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ABAP Keyword Documentation →  ABAP - Reference →  Declarations →  Local Declarations in Expressions 

let_exp - LET ... IN

Other versions: 7.31 | 7.40 | 7.54

Syntax


 ... LET {var1 = rhs1}|{<fs1> = wrexpr1} 
        {var2 = rhs2}|{<fs2> = wrexpr2} ... IN ...

Alternatives

1. ... var = rhs ...

2. ... <fs> = wrexp ...

Effect

A LET expression defines variables var1, var2, ... or field symbols <fs1>, <fs2>, ... as local auxiliary fields in an expression and assigned values to them. When declared, the auxiliary fields can be used in the operand positions of the expression. There is no way of accessing an auxiliary field statically outside its expression. The documentation of an expression specifies whether it contains a LET expression and in which positions. Any LET expressions in an expression or subexpression are evaluated first.

An auxiliary field specified in a LET expression is valid in the context in which the LET expression is specified. This can be a full expression or just part of an expression. All auxiliary fields of a full expression are in the same namespace. A previously specified auxiliary field can be specified in a further LET expression of the same expression. Furthermore, the auxiliary fields are in the same namespace as the data objects or field symbols of the current procedure or program. Auxiliary fields cannot be defined in a LET expression if a data object or field symbol with the same name already exists in the procedure or program of the expression. Conversely, no data objects or field symbols with names given to auxiliary fields can be declared after an expression with a LET expression.

When reusing auxiliary fields in different expressions, the following applies:

  • If an auxiliary field is defined for the first time in the current procedure or program, it is declared inline.
  • If an auxiliary field in the current procedure or program is defined again in a LET expression in a different expression and the derived data type matches, the auxiliary field is bound to this expression and can be used there.
  • If an auxiliary field in the current procedure or program is defined again in a LET expression in a different expression and the derived data type does not match, the auxiliary field cannot be used there and a syntax error occurs.


Notes

  • LET expressions can currently only be used in constructor expressions. Subexpressions in which LET expressions can be used are iteration expressions with FOR and results specified after THEN and ELSE in the conditional expressions COND and SWITCH.
  • In the assignment of a value operator VALUE to structures or internal tables, values from the left side can be saved to auxiliary fields using a LET expression before the left side is overwritten.
  • An auxiliary field defined in a LET expression can be addressed dynamically in the entire current context. This is not recommended, however, since expressions are not supposed to produce side effects.
  • It is a good idea to use LET expressions whenever auxiliary fields are required for performance reasons (by avoiding multiple calculations) or the readability of an expression or whenever values from the left side of an assignment in a VALUE operator are needed on the right side.


Example

See LET expression.

Alternative 1

... var = rhs ...

Effect

Defines a local auxiliary variable var as an auxiliary field in a LET expression. The value of the right side, rhs, is assigned to the auxiliary variable as an initial value.

For the right side, rhs, the same can be specified as in a regular assignment using the assignment operator =. The data type of the auxiliary is determined from the right side, rhs, in exactly the same way as when an inline declaration DATA(var) is specified on the left side of an assignment operator using the assignment operator =. It must be possible to determine the data type in full from the right side to avoid syntax errors.


Notes

  • The auxiliary variables declared in a LET expression generally retain the value defined by rhs while the expression is being calculated. It is possible to modify the value of an auxiliary variable in the expression, for example by binding to a CHANGING parameter of a method, but this would be unusual.
  • The auxiliary variables declared in a LET expression are a good example of where long readable names are not necessary and can even harm readability. The auxiliary variables can only be used in their own expression, which means that short names (possibly even single-character names) are enough.


Example

Defines three local auxiliary variables, x, y, and z, in a constructor expression to construct the values of a structure. The values of the auxiliary variables are used for the structure components.

TYPES: 
   BEGIN OF struc, 
     col1 TYPE i, 
     col2 TYPE i, 
   END OF struc. 

DATA(rnd) = cl_abap_random_int=>create( 
  seed = CONV i( sy-uzeit ) min = 1 max = 10 ). 

DO 5 TIMES. 
  DATA(struc) = VALUE struc( 
    LET x = rnd->get_next( ) 
        y = x * x 
        z = sy-index * 1000 IN col1 = x + z 
                               col2 = y + z ). 
  cl_demo_output=>write( struc ). 
ENDDO. 
cl_demo_output=>display( ).

Alternative 2

... <fs> = wrexp ...

Effect

Defines a local field symbol <fs> as an auxiliary field in a LET expression. Here, the result of the writable expression wrexp is assigned to the field symbol. The same applies here as in assignments of writable expressions using ASSIGN, which means that only the expressions specified there can be used. The typing of the field symbol is determined by the type of wrexp and is performed in the same way as in inline declarations of a field symbol using the statement FIELD-SYMBOL.


Example

Defines a field symbol and a variable as auxiliary fields in a LET expression in a conversion expression. The rows of an internal table are assigned to the field symbol.

TYPES: 
   BEGIN OF date, 
     year  TYPE c LENGTH 4, 
     month TYPE c LENGTH 2, 
     day   TYPE c LENGTH 2, 
   END OF date, 
   dates TYPE TABLE OF date WITH EMPTY KEY. 

DATA(dates) = VALUE dates( 
  ( year = '2013' month = '07' day = '16' ) 
  ( year = '2014' month = '08' day = '31' ) 
  ( year = '2015' month = '09' day = '07' ) ). 

DO lines( dates ) TIMES. 
  DATA(isodate) = CONV string( 
    LET <date>  = dates[ sy-index ] 
        sep   =   '-' 
     IN  <date>-year && sep && <date>-month && sep && <date>-day  ). 
  cl_demo_output=>write( isodate ). 
ENDDO. 
cl_demo_output=>display( ).

Continue

LET Expression - Example