python - How to go back to a specific line - 


print "1) add" print "2) substract" print "3) multiply" print "4) divide" print "5) exit"  x=input("choose operation: ") y=input("how many numbers need operate: ") op=1 lista=[]  while y>0:     a=input("value"+" "+str(op)+": ")     litlist=[a]     lista=lista+litlist     y=y-1     op=op+1  while x!=5:     if x==1:         b=0         n in lista:             b=b+n         print b     elif x==2:         b=0         n in lista:             if lista[0]==n:                 b=b+n             else:                 b=b-n         print b     elif x==3:         b=1         n in lista:             b=b*n         print b     elif x==4:         b=1         n in lista:             if lista[0]==n:                 b=b*n             else:                 b=b/float(n)         print b

this program designed to:

  • first ask operation user wants do
  • then ask how many numbers need operated
  • input numbers
  • and print result

i want ask operation needs done, how many numbers need operated again after printing result. input numbers , on.

i know can use input in while make ask numbers again , stop loop there 2 whiles , doesnt allow me ask y again, x. cool able go line 6 , start over
answers :)

you're looking goto statement. in 1968, dijkstra wrote famous paper called go statement considered harmful explained why should not looking goto.

the right thing structure code.

the simplest change this:

print "1) add" print "2) substract" print "3) multiply" print "4) divide" print "5) exit" while true:     x=input("choose operation: ")     # ...

however, can better. take isolated pieces of code , separate them functions can call. if 2 (or, in case, four) pieces of code identical, abstract them single function takes parameter, instead of repeating same code 4 times. , on.

but really, without functions, can rid of of repetition:

import operator  print "1) add" print "2) substract" print "3) multiply" print "4) divide" print "5) exit" while true:     x=input("choose operation: ")     if x==5:         break     y=input("how many numbers need operate: ")     operands=[input('value {}'.format(i+1)) in range(count)]     if x==1:         op, value = operator.add, 0     elif x==2:         op, value = operator.sub, 0     elif x==3:         op, value = operator.mul, 1     elif x==4:         op, value = operator.truediv, 1     operand in operands:         value = op(value, operand)     print value

the reason had import operator above add, sub, etc. functions. these trivial, write them yourself:

def add(x, y):     return x+y # etc.

then, instead of this:

op, value = operator.add, 0

… this:

op, value = add, 0

… , same other three.

or can define them in-place lambda:

op, value = (lambda x, y: x+y), 0

still, shouldn't either of these. simple defining add, sub, mul, , truediv is, it's simpler not define them. python comes "batteries included" reason, , if you're avoiding using them, you're making life (and lives of has read, maintain, etc. code) harder absolutely no reason.


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