The first four functions are complete. The last function, calc(), is the one I a

Question

The first four functions are complete. The last function, calc(), is the one I am having trouble with. It is supposed to use the previous functions to evaluate a string expression such as "4 - 3 + 5 / 4 * 2"

Comments explaining your thought process for each step and what the functions do would be great. This is on a very close time limit of the end of the night. I can't seem to get it to work. Thank you very much for your help.

def findNextOpr (s) must be a nonempty string. if len(s)0 or isinstance (s, str) ops= ['+', '-', '/', for i in range (len (s)) if s[i] in ops: return i return -1 #In this exercise +, -, *, / are all the 4 operators #The function returns -1 if there is no operator in s, #otherwise returns the position of the leftmost operator # code the rest of the function # - function code ends- def isNumber (s) #3 must be a non-empty string #returns True if it's convertible to float, else False if len(s)=-0 or not isinstance(s, str): print ("type mismatch error: isNumber") return "type mismatch error: isNumber" try: float (s) return True except: return False

Explanation / Answer

def findNextOpr(s):
if len(s)<=0 or not isinstance(s,str):
print("type mimatch error: findNextOpr")
return "type mimatch error: findNextOpr"
  
p = -1
for (j, i) in enumerate(list(s)):
if i == '+' or i == '-' or i == '*' or i == '/':
p = j
break

return p


def isNumber(s):
if len(s)==0 or not isinstance(s, str):
print("type mismatch error: isNumber")
return "type mismatch error: isNumber"

try:
float(s.strip())
return True
except ValueError:
return False



def getNextNumber(expr, pos):
if len(expr)==0 or not isinstance(expr, str) or pos<0 or pos>=len(expr) or not isinstance(pos, int):
print("type mismatch error: getNextNumber")
return None, None, "type mismatch error: getNextNumber"

a = findNextOpr(expr[pos:])
if a == -1:
a = None
b = None
c = expr[pos:]
elif a == 0:
pos += 1
a = findNextOpr(expr[pos:])
if a == -1:
a = None
b = None
c = expr[pos:]
else:
a += pos
b = expr[a]
c = "-" + expr[pos:a]
else:
a += pos
b = expr[a]
c = expr[pos:a]

if not isNumber(c):
c = None
else:
c = float(c)

return (c, b, a)

def exeOpr(num1, opr, num2):
if opr=="+":
return num1+num2
elif opr=="-":
return num1-num2
elif opr=="*":
return num1*num2
elif opr=="/":
return num1/num2
else:
return None


def calc(expr):
if len(expr)<=0 or not isinstance(expr,str):
print("argument error: line A in eval_expr")
return "argument error: line A in eval_expr"
newNumber, newOpr, oprPos = getNextNumber(expr, 0)
if newNumber is None:
print("input formula error: line B in eval_expr")
return "input formula error: line B in eval_expr"
elif newOpr is None:
return newNumber
elif newOpr=="+" or newOpr=="-":
mode="add"
addResult=newNumber
mulResult=None
elif newOpr=="*" or newOpr=="/":
mode="mul"
addResult=0
mulResult=newNumber
pos=oprPos+1
opr=newOpr
while True:
c, d, e = getNextNumber(expr, pos)
if d=="+" or d=="-":
f = "add"
elif d =="*" or d == "/":
f = "mul"
else:
f = None
if c is None:
print("c is None")
if mode == "add":
return addResult
else:
return mulResult
  
elif mode == "add" and f == "add":
addResult = exeOpr(addResult, opr, c)
return calc(str(addResult) + expr[e:])
elif mode == "add" and f == "mul":
if opr == "+":
return addResult + calc(expr[pos:])
elif opr == "-":
return addResult + calc("-" + expr[pos:])
elif mode == "mul" and f == "mul":
mulResult = exeOpr(mulResult, opr, c)
return calc(str(mulResult) + expr[e:])
elif mode == "mul" and f == "add":
mulResult = exeOpr(mulResult, opr, c)
return calc(str(mulResult) + expr[e:])
elif f == None:
if mode == "add":
return exeOpr(addResult, opr, c)
else:
return exeOpr(mulResult, opr, c)
break

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