# expr ::= term | term + expr
# term ::= factor | factor * term
# factor ::= (expr) | num
s="((4+5)*3+2)*8"
i=0
def gettoken ():
global i
j=i
print(s[i])
if i < len(s)-1:
i=i+1
return s[j]
else:
return 'e'
def tokenback ():
global i
i = i-1
def expr ():
y = 0
x = term ()
p = gettoken ()
if p == '+':
y = expr ()
elif p == 'e':
return x
else:
tokenback ()
return x+y
def term ():
y = 1
x = factor ()
if x == -1:
return 1
p = gettoken ()
if p == '*':
y = term ()
elif p == 'e':
return x
else:
tokenback ()
return x*y
def factor ():
p = gettoken ()
if p.isdigit():
x = int(p)
elif p == '(':
x = expr ()
if gettoken () != ')':
print ("error - Bracket not closed");
exit ()
elif p == 'e':
return -1
else:
print ("error - invalid character")
exit ()
return x
print(expr())
