13.  PROGRAMMING FEATURES

BRANCHES

Within BASIC programs  the  statement  GOTO  n  causes  an  unconditional 
transfer  of  control (branch) to line n, changing the normal sequence of 
execution.

For a conditional transfer of control the following statements are avail-
able:

                IF... THEN... ELSE 
                IF... GOTO... ELSE 
                ON... GOTO

All of these statements can be 'nested' (the process  of  introducing  an 
activity  within  an  activity) and details of the format of these state-
ments are given in Chapter 15.

LOOPS

Program loops for  repetitive  operations  can  be  generated  using  the 
FOR... NEXT  statement.  Loops can be nested in the same way as branches, 
the limitation being the available memory. A NEXT statement may  conclude 
nore  than one loop if the inner loop terminates at the same point as the 
outer loop. There is no error condition for a FOR without a matching NEXT 
statement,  but  if a NEXT is used without a preceding FOR the error code 
NF is displayed.

SUBROUTINES

A subroutine may be used to allow the same sequence of instructions to be 
carried  out at any point in a program.  The statement GOSUB n causes the 
progran .a junp to line n ahd to continue with execution of  the  subrou-
tine which starts at line n. The last statement of the subroutine must be 
RETURN which  transfers  orogram  control  back  to  the  line  following 
GOSUB n.

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MACHINE LANGUAGE SUBROUTINES

A machine language subroutine can be used within a BASIC program by using 
the  CALL  statement  to invoke the particular program required. A few of 
the subroutiness availahle cannot be started with a CALL  statement;  see 
CALL in the Directory of BASIC commands (Chapter 15).

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                               14.  FILES

Files can contain programs or data. BAS1C program files are automatically 
assigned the suffix .BA, whereas TEXT files and BASIC data files have the 
suffix .DO. Machine languaqe program files have the suffix .CO.  All data 
files are in a sequential form. Normally they are created in memory; they 
can then be transferred to a cassette tape or another peripheral.  It  is 
possible to write a file directly onto cassette but it is not possible to 
append data onto a cassette file. Anything written  to  a  cassette  file 
overwrites the original. To read or write to a data file it must first be 
opened using the statement OPEN.

OPENING A FILE

The OPEN statement is used to specify the direction of data  transferred. 
To read a file it must be opened in input mode, and to write to a file it 
must be opened in output mode.

Data files in memory can be opened in  the  append  mode  where  anything 
written to the file is added to the existing file and the original is not 
lost. A file in memory can be transferred to cassette tape using the SAVE 
statement, and a file on tape can be transferred to memory using LOAD.

A number is allocated to the program or data file as  part  of  the  OPEN 
statement.  The number allocated for a particular mode cannot be used for 
another anode unless the file is first closed.

READING A DATA FILE

Data from a file can be used in a program by first opening  the  file  in 
input  mode,  and then using the INPUT  statement to extract the items of 
data and assign them to program variables.

The EOF statement must be used after TNPUT  to  detect  the  end  of  the 
file.  If  this is not done an EF error will be displayed when the end of 
the file is reached and the program will  terminate  in  an  uncontrolled 
way.

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WRITING A DATA FILE

Data can be written to a file by first opening the file in  output  mode, 
and then using the PRINT or PRINT USING statements.

CLOSING A FILE

Some statements and comnands will automatically  close  all  open  files, 
e.g.  END.  To close a file independently of these the CLOSE statement is 
available.

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