def __returnstmt(self):
"""Parses a RETURN statement"""
self.__advance() # Advance past RETURN token
# Set up and return the flow signal
return FlowSignal(ftype=FlowSignal.RETURN)
def __ifstmt(self):
"""Parses if-then-else
statements
:return: The FlowSignal to indicate to the program
how to branch if necessary, None otherwise
"""
self.__advance() # Advance past IF token
self.__logexpr()
# Save result of expression
saveval = self.__operand_stack.pop()
# Process the THEN part and save the jump value
self.__consume(Token.THEN)
if self.__token.category == Token.GOTO:
self.__advance() # Advance past optional GOTO
self.__expr()
then_jump = self.__operand_stack.pop()
# Jump if the expression evaluated to True
if saveval:
# Set up and return the flow signal
return FlowSignal(ftarget=then_jump)
# See if there is an ELSE part
if self.__token.category == Token.ELSE:
self.__advance()
if self.__token.category == Token.GOTO:
self.__advance() # Advance past optional GOTO
self.__expr()
# Set up and return the flow signal
return FlowSignal(ftarget=self.__operand_stack.pop())
else:
# No ELSE action
return None
def __nextstmt(self):
"""Processes a NEXT statement that terminates
a loop
:return: A FlowSignal indicating that a loop
has been processed
"""
self.__advance() # Advance past NEXT token
return FlowSignal(ftype=FlowSignal.LOOP_REPEAT)
def __gotostmt(self):
"""Parses a GOTO statement
:return: A FlowSignal containing the target line number
of the GOTO
"""
self.__advance() # Advance past GOTO token
self.__expr()
# Set up and return the flow signal
return FlowSignal(ftarget=self.__operand_stack.pop())
def __gosubstmt(self):
"""Parses a GOSUB statement
:return: A FlowSignal containing the first line number
of the subroutine
"""
self.__advance() # Advance past GOSUB token
self.__expr()
# Set up and return the flow signal
return FlowSignal(ftarget=self.__operand_stack.pop(),
ftype=FlowSignal.GOSUB)
def __forstmt(self):
"""Parses for loops
:return: The FlowSignal to indicate that
a loop start has been processed
"""
# Set up default loop increment value
step = 1
self.__advance() # Advance past FOR token
# Process the loop variable initialisation
loop_variable = self.__token.lexeme # Save lexeme of
# the current token
if loop_variable.endswith('$'):
raise SyntaxError('Syntax error: Loop variable is not numeric' +
' in line ' + str(self.__line_number))
self.__advance() # Advance past loop variable
self.__consume(Token.ASSIGNOP)
self.__expr()
# Check that we are using the right variable name format
# for numeric variables
start_val = self.__operand_stack.pop()
# Advance past the 'TO' keyword
self.__consume(Token.TO)
# Process the terminating value
self.__expr()
end_val = self.__operand_stack.pop()
# Check if there is a STEP value
increment = True
if not self.__tokenindex >= len(self.__tokenlist):
self.__consume(Token.STEP)
# Acquire the step value
self.__expr()
step = self.__operand_stack.pop()
# Check whether we are decrementing or
# incrementing
if step == 0:
raise IndexError('Zero step value supplied for loop' +
' in line ' + str(self.__line_number))
elif step < 0:
increment = False
# Now determine the status of the loop
# If the loop variable is not in the set of extant
# variables, this is the first time we have entered the loop
# Note that we cannot use the presence of the loop variable in
# the symbol table for this test, as the same variable may already
# have been instantiated elsewhere in the program
if loop_variable not in self.__loop_vars:
self.__symbol_table[loop_variable] = start_val
# Also add loop variable to set of extant loop
# variables
self.__loop_vars.add(loop_variable)
else:
# We need to modify the loop variable
# according to the STEP value
self.__symbol_table[loop_variable] += step
# If the loop variable has reached the end value,
# remove it from the set of extant loop variables to signal that
# this is the last loop iteration
stop = False
if increment and self.__symbol_table[loop_variable] > end_val:
stop = True
elif not increment and self.__symbol_table[loop_variable] < end_val:
stop = True
if stop:
# Loop must terminate, so remove loop vriable from set of
# extant loop variables and remove loop variable from
# symbol table
self.__loop_vars.remove(loop_variable)
del self.__symbol_table[loop_variable]
return FlowSignal(ftype=FlowSignal.LOOP_SKIP,
ftarget=loop_variable)
else:
# Set up and return the flow signal
return FlowSignal(ftype=FlowSignal.LOOP_BEGIN)
def __stopstmt(self):
"""Parses a STOP statement"""
self.__advance() # Advance past STOP token
return FlowSignal(ftype=FlowSignal.STOP)