Hash vs Table

Consider a table for customers records with two character fields : Customer ID and customer name:

Cust_ID Cust_Name
CC001 Pierce Firth
CC002 Stellan Taylor
CC003 Chris Cherry
CC004 Amanda Baranski

 It’s known all possible and necessary operations on a table: APPEND, DELETE, SEEK and so on; by the way, for SEEK we need an index file also.

Listing this table is quite simple:

USE CUSTOMER
WHILE .NOT. EOF()
   ? CUST_ID, CUST_NAME
   DBSKIP()
ENDDO

 If our table is sufficiently small, we can find a customer record without index and SEEK :

LOCATE FOR CUST_ID = “CC003”
? CUST_ID, CUST_NAME

If we want all our data will stand in memory and we could manage it more simple and quick way, we would use an array ( with some considerations about size of table; if it is too big, this method will be problematic ) :

aCustomer := {}    // Declare / define an empty array
USE CUSTOMER
WHILE .NOT. EOF()
   AADD(aCustomer, { CUST_ID, CUST_NAME } )
   DBSKIP()
ENDDO
Traversing this array is quite simple :

FOR nRecord := 1 TO LEN( aCustomer )

    ? aCustomer[ nRecord, 1 ], aCustomer[ nRecord, 2 ]
NEXT
or :

a1Record := {}

FOR EACH a1Record IN aCustomer
   ? a1Record[ 1 ], a1Record[ 2 ]
NEXT

And locating a specific record too:

nRecord := ASCAN( aCustomer, { | a1Record | a1Record[ 1 ] == “CC003” } )

? aCustomer[ nRecord, 1 ], aCustomer[ nRecord, 2 ]

A lot of array functions are ready to use for maintain this array : ADEL(), AADD(), AINS() etc …

Now, let’s see how we could use a hash for achieve this job :

hCustomer := { => } // Declare / define an empty hash

USE CUSTOMER
WHILE .NOT. EOF()
   hCustomer[ CUST_ID ] := CUST_NAME
   DBSKIP()
ENDDO
Let’s traversing :

h1Record := NIL

FOR EACH h1Record IN hCustomer
   ? h1Record: __ENUMKEY(),h1Record:__ENUMVALUE()
NEXT

Now, we have a bit complicate our job; a few field addition to the table :

No: Field Name Type Width  Dec Decription

1

 CUST_ID

C

 5

0

Id ( Code )

2

 CUST_NAME

C

10

0

Name

3

 CUST_SNAM

C

10

0

Surname

4

 CUST_FDAT

D

 8

0

First date

5

 CUST_ACTV

L

 1

0

Is active ?

6

 CUST_BLNCE

N

11

2

Balance

 While <key> part of an element of a hash may be C / D / N / L type; <xValue> part of hash too may be ANY type of data, exactly same as arrays.

So, we can make fields values other than first ( ID) elements of an array:

hCustomer := { => } // Declare / define an empty hash
USE CUSTOMER
WHILE .NOT. EOF()
   a1Data:= { CUST_NAME, CUST_SNAM, CUST_FDAT, CUST_ACTV, CUST_BLNCE }
   hCustomer[ CUST_ID ] := a1Data
   DBSKIP()
ENDDO
Let’s traversing :

h1Record := NIL

FOR EACH h1Record IN hCustomer
   a1Key  := h1Record:__ENUMKEY()
   a1Data := h1Record:__ENUMVALUE()
   ? a1Key
   AEVAL( a1Data, { | x1 | QQOUT( x1 ) } )
NEXT
*-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._
/*
Hash vs Tables
 
*/
#define NTrim( n ) LTRIM( STR( n ) )
#define cLMarj SPACE( 3 )
PROCEDURE Main()

  SET DATE GERM
  SET CENT ON
  SET COLO TO "W/B"
  SetMode( 40, 120 )
 
  CLS
 
  hCustomers := { => } // Declare / define an empty PRIVATE hash
 
  IF MakUseTable() 
 
     Table2Hash()
 
     * Here the hash hCustomers may be altered in any way
 
     ZAP
 
     Hash2Table()
 
  ELSE
      ? "Couldn't make / USE table"
  ENDIF
 
  ?
  @ MAXROW(), 0
  WAIT "EOF HashVsTable.prg"
 
RETURN // HashVsTable.Main()
*-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.
PROCEDURE Table2Hash()
   hCustomers := { => } 
   WHILE .NOT. EOF()
     hCustomers[ CUST_ID ] := CUST_SNAM
     DBSKIP()
   ENDDO
 
   ListHash( hCustomers, "A hash transferred from a table (single value)" )
 
   hCustomers := { => } // Declare / define an empty hash
   DBGOTOP()
   WHILE .NOT. EOF()
      hCustomers[ CUST_ID ] := { CUST_NAME, CUST_SNAM, CUST_FDAT, CUST_ACTV, CUST_BLNCE }
      DBSKIP()
   ENDDO
 
   ListHash( hCustomers, "A hash transferred from a table (multiple values)" )
 
RETURN // Table2Hash()

*-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.

PROCEDURE Hash2Table()
   LOCAL h1Record,;
         c1Key,;
         a1Record,;
         n1Field
 
   FOR EACH h1Record IN hCustomers
      c1Key := h1Record:__ENUMKEY()
      a1Record := h1Record:__ENUMVALUE()
      DBAPPEND()
      FIELDPUT( 1, c1Key )
      AEVAL( a1Record, { | x1, n1 | FIELDPUT( n1 + 1 , x1 ) } )
   NEXT h1Record
   DBGOTOP()
 
   ?
   ? "Data trasferred from hash to table :"
   ?
   WHILE ! EOF()
      ? STR( RECN(), 5), ''
      FOR n1Field := 1 TO FCOUNT()
         ?? FIELDGET( n1Field ), ''
      NEXT n1Field
      DBSKIP()
   ENDDO 
 
RETURN // Hash2Table()

*-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.

PROCEDURE ListHash( hHash, cComment )
 
  LOCAL x1Pair
 
  cComment := IF( HB_ISNIL( cComment ), '', cComment )
 
  ? 
  ? cComment // , "-- Type :", VALTYPE( hHash ), "size:", LEN( hHash )
  ?
  IF HB_ISHASH( hHash ) 
     FOR EACH x1Pair IN hHash
        nIndex := x1Pair:__ENUMINDEX()
        x1Key := x1Pair:__ENUMKEY()
        x1Value := x1Pair:__ENUMVALUE()
        ? cLMarj, NTrim( nIndex ) 
*       ?? '', VALTYPE( x1Pair )
        ?? '', x1Key, "=>"
*       ?? '', VALTYPE( x1Key ) 
*       ?? VALTYPE( x1Value ) 
        IF HB_ISARRAY( x1Value ) 
           AEVAL( x1Value, { | x1 | QQOUT( '', x1 ) } )
        ELSE 
           ?? '', x1Value
        ENDIF 
     NEXT
  ELSE
    ? "Data type error; Expected hash, came", VALTYPE( hHash ) 
  ENDIF HB_ISHASH( hHash )
RETURN // ListHash()
*-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.

FUNCTION MakUseTable() // Make / USE table
 
 LOCAL cTablName := "CUSTOMER.DBF"
 LOCAL lRetval, aStru, aData, a1Record 
 
 IF FILE( cTablName ) 
    USE (cTablName)
 ELSE
    aStru := { { "CUST_ID", "C", 5, 0 },;
               { "CUST_NAME", "C", 10, 0 },;
               { "CUST_SNAM", "C", 10, 0 },;
               { "CUST_FDAT", "D", 8, 0 },;
               { "CUST_ACTV", "L", 1, 0 },;
               { "CUST_BLNCE", "N", 11, 2 } }
    * 
    * 5-th parameter of DBCREATE() is alias - 
    * if not given then WA is open without alias 
    *                              ^^^^^^^^^^^^^ 
    DBCREATE( cTablName, aStru, , .F., "CUSTOMER" ) 
 
    aData := { { "CC001", "Pierce", "Firth", 0d20120131, .T., 150.00 },; 
               { "CC002", "Stellan", "Taylor", 0d20050505, .T., 0.15 },;
               { "CC003", "Chris", "Cherry", 0d19950302, .F., 0 },;
               { "CC004", "Amanda", "Baranski", 0d20011112, .T., 12345.00 } }
 
    FOR EACH a1Record IN aData
        CUSTOMER->(DBAPPEND())
        AEVAL( a1Record, { | x1, nI1 | FIELDPUT( nI1, X1 ) } )
    NEXT a1Record 
    DBGOTOP()
 
 ENDIF 
 
 lRetval := ( ALIAS() == "CUSTOMER" )
 
RETURN lRetval // MakUseTable()

*-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._.-._
 
HashVsTable
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