Holding until next value change

[Arshpreet Singh]

On Saturday, 20 August 2016 11:38:03 UTC+5:30, Steve D'Aprano  wrote:
> On Sat, 20 Aug 2016 02:53 pm, Arshpreet Singh wrote:
> 
> > I am writing a function as main_call() which is continuously producing
> > values. (+ve or -ve) I want to print on screen only for first +ve value
> > and hold until -ve value comes around. here is my code:
> > 
> > 
> > def main_call():
> >     while True:
> >         yield strategy()
> >  
> > for value in main_call():
> >     if(value>0):
> >         print '+ve'
> >     elif(value>0):
> >         print '-ve'
> >     else:
> >         pass
> > 
> > Do I need to use threads or processes?
> 
> Only if you want a major headache.
> 
> 
> Your code doesn't do what you want it to do. It prints "+ve" every time it
> sees a positive value, not just the first time.
> 
> One solution to this is to think of a state machine: your machine starts off
> in the state:
> 
> (1) Ignore negative values, print the first positive value you see, 
>     then change to the next state.
> 
> The second state is:
> 
> (2) Ignore positive values, print the first negative value you see,
>     then change to the next state.
> 
> The third state is unspecified. You don't know say what it is, so I'm going
> to guess that you change to a third state:
> 
> (3) Don't ignore anything, print every value you see.
> 
> 
> So here are our three machines:
> 
> def ignore_negative(x):
>     if x < 0:
>         return False
>     else:
>         print("+ve")
>         return True
> 
> def ignore_positive(x):
>     if x > 0:
>         return False
>     else:
>         print("-ve")
>         return True
> 
> def ignore_nothing(x):
>     if x > 0:
>         print("+ve")
>     else:
>         print("-ve")
>     return False
> 
> 
> Here is a table that specifies the changes in state:
> 
> TABLE = { # current state: next state
>     ignore_negative: ignore_positive,
>     ignore_positive: ignore_nothing,
>     }
> 
> 
> And some code to drive it:
> 
> 
> state = ignore_negative  # DON'T call the function yet
> for value in main_call():
>     print(value)  # for testing
>     if state(value):
>         print("changing state")
>         state = TABLE[state]
Hi Steve, my third state is if x==0 , I tired the same code but here is output I am getting, so we are changing the state even it is printing out the +ve as well as -ve values

8.7664352813e-05
+ve
changing state
8.7664352813e-05
8.7664352813e-05
8.7664352813e-05
8.7664352813e-05
8.7664352813e-05
8.76628243948e-05
8.30490294982e-05
8.30490294982e-05
6.45892873188e-05
6.45892873188e-05
6.45892873188e-05
6.45892873188e-05
-4.61232547941e-06
-ve
changing state
-4.61232547941e-06
-ve
-4.61232547941e-06
-ve
-4.61232547941e-06
-ve
-4.61232547941e-06
-ve
-4.61232547941e-06
-ve
-4.61232547941e-06
-ve
-4.61232547941e-06
-ve
-4.61232547941e-06
-ve
-4.61232547941e-06
-ve
-4.61232547941e-06
-ve
-4.61232547941e-06
-ve