type
TPhoneData = class
private
fPhone:string;
fFirstCalledDate:TDateTime;
fLastCalledDate:TDateTime;
fCallCount:integer;
public
constructor Create(phone:string; firstDate, lastDate:TDateTime);
procedure updateCallData(date:TDateTime);
property phoneNumber:string read fPhone write fPhone;
property firstCalledDate:TDateTime read fFirstCalledDate write fFirstCalledDate;
property lastCalledDate:TDateTime read fLastCalledDate write fLastCalledDate;
property callCount:integer read fCallCount write fCallCount;
end;
{ TPhoneData }
constructor TPhoneData.Create(phone: string; firstDate, lastDate: TDateTime);
begin
fCallCount:=1;
fFirstCalledDate:=firstDate;
fLastCalledDate:=lastDate;
fPhone:=phone;
end;
procedure TPhoneData.updateCallData(date: TDateTime);
begin
inc(fCallCount);
if fFirstCalledDate<date then fFirstCalledDate:=date;
if date>fLastCalledDate then fLastCalledDate:=date;
end;
and then fill it, report on it:
procedure TForm1.btnSortExampleClick(Sender: TObject);
const phoneSeed:array[0..9] of string = ('111-111-1111','222-222-2222','333-333-3333','444-444-4444','555-555-5555','666-666-6666','777-777-7777','888-888-8888','999-999-9999','000-000-0000');
var TSL:TStringList;
TPD:TPhoneData;
i,index:integer;
phone:string;
begin
randseed;
TSL:=TStringList.Create;
TSL.Sorted:=true;
for i := 0 to 100 do
begin
phone:=phoneSeed[random(9)];
if TSL.Find(phone, index) then
TPhoneData(TSL.Objects[index]).updateCallData(now-random(100))
else
TSL.AddObject(phone,TPhoneData.Create(phone,now,now));
end;
for i := 0 to 9 do
begin
if TSL.Find(phoneSeed[i], index) then
begin
TPD:=TPhoneData(TSL.Objects[index]);
ShowMessage(Format('Phone # %s, first called %s, last called %s, num calls %d', [TPD.PhoneNumber, FormatDateTime('mm-dd-yyyy',TPD.firstCalledDate), FormatDateTime('mm-dd-yyyy',TPD.lastCalledDate), TPD.callCount]));
end;
end;
end;
Instead of a TStringList I would recommend using DeCAL's (on sf.net) DMap to store the items in memory. You could specify the phone is the key and store a Record/Class structure containing the rest of the record.
So your Record class will be:
TPhoneData = class
number: string;
access_count: integer;
added: TDateTime.
...
end;
Then in code:
procedure TSomeClass.RegisterPhone(number, phoneData);
begin
//FStore created in Constructor as FStore := DMap.Create;
FStore.putPair([number, phoneData])
end;
...
procedure TSoemClass.GetPhoneAndIncrement(number);
var
Iter: DIterator;
lPhoneData: TPhoneData;
begin
Iter := FStore.locate([number]);
if atEnd(Iter) then
raise Exception.CreateFmt('Number %s not found',[number])
else
begin
lPhoneData := GetObject(Iter) as TPhoneData;
lPhoneData.access_count = lPhoneData.access_count + 1;
//no need to save back to FStore as it holds a pointer to lPhoneData
end;
end;
DMap implements a red/black tree so the data structure sorts the keys for you for free. You can also use a DHashMap for the same affect and (arguably) increased speed.
DeCAL is one of my favourite data structure libraries and would recommend anybody doing in-memory storage operations to have a look.
Hope that helps