beginner question (True False help)

[eschneider92]

I'm trying to create an option for the program to repeat if the user types 'y' or 'yes', using true and false values, or otherwise end the program. If anyone could explain to me how to get this code working, I'd appreciate it.

letters='abcdefghijklmn'
batman=True
def thingy():
print('type letter from a to n')
typedletter=input()
if typedletter in letters:
print('yes')
else:
print('no')
def repeat():
print('go again?')
goagain=input()
if goagain in ('y', 'yes'):
print('ok')
else:
print('goodbye')
batman=False
while batman==True:
thingy()
repeat()
print('this is the end')

 

[Joshua Landau]

I'm trying to create an option for the program to repeat if the user types 'y' or 'yes', using true and false values, or otherwise end the program. If anyone could explain to me how to get this code working, I'd appreciate it.

Always tell people what in particular you don't understand (*ducks*)
because it wasn't obvious what part of the problem you were unable to
fulfil.

letters='abcdefghijklmn'
batman=True
def thingy():
print('type letter from a to n')
typedletter=input()
if typedletter in letters:
print('yes')
else:
print('no')
def repeat():
print('go again?')
goagain=input()
if goagain in ('y', 'yes'):
print('ok')
else:
print('goodbye')
batman=False

This doesn't do what you want it to.

x = "old thing"

def change_x():
x = "new thing"

change_x()

print(x) # Not changed!

The solution is to put "global x" at the start of the function.

while batman==True:
thingy()
repeat()
print('this is the end')

Note that this isn't actually a good way to do it. Imagine you had
several hundred function -- would you really want to have an
equivalent number of names floating around that you have to look
after?

The solution is to make the functions simply return values:

x = "old thing"

def return_thing():
x = "new thing"
return "new thing" # You can do this in one line

x = return_thing() # Get the value from the function and change x with it


Does this make sense?

 

[eschneider92]

What I wanted to happen is when the user typed something other than 'y' or 'yes' after being asked 'go again?', the batman==False line would causethe program to stop asking anything and say 'this is the end'. Instead, what is happening is that the program just keeps going. I figured that after defining the function (thingy(), repeat()), that the while statement would repeat until the 'go again' user input was something other than 'y' or 'yes', and the batman==False part of the repeat() function would cause the 'while batman==True' part to become False and end. You probably answered my question and I'm too dumb to see it, but that's a slight elaboration on my problem.

 

[Dave Angel]

What I wanted to happen is when the user typed something other than 'y' or 'yes' after being asked 'go again?', the batman==False line would cause the program to stop asking anything and say 'this is the end'. Instead, what is happening is that the program just keeps going. I figured that after defining the function (thingy(), repeat()), that the while statement would repeat until the 'go again' user input was something other than 'y' or 'yes', and the batman==False part of the repeat() function would cause the 'while batman==True' part to become False and end. You probably answered my question and I'm too dumb to see it, but that's a slight elaboration on my problem.

When you assign a variable inside a function, it has no effect on a
global variable with similar name. In order to make it change the
global, you'd have needed the global declaration.

Try this:

var = 42
def myfunc():
var = 90


print "before:", var
myfunc()
print "after:", var

Now, change the function, by adding a declaration:

def myfunc():
global var
var = 90

and the result will change.

 

[wxjmfauth]

Le mercredi 7 août 2013 10:17:21 UTC+2, (e-mail address removed) a écrit :

I'm trying to create an option for the program to repeat if the user types 'y' or 'yes', using true and false values, or otherwise end the program. If anyone could explain to me how to get this code working, I'd appreciate it.



letters='abcdefghijklmn'

batman=True

def thingy():

print('type letter from a to n')

typedletter=input()

if typedletter in letters:

print('yes')

else:

print('no')

def repeat():

print('go again?')

goagain=input()

if goagain in ('y', 'yes'):

print('ok')

else:

print('goodbye')

batman=False

while batman==True:

thingy()

repeat()

print('this is the end')

-----------

Your loop is not very well organized. It should be
at the same time the "loop" and the "condition tester".
Compare your code with this and note the missing and
unnecessary "batman":

.... letters = 'abc'
.... c = input('letter: ')
.... while c in letters:
.... print('do stuff')
.... c = input('letter: ')
.... print('end, fin, Schluss')
.... letter: a
do stuff
letter: b
do stuff
letter: b
do stuff
letter: c
do stuff
letter: n
end, fin, Schlussletter: q
end, fin, Schluss


Variant
It is common to use a infinite loop and to break it
in order to end the job.

.... letters = 'abc'
.... while True:
.... c = input('letter: ')
.... if c not in letters:
.... print('end, fin, Schluss')
.... break
.... else:
.... print('do stuff')
.... letter: a
do stuff
letter: b
do stuff
letter: a
do stuff
letter: q
end, fin, Schluss


jmf

 

[Chris Angelico]

... letters = 'abc'
... while True:
... c = input('letter: ')
... if c not in letters:
... print('end, fin, Schluss')
... break
... else:
... print('do stuff')

Minor quibble: I don't like having a hard exit followed by an "else".
If the "if" branch will unconditionally quit the loop (with a break,
here, but could also be a return, a thrown exception, etc etc), I
would prefer to see the "else" removed and its code unindented one
level. Maybe this is just personal preference, though, learned from
assembly language programming where a "block if" looks something like
this:

; if x == y:
CMP x,y
JNZ .else
; Code for "x == y"
JMP .endif
..else:
; Code for "else"
..endif

Putting an unconditional departure in the "x == y" branch makes the
"JMP .endif" redundant.

ChrisA

 

[Terry Reedy]

Minor quibble: I don't like having a hard exit followed by an "else".

Whereas I tend to prefer to have the two alternatives cleanly marked as
alternatives by both being indented the same.

Many alternatives are not so trivial as the above. I remember reading
one snippet in the CPython codebase where the 'else' was omitted and the
if clause subdivided into about three paths. It took at least a minute
to determine that all paths terminated in such a way that there really
was an inplied else. How much easier it would have been to read the code
if the author had explicitly types the 'else'.

If the "if" branch will unconditionally quit the loop (with a break,
here, but could also be a return, a thrown exception, etc etc), I
would prefer to see the "else" removed and its code unindented one
level. Maybe this is just personal preference, though, learned from
assembly language programming where a "block if" looks something like
this:

; if x == y:
CMP x,y
JNZ .else
; Code for "x == y"
JMP .endif
.else:
; Code for "else"
.endif

Putting an unconditional departure in the "x == y" branch makes the
"JMP .endif" redundant.

Python is not assembly ;-). 3.3 effectively ignores the extraneous
'else:'. Either way, if the condition is false, control jumps to the
second print. For what little it matters, the bytecode is the same length.

def f():
while True:
if a:
b = 1
break
else:
b = 2
2 0 SETUP_LOOP 25 (to 28)

3 >> 3 LOAD_GLOBAL 0 (a)
6 POP_JUMP_IF_FALSE 19

4 9 LOAD_CONST 1 (1)
12 STORE_FAST 0 (b)

5 15 BREAK_LOOP
16 JUMP_ABSOLUTE 3

7 >> 19 LOAD_CONST 2 (2)
22 STORE_FAST 0 (b)
25 JUMP_ABSOLUTE 331 RETURN_VALUE

def f():
while True:
if a:
b = 1
break
b = 2
2 0 SETUP_LOOP 25 (to 28)

3 >> 3 LOAD_GLOBAL 0 (a)
6 POP_JUMP_IF_FALSE 19

4 9 LOAD_CONST 1 (1)
12 STORE_FAST 0 (b)

5 15 BREAK_LOOP
16 JUMP_FORWARD 0 (to 19)

6 >> 19 LOAD_CONST 2 (2)
22 STORE_FAST 0 (b)
25 JUMP_ABSOLUTE 331 RETURN_VALUE

 

[wxjmfauth]

Le jeudi 8 août 2013 22:29:00 UTC+2, Terry Reedy a écrit :

Whereas I tend to prefer to have the two alternatives cleanly marked as

alternatives by both being indented the same.



Many alternatives are not so trivial as the above. I remember reading

one snippet in the CPython codebase where the 'else' was omitted and the

if clause subdivided into about three paths. It took at least a minute

to determine that all paths terminated in such a way that there really

was an inplied else. How much easier it would have been to read the code

if the author had explicitly types the 'else'.















Python is not assembly ;-). 3.3 effectively ignores the extraneous

'else:'. Either way, if the condition is false, control jumps to the

second print. For what little it matters, the bytecode is the same length..



def f():

while True:

if a:

b = 1

break

else:

b = 2




2 0 SETUP_LOOP 25 (to 28)



3 >> 3 LOAD_GLOBAL 0 (a)

6 POP_JUMP_IF_FALSE 19



4 9 LOAD_CONST 1 (1)

12 STORE_FAST 0 (b)



5 15 BREAK_LOOP

16 JUMP_ABSOLUTE 3



7 >> 19 LOAD_CONST 2 (2)

22 STORE_FAST 0 (b)

25 JUMP_ABSOLUTE 3


31 RETURN_VALUE



def f():

while True:

if a:

b = 1

break

b = 2




2 0 SETUP_LOOP 25 (to 28)



3 >> 3 LOAD_GLOBAL 0 (a)

6 POP_JUMP_IF_FALSE 19



4 9 LOAD_CONST 1 (1)

12 STORE_FAST 0 (b)



5 15 BREAK_LOOP

16 JUMP_FORWARD 0 (to 19)



6 >> 19 LOAD_CONST 2 (2)

22 STORE_FAST 0 (b)

25 JUMP_ABSOLUTE 3


31 RETURN_VALUE

-----

The problem of this guy is not at this level.
His problem is more simply, he most probably
does not understand how to build a correct,
proper loop.

"What I wanted to happen is when the user typed something ...
.... would cause the program to stop ... "


jmf

 

[eschneider92]

This is what I ended up with btw. Does this insult anyone's more well attuned Python sensibilities?

letters='abcdefghijkl'
def repeat():
print('wanna go again?')
batman=input()
if batman in ('y','yes'):
main()
else:
return
def main():
print('guess a letter')
batman=input()
if batman in letters:
print('ok that letter was in letters')
repeat()
else:
print('asdasdasd')
repeat()
main()
print('how ya doin')

 

[eschneider92]

This is what I ended up with btw. Does this insult anyone's more well-attuned Pythonic sensibilities?

letters='abcdefghijkl'
def repeat():
print('wanna go again?')
batman=input()
if batman in ('y','yes'):
main()
else:
return
def main():
print('guess a letter')
batman=input()
if batman in letters:
print('ok that letter was in letters')
repeat()
else:
print('asdasdasd')
repeat()
main()
print('how ya doin')

 

[Joshua Landau]

This is what I ended up with btw. Does this insult anyone's more well attuned Python sensibilities?

....

Yes.

You didn't listen to any of the advice we've been giving you. You've
had *much* better answers given than this.


Start from the top.

We need letters, so define that:

letters = "abcdefghijkl"

We then want to loop, possibly forever. A good choice is a "while" loop.

# True is always True, so will loop forever
while True:

We then want to ask for a letter. We want to use "input". Write
"help(input)" in the Python Shell and you get
Help on built-in function input in module builtins:

input(...)
input([prompt]) -> string

Read a string from standard input. The trailing newline is stripped.
If the user hits EOF (Unix: Ctl-D, Windows: Ctl-Z+Return),
raise EOFError.
On Unix, GNU readline is used if enabled. The prompt string, if given,
is printed without a trailing newline before reading.

So our line should be:

letter = input("Type a letter from 'a' to 'n' in the alphabet: ")

Then we want to test if it's on of our letters:

if letter in letters:

And if so we want to say something positive:

print("That's right.")

If not we want to say something negative:

else:
print("That's wrong.")

And then we want to ask if we should go again:

go_again = input("Do you want to do this again? ")

If the response is "y" or "yes", we want to continue looping. The
while loop will do that automatically, so we can do nothing in this
circumstance.

If the response in *not* "y" or "yes", we want to stop:

if go_again not in ("y", "yes"):
break


That's it. No need to complicate things. Just take it one step at a time.

 

[eschneider92]

I don't understand any of the advice any of you have given.

 

[eschneider92]

Thanks, though me not utilizing any of the other advice wasn't from lack of trying; I couldn't understand any of it. I get it now that I have a corrrect example code in front of me.

 

[Joshua Landau]

I don't understand any of the advice any of you have given.

What about it don't you understand? Pick a sentence you don't
understand and throw it back at us. If you understand all the
sentences but not how they come together, say so. If there's a leap
that you don't understand, say that you don't get it.

We've tried rephrasing things a few ways but without any interaction
we can't really help.

---

You have said "I figured that ... the batman==False part of the
repeat() function would cause the 'while batman==True' part to become
False and end."

We have said this is untrue. The "batman = False" inside the function
does not affect "batman" outside of the function. You need to put
"global batman" in the function for it to change "batman" on a global
scope.

You've not once explained what part of this explanation confuses you.

 

[eschneider92]

What does global mean?

 

[eschneider92]

(I forgot to post this with my last post.)
Also, I don't understand any part of the following example, so there's no specific line that's confusing me. Thanks for the help btw.

var = 42
def myfunc():
var = 90

print "before:", var
myfunc()
print "after:", var

def myfunc():
global var
var = 90

 

[eschneider92]

(I forgot to post this with my last post.)
Also, I don't understand any part of the following example, so there's no specific line that's confusing me. Thanks for the help btw.

var = 42
def myfunc():
var = 90

print "before:", var
myfunc()
print "after:", var

def myfunc():
global var
var = 90

 

[MRAB]

(I forgot to post this with my last post.)
Also, I don't understand any part of the following example, so there's no specific line that's confusing me. Thanks for the help btw.

You don't understand _any_ of it?

var = 42

Here you're assigning to 'var'. You're not in a function, so 'var' is a
global variable.

def myfunc():
var = 90

Here you're assigning to 'var'. If you assign to a variable anywhere in
a function, and you don't say that that variable is global, then it's
treated as being local to that function, and completely unrelated to
any other variable outside that function.

print "before:", var
myfunc()
print "after:", var

def myfunc():
global var
var = 90

Here you're assigning to 'var', but this time you've declared that it's
global, so you're assigning to the global variable called 'var'.

 

[eschneider92]

I'm sorry, but I still don't understand how it applies to my problem. Thanks for everyone's patience.

 

[Joshua Landau]

What does global mean?

Python has "scopes" for its variables. Most programming languages do.
A "scope" is a restriction on where variables exist -- they exist only
within the scope.

This can be seen in this example:

def function():
# A new "scope" is made when you enter a function
variable = 100

function()
print(variable)
# Error, as variable doesn't exist outside of "function"'s scope

There are lots of different "scopes" in code. Every function has one,
and there are some more too.

One of the scopes is the "global" scope. This is the scope *outside*
of all the functions and other scopes. Everything in the file is
within this sope:

# Make in global scope
variable = 100

def function():
# Works because we're inside the global scope
print(variable)

# Prints "100"
function()

So "a = b" inside the function applies to the function's scope, but
when accessing variables (such as "print(variable)") it will look in
all of the outer scopes too.

If you want to write "a = b" inside the function and change the global
scope, you need to say that "a" refers to the "a" in the global scope.
You do that like this:

def function():
# "variable" is in the global scope, not the functions'
global variable
variable = 100

function()
# Prints "100"
print(variable)


Does that help you understand what "global" means?