ich habe ein Problem bei Python. Ich muss einen vorgegebenen Quelltext mit folgendem abstakten Code editieren.
Ich weiss, es schaut nach sehr viel Code aus, aber es geht nur um eine Zeile...Bitte helft mir.
Abstrakter Code:
Folgende zeile bereitet mir Probleme:< command > = < assignment > | < expression >
< assignment > = < variable > ‘=’ < expression >
< expression > = < term >
| < expression > ‘+’ < term >
| < expression > ‘−’ < term > ;
< term > = < factor >
| < term > ‘×’ < factor > ;
< factor > = < number > | ‘(’ < expression > ‘)’ | < variable >
< number > = ( ‘0’ | ‘1’ | ‘2’ | ‘3’ | . . . | ‘9’ )+
< variable > = ‘a’ | ‘b’ | ‘c’
Ich weiss einfach nicht, wie ich die in Python Code umsetzten muss...< factor > = < number > | ‘(’ < expression > ‘)’ | < variable >
Folgendes habe ich bis jetzt selber geschafft:
Code: Alles auswählen
# Simple Calculator Parser
#
#
# Description:
# This simple calculator parser evaluates any expression
# typed in, and outputs the result.
#
# Example:
# > 2 + 3 - 1
# Result: 4
def scanCommand():
ok, x = assignment()
if ok:
print "Result:", x
else:
ok, x = scanExpression()
if ok:
print "Result:", x
else:
return False, 0
def assignment():
global scanPosition
ok, x = scanLetter()
if not ok:
return False, 0
while True:
lastPosition = scanPosition
if scanChar('='):
ok, y = scanExpression()
if ok:
x = y
else:
scanPosition = lastPosition
break
else:
break
return True, x
def scanExpression():
global scanPosition
ok, x = scanTerm()
if not ok:
return False, 0
while True:
lastPosition = scanPosition
if scanChar('+'):
ok, y = scanTerm()
if ok:
x = x + y
else:
scanPosition = lastPosition
break
elif scanChar('-'):
ok, y = scanTerm()
if ok:
x = x - y
else:
scanPosition = lastPosition
break
else:
break
return True, x
def scanTerm():
global scanPosition
ok, x = scanFactor()
if not ok:
return False, 0
while True:
lastPosition = scanPosition
if scanChar('x'):
ok, y = scanFactor()
if ok:
x = x*y
else:
scanPosition = lastPosition
break
else:
break
return True, x
def scanFactor():
global scanPosition
ok, x = scanNumber()
if ok:
return True, x
while True:
lastPosition = scanPosition
if scanChar('('):
ok, y = scanExpression()
else:
break
return True, y
# Return whether the character 'char' could be scanned
def scanChar(char):
skipWhitespace()
ok, c = scanChars(char)
return ok
# Return whether any sequence of digits could be scanned,
# together with its numerical value, as an integer number
def scanNumber():
skipWhitespace()
ok, c = scanDigit()
if not ok: return False, 0
s = ''
while ok:
s = s + c
ok, c = scanDigit()
return True, int(s)
# Return whether any digit could be scanned,
# together with the scanned digit
def scanDigit():
return scanChars(['0', '1', '2', '3', '4', '5', '6', '7', '8', '9'])
# Return whether any letter (a, b or c) could be scanned,
# together with the scanned letter
def scanLetter():
skipWhitespace()
return scanChars(['a', 'b', 'c'])
# Skip whitespace characters
def skipWhitespace():
while True:
ok, c = scanChars([' '])
if not ok:
break;
# Return whether any character from the list 'chars' could be scanned,
# together with the scanned character
def scanChars(chars):
global scannedString, scanPosition
if scanPosition >= len(scannedString):
return False, ''
for c in chars:
if scannedString[scanPosition] == c:
scanPosition += 1
return True, c
else:
return False, ''
# Initialize the sttring to be scanned
def setScannedString(string):
global scannedString, scanPosition
scannedString = string
scanPosition = 0
# Set the content of variable 'var' to x
def setVariable(var, x):
global memory
memory[var] = x
# Get the content of variable 'var'
def getVariable(var):
global memory
return memory[var]
memory = {}
# Main part of the program
print "(Type q to quit)"
while True:
input = raw_input("> ") # Prompt user for input
if input[:1] == 'q':
break # Stop if user types 'q'
elif len(input) > 0: # If user typed something
setScannedString(input)
scanCommand()
Und dies ist die Vorlage(welche ich editieren musst, sodass der Rechner auch Speichern und Punkt vor Strich unterstützt):
Code: Alles auswählen
# Simple Calculator Parser
# FGII, Fall Semester 2008, Simon Bovet
#
# Description:
# This simple calculator parser evaluates any expression
# typed in, and outputs the result.
#
# Example:
# > 2 + 3 - 1
# Result: 4
def scanCommand():
ok, x = scanExpression()
if ok:
print "Result:", x
def scanExpression():
global scanPosition
ok, x = scanFactor()
if not ok:
return False, 0
while True:
lastPosition = scanPosition
if scanChar('+'):
ok, y = scanFactor()
if ok:
x = x + y
else:
scanPosition = lastPosition
break
elif scanChar('-'):
ok, y = scanFactor()
if ok:
x = x - y
else:
scanPosition = lastPosition
break
else:
break
return True, x
def scanFactor():
ok, x = scanNumber()
if ok:
return True, x
return False, 0
# Return whether the character 'char' could be scanned
def scanChar(char):
skipWhitespace()
ok, c = scanChars(char)
return ok
# Return whether any sequence of digits could be scanned,
# together with its numerical value, as an integer number
def scanNumber():
skipWhitespace()
ok, c = scanDigit()
if not ok: return False, 0
s = ''
while ok:
s = s + c
ok, c = scanDigit()
return True, int(s)
# Return whether any digit could be scanned,
# together with the scanned digit
def scanDigit():
return scanChars(['0', '1', '2', '3', '4', '5', '6', '7', '8', '9'])
# Return whether any letter (a, b or c) could be scanned,
# together with the scanned letter
def scanLetter():
skipWhitespace()
return scanChars(['a', 'b', 'c'])
# Skip whitespace characters
def skipWhitespace():
while True:
ok, c = scanChars([' '])
if not ok:
break;
# Return whether any character from the list 'chars' could be scanned,
# together with the scanned character
def scanChars(chars):
global scannedString, scanPosition
if scanPosition >= len(scannedString):
return False, ''
for c in chars:
if scannedString[scanPosition] == c:
scanPosition += 1
return True, c
else:
return False, ''
# Initialize the sttring to be scanned
def setScannedString(string):
global scannedString, scanPosition
scannedString = string
scanPosition = 0
# Set the content of variable 'var' to x
def setVariable(var, x):
global memory
memory[var] = x
# Get the content of variable 'var'
def getVariable(var):
global memory
return memory[var]
memory = {}
# Main part of the program
print "(Type q to quit)"
while True:
input = raw_input("> ") # Prompt user for input
if input[:1] == 'q':
break # Stop if user types 'q'
elif len(input) > 0: # If user typed something
setScannedString(input)
scanCommand()
< command > = < expression > ;
< expression > = < factor >
| < expression > ‘+’ < factor >
| < expression > ‘−’ < factor > ;
< factor > = < number > ;
< number > = ( ‘0’ | ‘1’ | ‘2’ | ‘3’ | . . . | ‘9’ )+ ;
(
"sma" hat da mal schon eine vernünftige Vorlage geliefert, konnte es auf die Schnelle aber nicht finden...