This document describes the structure and the syntax of the src/FILENAME.t source files, and also provides explanation to the most common expressions. A brief summary at the end is provided as a quick reference guide.

A more general and possibly more enlightening description of the source language can be found in a paper on the LASY Preprocessor.

This page:
[STRUCTURE] [SYNTAX]: [Dimensions] [Limits] [Asymmetry] [Special] [Modules] [Patterns] [Expressions]


BHAC does both the initialization as well as the advancing of the variables in the governing PDEs. The program consists of several modules. Modules are simply sets of subroutines and functions that belong together and they are put in a single folder.

The main program is in the file amrvac.t and all time-advancing actually happens in advance.t. Input and output routines are in amrio.t and also in the postprocess conversion part collected in convert.t. Subroutines likely to be modified by the user are to be collected in amrvacusr.t in your run directory. Similarly, user-specific parameters can be added, for which the file amrvacusrpar.t is there.

Independence of equations is ensured by putting all flow variables in a single array w, and the variables are distinguished by their named indices, e.g. w(ix1,ix2,rho_) would read rho(ix1,ix2) in a typical code designed for a single equation. Here rho_ is an integer parameter defined amrvacpar.t file in a given physics folder (e.d. rmhd). Thus the AMRVACPAR module contains the EQUATION dependent parameters. The amrvacusrpar.t contains the extra PROBLEM dependent equation parameters.

The explicit temporal discretizations are in the main advancing module advance.t.

Finally the AMRINI module takes care of actually calling the initonegrid_usr subroutine, and allows to use the same subroutine to modify initial conditions read in from a pre-existing *.dat file. The info on the grid related quantities is all in the geometry module.


  1. Dimensions and vector components
  2. Since we aim to solve equations independent of the number of dimensions, arrays with unknown number of dimensions have to be declared and manipulated. This is achieved by the extensive use of the preprocessor VACPP.

    The basic idea of the Loop Annotation Syntax (LASY) is defining loops in the source text. A first example may be an array with 3 indices:

    a({ix^D|,})   --->     a(ix1,ix2,ix3)
    The string ix^D between the { and | characters is repeated NDIM=3 times with the pattern string ^D replaced by 1, 2 and 3, and the three resulting strings are separated by the string between the | and } characters, in this case a single comma. As you may observe, some special characters are used to help the preprocessor in recognizing the loop. The loop is enclosed between curly brackets, the pattern consists of a ^ character followed by uppercase letters (or %, or &), and the separator string is preceded by the | character. Thus the full syntax of the loop is
    { text ^PATTERN text ^PATTERN ... | separator }
    Fortunately some default values can be introduced to simplify the notation. The default separator is the comma, thus the above example can be written as a({ix^D}). Furthermore the preprocessor can expand patterns without enclosing curly brackets. When it sees a pattern, it looks for bounding characters on both sides. These are one of comma, space, newline, semicolon and enclosing parentheses. Since our ^D pattern is enclosed by a left and a right parenthesis, we may simply type
    which will expand to a(ix1), a(ix1,ix2), and a(ix1,ix2,ix3) for the choices NDIM=1, 2, and 3, respectively.

    The curly brackets are used only when the repetition should expand over some bounding characters. The required separator is very often a single character, such as in the calculation of sums or products:

    ix^D*    --->   ix1*ix2*ix3
    Here the ^D pattern is enclosed by a new line and a space character, and the preprocessor checks the last character of the repeated string. If it is one of comma, space, +, -, *, /, :, ;, or \, it is taken to be a separator character. The \ is replaced by a new line. A simple use may be nested DO loops:
    {do ix^D=1,100\}                    do ix1=1,100
       a(ix^D)= ix^D*       --->        do ix2=1,100
    {enddo\}                            a(ix1,ix2)= ix1*ix2
    Here NDIM=2 was assumed. Note the need for curly brackets in the first line to override the space and the comma, and also note the space in a(ix^D)= ix^D* which is needed to bound the ix^D* loop. In the final {enddo\} line the number of repetitions was assumed to be NDIM, as we shall see this may not always be the case.

    Suppose we want to assign the same value to NDIM variables:

    ix^D=1;     --->     ix1=1;ix2=1;
    The semicolon is a bounding character, thus strictly speaking it does not belong to the ix^D=1 loop. The preprocessor, however, checks wether the right bounding character is a semicolon, and if it is, it becomes the separator. The original trailing semicolon is also preserved, which turns out to be a useful feature.

    Up to this point we used a single pattern ^D only, which is replaced by the numbers 1..NDIM. During the code development it became obvious that the preprocessor can be used for many other things than repeated indices for the dimensions. The first new application is the components for vector variables, which run from 1 to NDIR, and in general 1<=NDIM<=NDIR<=3. The ^C (mnemonic: Component as opposed to ^D for Dimension) pattern is thus replaced by 1..NDIR. When more than one pattern is found in a loop, the first one determines the number of repetitions, and only patterns with the same first letter are replaced by their substitutes.

    Look at the following typical case construct for NDIM=2 and NDIR=2:

    select case(iw)
         mom(ixmin^D:ixmax^D)=w(ixmin^D:ixmax^D,m^C_) \}
    end select
    select case(iw)
    end select
    select case(iw)
    end select
    Notice that the preprocessor first expanded the loop for the first ^C pattern and substituted ^C in the third index of the w array but the other indices with the ^D patterns were left alone. In the second iteration the loops with ^D patterns were expanded. The resulting lines may become extremely long, thus the preprocessor breaks them into continuation lines.

  3. Limits, segments, and nulpatterns
  4. To further shorten the notation the often used min^D and max^D strings were interpreted as a loop of the ^L pattern (mnemonic: Limits). Yes, a pattern may expand into other pattern(s) thus producing another loop. To declare the limits of the array sections one may say

    integer:: ix^L  --->  integer:: ixmin^D,ixmax^D  --->
    integer:: ixmin1,ixmin2,ixmax1,ixmax2
    The intermediate ^D pattern becomes very important in the following typical example:
    jx^L=ix^L+kr(idim,^D);                                       --->
    jxmin^D=ixmin^D+kr(idim,^D);jxmax^D=ixmax^D+kr(idim,^D);     --->
    The purpose is to shift the limits by 1 in the idim direction. The kr array is a Kronecker delta, it is 1 for the diagonal elements where the two indices are the same, and 0 otherwise. The j in the jx is the next letter in the alphabet after the i, thus jx is the mnemonic for ix+1. Similarly hx is consistently used for ix-1. The semicolon remains the separator for both loop expansions thanks to the trailing semicolon in the intermediate step. Also note that the parentheses of kr(idim,^D) do not bound the initial loop for jx^L=... because they do not enclose it. Once we get used to the notation it becomes natural to think of an ix^L type loop as a Pascal record or C structure consisting of 2, 4 or 6 integers. Therefore ^LADD, ^LSUB and ^LT are introduced to do operations and comparisons on them. ^LADD (mnemonic: add to limits) is replaced by - and + to decrease lower and increase upper limits, ^LSUB does the opposite by expanding to + and -, finally ^LT has substitutes > and < ensuring that the limits on the left are within the limits on the right. Thus we may extend the ix^L limits by 1 with
    ix^L=ix^L^LADD1;  --->  ixmin^D=ixmin^D-1;ixmax^D=ixmax^D+1;
    To check if the ixI^L input limits are not narrower than the ix^L limits
    if(ixI^L^LTix^L|.or.|.or.)stop --->
    if(ixImin^D>ixmin^D|.or. .or. ixImax^D<ixmax^D|.or.)stop  --->
    if(ixImin1>ixmin1.or.ixImin2>ixmin2 .or. ixImax1<ixmax1.or.&
    where note the repeated use of the .or. separator string.

    It is possible to form array sections from the ^L pattern by making use of the : as a separator, but it turns out that introducing the new patterns ^LIM (mnemonic: three letter LIMits min and max) and ^S (mnemonic: Sections) is a better solution. The typically internally used ^LIM pattern expands to min and max, while ^S expands to ^LIM1:..^LIMndim:, and therefore

    a(ix^S)  --->  a(ix^LIM1:,ix^LIM2:)  --->  a(ixmin1:ixmax1,ixmin2:ixmax2)
    As you may suspect the whole exercise of introducing ^LIM and ^S served the purpose of achieving this extremely compact notation for array sections. In array declarations the lo and hi absolute limits are used instead of the min and max actual limits. The ^LL pattern (mnemonic: Lowest/highest Limits) thus expands to lo^D and hi^D similarly to ^L, while ^LLIM gives simply lo and hi, and ^T (mnemonic: total segment) is used in most array declarations:
    a(ix^T)  --->  a(ix^LLIM1:,ix^LLIM2:)  --->  a(ixlo1:ixhi1,ixlo2:ixhi2)
    Now that we have many different patterns with different number of substitutes it becomes useful and necessary to introduce patterns which are replaced by nulstrings, but they determine the number and kind of substitutions. The ^D& and ^C& patterns produce NDIM and NDIR repetitions respectively. (The alternative names ^DLOOP and ^CLOOP can also be used to avoid syntax problems at the end of a line, where the & means continuation according to FORTRAN 90.) A trivial application is the enddo-s at the end of NDIM nested do loops:
    enddo^D&;     --->    enddo;enddo;
    Obviously the ^D pattern would not work here: enddo^D; --> enddo1;enddo2;. For scalar products of vector variables the ^C& is used at the head of the loops to tell VACPP that first the ^C patterns should be expanded. For example calculate the magnetic pressure from the pb=0.5*B.B formula:
    pb(ix^S)=0.5*(^C&w(ix^S,b^C_)**2+)   --->
    for NDIM=1, NDIR=2. The interpretation of the original code is easiest by thinking of ^C& as the index for the loop and of the + at the end as the operator applied for the elements.

  5. Breaking the symmetry
  6. The symmetry of the indices is sometimes broken. In the code it is mostly related to the assumption of axial symmetry, when the first dimension becomes special. The rest of the dimensions and sections are expanded from the ^DE (mnemonic: Dimensions Extra) and ^SE (Sections Extra) patterns. The radial distance may be related to the x coordinate array by

    r(ix^LIM1:)=x(ix^LIM1:,ixmin^DE,1)   --->
    then r may be a weight function for an array
    forall(ix= ix^LIM1:) a(ix,ix^SE)=r(ix)*a(ix,ix^SE)      --->
    forall(ix= ixmin1:ixmax1) a(ix,ixmin2:ixmax2)=r(ix)*a(ix,ixmin2:ixmax2)
    If NDIM=1 the preprocessor removes the separator, in this case a comma in the ix,ix^SE string, and the ix^SE loop is repeated 0 times, thus we get forall(ix= ixmin1:ixmax1) a(ix)=r(ix)*a(ix) as expected.

    Another type of symmetry breaking occurs when something is done for all the indices separately, e.g. the boundary elements of an NDIM dimensional array are filled up for each boundary separately. The ^D% pattern is replaced by ^%1..^%NDIM patterns. The ^%N pattern substitutes the text in front of it in the N-th repetition, and the text behind in the other substitutions. In general head^%Ntail ---> tail,..,tail,head,tail,..,tail with head being at the N-th position. The number of repetitions is determined by the first pattern in head and tail. Here is an application for boundary conditions with ixB^L enclosing the boundary region and ixMmin^D being the edge of the mesh:

       do ix= ixBmin^D,ixBmax^D
       end do \}
    ---> /loop for ^D/
       do ix= ixBmin1,ixBmax1
       end do
       do ix= ixBmin2,ixBmax2
       end do
    ---> /loops for ^S taking ^%1 and ^%2 into account/
       do ix= ixBmin1,ixBmax1
       end do
       do ix= ixBmin2,ixBmax2
       end do
    ---> /loops for ^LIM/
       do ix= ixBmin1,ixBmax1
       end do
       do ix= ixBmin2,ixBmax2
       end do

  7. Special features: switches, include files, common variables
  8. Depending on the actual use of BHAC some subroutines may or may not be present, so something where you really need to use a 1D construct is best embedded as

    {^IFONED   (something where x(ix1) occurs e.g.) }
    This should be done with care, as the idea of the code is to use it for any dimensionality. Still, certain subroutines are not used at all in 1D, or they may be needed only in 2D, and the ^NOONED, ^IFTWOD etc. switches are then used to make a part of the code conditional on the value of the NDIM parameter.

    Another way of switching between different versions of some part of the code is including a file filename.t which can be a link to the particular realization. The following expression, placed to the beginning of the line,

    will be replaced by the content of the file. Include files can be nested. The INCLUDE: statement is often combined with conditional pattern(s). There is a restriction, which follows from the simple syntax rules: two conditional patterns can only be combined if their initial letters differ.

    Having a powerful preprocessor one is tempted to eliminate all the annoying repetitions inherent in the restricted Fortran 90 syntax. Variables in common blocks have to be defined twice, once giving their type, and once listing them in the common blocks. It would be very convenient to give them a COMMON attribute in the type declaration and that's exactly what VACPP lets you do if you insert 'COMMON,' to the beginning of the line:

    COMMON, INTEGER:: a(10),b
    COMMON, INTEGER:: c(20)
    INTEGER:: a,b
    LOGICAL:: l
    INTEGER:: c
    COMMON /INTE/  a(10),b, c(20)
    COMMON /LOGI/  l
    The dimensions are stripped from the array declarations, and they are declared in the COMMON /NAME/ blocks collected at the end of the file. The /NAME/ is the first four letters of the type, thus only variables of identical types get into the same common block, which avoids alignment problems.

  • VACPP Patterns
  • We assume NDIM=2 and NDIR=3, so you may try this interactively with -d=23 -. For a full list of defined patterns read the &patdef definitions in

    Pattern   Substitutes                              Mnemonic
    ^D --> 1,2 Dimensions ^DE --> 2 Dimensions Extra ^D& --> , NDIM repetitions ^DLOOP--> , NDIM repetitions ^DD --> ^D,^D --> 1,2,1,2 NDIM*NDIM dimensions ^D% --> ^%1,^%2 --> head,tail,tail,head NDIM*NDIM head-tail matrix ^C --> 1,2,3 Components ^C& --> ,, NDIR repetitions ^CLOOP--> ,, NDIR repetitions ^L --> min^D,max^D --> min1,min2,max1,max2 Limits in all D ^LIM --> min,max LIMits min and max ^LLIM --> lo,hi Low and high LIMits ^LADD --> -,+ Add to ^L (extend) ^LSUB --> +,- Subtract from ^L (shrink) ^LT --> >,< Less Than (compare ^L) ^LL --> lo^D,hi^D --> lo1,lo2,hi1,hi2 Lowest/highest Limits ^S --> ^LIM1:,^LIM2: --> min1:max1,min2:max2 Segments ^SE --> ^LIM2: --> min2:max2 Segments Extra ^T --> ^LLIM1:,^LLIM2: --> lo1:hi1,lo2:hi2 Total segments

  • VACPP Expressions
  • Source VACPP notation     Expanded Fortran 90 code
    integer:: ix^D --> integer:: ix1,ix2 integer:: ix^L --> integer:: ixmin1,ixmin2,ixmax1,ixmax2 real:: w(ixG^T,nw) --> real:: w(ixGlo1:ixGhi1,ixGlo2:ixGhi2,nw) call subr(w,ix^L) --> call subr(w,ixmin1,ixmin2,ixmax1,ixmax2) pb(ix^S)=half*& --> pb(ixmin1:ixmax1,ixmin2:ixmax2)=half*& (^C&w(ix^S,b^C_)**2+) (w(ixmin1:ixmax1,ixmin2:ixmax2,b1_)**2+& w(ixmin1:ixmax1,ixmin2:ixmax2,b2_)**2+& w(ixmin1:ixmax1,ixmin2:ixmax2,b3_)**2) ixI^L=ix^L^LADD1; --> ixImin1=ixmin1-1;ixImin2=ixmin2-1; ixImax1=ixmax1+1;ixImax2=ixmax2+1; jx^L=ix^L+kr(2,^D); --> jxmin1=ixmin1+kr(2,1);jxmin2=ixmin2+kr(2,2); jxmax1=ixmax1+kr(2,1);jxmax2=ixmax2+kr(2,2); select case(iw) --> select case(iw) {case(m^C_) case(m1_) a(ix^S)=w(ix^S,m^C_)\} a(ixmin1:ixmax1,ixmin2:ixmax2)=& end select w(ixmin1:ixmax1,ixmin2:ixmax2,m1_) case(m2_) a(ixmin1:ixmax1,ixmin2:ixmax2)=& w(ixmin1:ixmax1,ixmin2:ixmax2,m2_) case(m3_) a(ixmin1:ixmax1,ixmin2:ixmax2)=& w(ixmin1:ixmax1,ixmin2:ixmax2,m3_) end select {^IFTVD text} --> text or '' (depending on the TVD module being on) {^IFTWOD text} --> text or '' (depending on ndim being equal to 2) {^NOONED text} --> '' or text (depending on ndim being equal to 1} INCLUDE:filename --> content_of_file COMMON, REAL:: c(20) --> REAL:: c text text COMMON /REAL/ c(20)