HReadSeekFirst (Function)

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Generic search/Exact-match search
Search performed on a key item or on a non-key item
Performing a search on a composite key
Search on an array item
Search and filter
Looping through records that match a condition
Exact-match search in Access
Locks
Optimizing iterations
Native XML Connector

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HReadSeekFirst (Function)

In french: HLitRecherchePremier

Not available with this kind of connection

Positions on the first record of a data file whose value for a specific item is strictly equal to a sought value (exact-match search). The record is read and the corresponding HFSQL variables are updated.

In most cases, HReadSeekFirst sets the position in the data file to loop through the records that match a condition. HReadNext is used to read the next record corresponding to the condition.

Several cases may occur after the call to HReadSeekFirst:

a record corresponding to the condition was found, locked (if necessary) and loaded in memory: HFound returns True.
if no record corresponding to the condition exists but if a record with a greater value is found: the record is read, HOut returns False and HFound returns False.
the data file is empty: no read operation is performed, HOut returns True and HFound returns False.
the function tries to lock a record that is already locked in read-only: no read operation is performed, HErrorLock returns True and HOut returns True.
Remark: An automatic management of lock errors and modification conflicts is performed by default (except in the code of stored procedures). This assisted management of errors can be customized or disabled at any time by HOnError.

HReadSeekFirst can be used with the data files, HFSQL views or queries.

Remark: the search can be canceled by HCancelSeek.

Special case: management of locks:

In PHP, the management of locks is not available.
In Java, during an access by JDBC, the management of locks is not available for the databases accessed by JDBC.

Example

See additional examples

// Find the first record for which 
// the CUSTOMER name is MOORE
HReadSeekFirst(CUSTOMER, NAME, "MOORE")
IF HFound(CUSTOMER) = False THEN
	Error("Customer not found")
	RETURN
ELSE
	// Continue the process on the customer named MOORE
END

Syntax

<Result> = HReadSeekFirst(<Data file> , <Item> , <Search value> [, <Options>])

<Result>: Boolean

True if the record was found (corresponds to the value returned by HFound).
False if a problem occurs. This problem can be caused by:
a positioning problem (empty data file, etc.): HFound returns False and HError returns 0.
an error: HError returns an integer other than 0. HErrorInfo returns more details.

<Data file>: Character string

Name of the HFSQL data file, view or query used.

<Item>: Character string

Name of item on which the search will be performed. This item can be a search key or not.
The search can only be performed on a key item.

<Search value>: Type corresponding to the value

Value of the sought item.

<Options>: Optional constant (or combination of constants)

Used to configure:
the lock mode applied to the sought record
the type of search performed.
hForwardOnly
This constant can only be used with Native Connectors.
Optimizes simple iterations that do not use the following features:
Reading the previous record.
Modifying a record.
Saving position.
If one of these features is used, the result may differ from the expected one.
For example, this constant can be used to populate a Table control programmatically.
hGeneric Generic search (see the Notes)
An exact-match search is performed by default (constant not specified).
hKeepFilter The filter set by HFilter will be taken into account, even if the search key is not optimized for the filter. Reminder: HFilter returns the search key optimized for the filter.
Caution: in this case, performance issues may arise for large files.
This variable cannot be used.
By default, the iteration performed after HReadSeekFirst ignores the filter.
hLimitParsing The iteration will stop when the last search value is found or if no value matches the search.
The current record will correspond to this last record found.
HFound will return False and HOut will return True.
This constant is used to optimize search speed in Client/Server mode and on external databases (accessed via OLE DB or via Native Connectors).
hLockNo No lock: the record can be read or modified by another application during the reading.
This constant is not available.
Access by JDBC: This constant is not available.
hLockReadWrite Lock in read/write mode: the record currently read cannot be read or modified by another application.
The lock mode is ignored if a query is used.
Lock in write-only. Operating mode equivalent to the hLockWrite constant.
This constant is not available.
Access by JDBC: This constant is not available.
hLockWrite Lock in write mode: the record currently read can be read by another application but it cannot be modified.
The lock mode is ignored if a query is used.
This constant is not available.
Access by JDBC: This constant is not available.
hNoRefresh
HReadSeekFirst does not refresh the content of the table or query. If possible, the query is not re-run. All the saved positions are stored.
The management of locks is not available.
Access by JDBC: The management of locks is not available for databases accessed by JDBC.
The lock options will have no effect if the locks are not supported by the OLE DB provider or by the Native Connector.
The lock mode specified by HReadSeekFirst will remain effective during the calls to HReadPrevious and HReadNext.
To change the lock mode, use:
HReadSeekFirst or HReadSeekLast,
HReadLast or HReadFirst.

Remarks

Generic search/Exact-match search

Generic search (mainly on the Character String items): Finds all records starting with the specified value.
For example: When a generic search is performed on "Smith" (for the NAME item), all records whose Name item starts with "Smith" will match the search. Therefore, the record that contains "Smither" will match the search (HFound returns True).
Remark: For backward compatibility with WINDEV 5.5, the generic search of an empty string ("") is equivalent to the use of HReadFirst.
Exact-match search: Finds all records that exactly correspond to the specified value.
For example: When an exact-match search is performed on "Smith" (for the NAME item), HFound returns True for the records whose item exactly matches "Smith".
Examples of searches on the CUSTOMER data file sorted by name:


Search value	Options	HReadSeekFirst sets the position on the record.	HFound returns	HOut returns	Explanations
Davon		1	True	False	Davon exists. The end of data file was not reached yet.
Davo		1	False	False	Davo does not exist. Position on the first greater value (Davon). The end of data file was not reached yet.
Moor	hGeneric	8	True	False	Moor does not exist but the search is a generic search and Moore is found (among others). The end of data file was not reached yet.
Moor		The record was not found (no move, the current record does not change).	False	False	Moor does not exist. The end of data file was not reached yet.
Norbert		The record was not found (no move, the current record does not change).	False	True	Norbert does not exist. Position on the first greater value (this value does not exist): the end of data file has been reached.

For more details, see the Hyper File 5.5 and 7: How to manage the space characters in the searches? table

Search performed on a key item or on a non-key item

The search can be performed on a key item or a non-key item.

If the search is performed on a key item:

the search is fast and the result is sorted.
if the iteration operation is continued by HReadNext, the next records will correspond to the values greater than or equal to the search value. In this case, HOut must be checked after each read operation to find out whether the end of the data file has been reached.

If the search is performed on a non-key item:

the selected item will appear in red in the code editor and a warning will appear in the "Compilation errors" pane.
Remark: The automatic completion proposes the key items only.
if the iteration is continued by HReadNext, the next records will correspond to the values equal to the search value.

Performing a search on a composite key

Several methods can be used to perform a search on a composite key:

Using a list of values.
Using the HBuildKeyValue function.

1. Using a list of values

The following syntax is used to perform a search on a composite key:

HReadSeekFirst(<File Name>, <Name of Composite Key>,
[<Search value of first element of composite key>,
<Search value of first element of composite key>, ...])

Example:

// Find the record
HReadSeekFirst(CUSTOMER, LASTNAME_FIRSTNAME, ["MOORE", "Vince"])

2. Using HBuildKeyValue

Example:

bufSoughtVal is Buffer = HBuildKeyValue(CUSTOMER, LASTNAME_FIRSTNAME, sLastName, sFirstName)
HSeekFirst(CUSTOMER, LASTNAME_FIRSTNAME, bufSoughtVal)
WHILE HFound(CUSTOMER)
	HDelete(CUSTOMER)
	HNext(CUSTOMER, LASTNAME_FIRSTNAME)
END

To perform generic searches on a composite key, all the components of composite key must be Text components. Otherwise, an exact-match search is performed.

Search on an array item

The search is performed on the first array element (element with index 1). To perform a search on the other array elements, use the filters or queries.

Search and filter

If a filter is enabled (HFilter), the filter is taken into account by the search only if the key used is identical.

To apply this filter in the rest of the iteration (even if the search key is not optimized for the filter), use the hKeepFilter constant.

If a filter is enabled, the filter is ignored by the search.

Looping through records that match a condition

In most cases, HReadSeekFirst sets the position in the data file to loop through the records that match a condition. HReadNext and HReadPrevious are used to read the next and previous matching records.

To ignore the search while going to the next or previous record, use one of the following functions:

Exact-match search in Access

To perform an exact-match search on an ACCESS database, it is recommended to use NoSpace if there are space characters at the end of the sought value.

Locks

The locks apply only when a record was found.

By default (no lock mode specified in <Options>), the record is not locked.

If a lock is requested (hLockWrite or hLockReadWrite constant), the record will be read only if this record is not already locked.

Reminder: A locked record can be unlocked by HUnlockRecNum.

Lock options will have no effect if the OLE DB provider or Native Connector (also called Native Access) does not support locks.

Optimizing iterations

To optimize the first iterations on a data file, use HOptimize on this data file.

Native XML Connector

The behavior of HReadSeekFirst depends on HActivateAutoFilter and HDeactivateAutoFilter.

HActivateAutoFilter is enabled by default.

Therefore, to read the content of XML file, read the content of main file (the parent) then read the content of linked files (the children).

By default, when reading a file, a filter is automatically applied to the linked files in order to only read the records corresponding to the main file.

For example:

The email of this person can be retrieved when looping through the Person file.

To do so, simply set the position on the "Person" file and apply HReadSeekFirst to the "Email" file.

In this case, the record read in the "Email" file will correspond to the first email associated with the current record in the "Person" file.

If this mechanism is disabled (HDeactivateAutoFilter), the record read in the "Email" file will correspond to the first record found in the Email file (and not to the child of the record read in the "Person" file).

Component: wd300hf.dll