Control Flow Statements

With the IF statement you can test a condition and then run code based on whether the condition test passed or failed.

IF <condition> THEN
    <statements>
END IF

The IF clause runs the statements under it when the <condition> is evaluated to be TRUE (pass). If the <condition> evaluates to FALSE (fail), nothing is run.

You can optionally include <statements> that are run when the <condition> fails by adding an ELSE clause

IF <condition> THEN
    <statements>
ELSE  
    <statements>
END IF

XC=BASIC doesn't contain a boolean data type to represent true or false, it uses a BYTE instead. The value of 0 represents FALSE. Any other value greater than 0 represents TRUE.

When the <condition> contains a comparison operator, such as = or >, the result of the comparison is set to 255 when the comparison passes, and 0 when it fails. The result is then processed as normal, with 0 representing FALSE and any other number representing TRUE.

If the <condition> is a numerical expression, such as 1 or 10 - 10, the result is evaluated the same way as previously described.

The IF statement can be written on a single line:

IF <condition> THEN <statements> [ELSE <statements>]

One or more code <statements> must be added to the THEN clause. If multiple statements are added, they must be separated by a colon.

The ELSE clause is optional, and must contain at least one code <statement>. Multiple statements can be used if separated by a colon.

The single line IF statement runs the same as the normal multi line IF statement described in the previous section.

The ON statement allows you to define multiple branches and an index expression. This expression will be evaluated as a number N and the program will continue at the Nth label in the list of branches.

DIM x$ AS STRING * 8
INPUT "enter a number "; x$
ON SGN(VAL(x$)) + 1 GOTO negative, zero, positive
negative:
PRINT "you entered a negative number"
END
zero:
PRINT "you entered zero"
END
positive:
PRINT "you entered a positive number"

ON can also be used in combination with GOSUB.

Syntax:

FOR <variable> [AS <type>] = <start_value> TO <end_value> [STEP <step_value>]
  <statements>
NEXT [<variable>]

The FOR … NEXT loop initializes a counter variable and executes the statements until the counter variable equals the value in the TO clause. After each iteration, the counter variable is incremented by the step value or 1 in case the step value was not specified. For example:

DIM i AS BYTE
FOR i = 1 TO 30 STEP 3
  PRINT i
NEXT i

In the above, the counter variable is pre-defined. If the variable is not pre-defined, you can use the AS keyword to specify the type and this way the compiler will automatically define the variable before starting the loop. The following code is identical to the one above:

FOR i AS BYTE = 1 TO 30 STEP 3
  PRINT i
NEXT i

You can use the CONTINUE FOR command to skip the rest of the statements in the FOR … NEXT block and go to the next iteration, for example:

REM -- print numbers from 1 to 10, except 5 and 7
FOR num AS BYTE = 1 TO 10
  IF num = 5 OR num = 7 THEN CONTINUE FOR
  PRINT num
NEXT

Finally, you can use the EXIT FOR statement to early exit a FOR … NEXT loop.

DIM a$ AS STRING * 1
FOR i AS BYTE = 1 TO 10
  PRINT "iteration #"; i : INPUT "do you want to continue? (y/n)"; a$
  IF a$ = "n" THEN EXIT FOR
  PRINT i
NEXT i

The DO … LOOP loop can be used as either a pre-test or post test loop.

DO WHILE|UNTIL <condition>
  <statements>
LOOP

DO
  <statements>
LOOP WHILE|UNTIL <condition>

The former is used to test the condition before entering the loop body and the latter is used to test the condition after each iteration. This effectively means that post-test loop will be executed at least once, whereas in a pre-test loop the condition may fail the very first time and therefore it may happen that the statements in the loop will not be executed at all.

In both types, you can either use the WHILE or the UNTIL keywords. WHILE means that the loop is entered if the condition evaluates to true, UNTIL means that the loop is exited if the condition evaluates to true.

Similarly to EXIT FOR, you can use the EXIT DO command to prematurely exit a DO … LOOP block, or CONTINUE DO to skip rest of the block and go to the next iteration.