How do you make a loop where the program keeps asking for
an integer or float until it gets one? (No letters or other input.)
an integer or float until it gets one? (No letters or other input.)
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Re: Input Types
Here's a function that can do the job.
def typed_input(pro mpt="", convert=int, catch=ValueErro r,
eprompt="Invali d input."):
while 1:
s = raw_input(promp t)
try:
return convert(s)
except catch:
print eprompt
Usage:[color=blue][color=green][color=darkred]
>>> typed_input("En ter an integer: ")[/color][/color][/color]
Enter an integer: asdf
Invalid input
Enter an integer: 3
3[color=blue][color=green][color=darkred]
>>> typed_input("En ter a number: ", float)[/color][/color][/color]
Enter a number:
Invalid input
Enter a number: 3.14
3.1400000000000 001
How does it work?
1. The "while 1" loop is repeated until the "return" statement is
successfully executed.
2. int(s) and float(s) convert a string argument s to the specified
type, or raise the ValueError exception
3. When there is a ValueError exception, the "error prompt" is printed,
and the while loop returns to the top to try again.
You can also write your own "convert" function to restrict values to a
range, etc:
[color=blue][color=green][color=darkred]
>>> def int_0_100(s):[/color][/color][/color]
.... i = int(s)
.... if i < 0 or i > 100:
.... raise ValueError # out of range
.... return i
....[color=blue][color=green][color=darkred]
>>> typed_input("En ter a number from 0 to 100: ", int_0_100)[/color][/color][/color]
Enter a number from 0 to 100: asdf
Invalid input.
Enter a number from 0 to 100: 101
Invalid input.
Enter a number from 0 to 100: 37
37
You can also specify the catch= or eprompt= arguments to change the
exception that is caught or the error prompt string.
Jeff
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Re: Input Types
EAS wrote:[color=blue]
> How do you make a loop where the program keeps asking for
> an integer or float until it gets one? (No letters or other input.)[/color]
import sys # We need sys.stdin for input
while True: # Infinite loop. We break out of it after getting a float.
try: # For catching the exception of getting an invalid float.
num = float(sys.stdin .readline().rst rip("\r\n")) # Try
# parsing it as a float.
break # If we reach this line, it's valid. Exit the loop
except ValueError: # It's an invalid float.
pass # Just let the loop run again.
Comment
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Re: Input Types
Jeff Epler wrote:[color=blue]
> Here's a function that can do the job.
>
> def typed_input(pro mpt="", convert=int, catch=ValueErro r,
> eprompt="Invali d input."):
> while 1:
> s = raw_input(promp t)
> try:
> return convert(s)
> except catch:
> print eprompt
>[/color]
You can send functions (int, float) and exceptions as parameters?
Amazing.
[color=blue]
> Usage:[color=green][color=darkred]
>>>>typed_input ("Enter an integer: ")[/color][/color]
>
> Enter an integer: asdf
> Invalid input
> Enter an integer: 3
> 3
>[color=green][color=darkred]
>>>>typed_input ("Enter a number: ", float)[/color][/color]
>
> Enter a number:
> Invalid input
> Enter a number: 3.14
> 3.1400000000000 001
>
> How does it work?
>
> 1. The "while 1" loop is repeated until the "return" statement is
> successfully executed.
>
> 2. int(s) and float(s) convert a string argument s to the specified
> type, or raise the ValueError exception
>
> 3. When there is a ValueError exception, the "error prompt" is printed,
> and the while loop returns to the top to try again.
>
> You can also write your own "convert" function to restrict values to a
> range, etc:
>
>[color=green][color=darkred]
>>>>def int_0_100(s):[/color][/color]
>
> ... i = int(s)
> ... if i < 0 or i > 100:
> ... raise ValueError # out of range
> ... return i
> ...
>[color=green][color=darkred]
>>>>typed_input ("Enter a number from 0 to 100: ", int_0_100)[/color][/color]
>
> Enter a number from 0 to 100: asdf
> Invalid input.
> Enter a number from 0 to 100: 101
> Invalid input.
> Enter a number from 0 to 100: 37
> 37
>
> You can also specify the catch= or eprompt= arguments to change the
> exception that is caught or the error prompt string.
>
> Jeff
>[/color]
Comment
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Re: Input Types
On Sat, May 15, 2004 at 08:23:51PM +0200, moma wrote:[color=blue]
> You can send functions (int, float) and exceptions as parameters?
> Amazing.[/color]
You sure can.
numbers, sequences, dicts, functions, exceptions, classes, modules: all
these things are objects, and you can pass any object you like as a
parameter. Of course, not all object types *make sense* everywhere!
One "exception" to this rule is thinking that print is a function, and
trying to write
def send_to(value, action):
return action(value)
def f(x): return x*x
[color=blue][color=green][color=darkred]
>>> send_to(3, f) # square the argument: works[/color][/color][/color]
9[color=blue][color=green][color=darkred]
>>> send_to(3, print) # Print the argument: error[/color][/color][/color]
File "<stdin>", line 1
send_to(3, print)
^
SyntaxError: invalid syntax
because print is a statement, not a function, this isn't really an
exception to the rule at all.
Jeff
Comment
Comment