Is there no switch function in Python

[Rudi Hansen]

I dont seem to be able to find the switch statement in Python.

I would like to be able to do

switch(var)
case 1 :
print "var = 1"
case 2:
print "var = 2"

But it seems that i have to do.

if(var=1)
print "var =1"
elseif(var=2)
print "var=2"

Is ther no easier way??

[Marius Bernklev]

"Rudi Hansen" <rs************ **@pobox.dk> writes:

I would like to be able to do

switch(var)
case 1 :
print "var = 1"
case 2:
print "var = 2"

But it seems that i have to do.

if(var=1)
print "var =1"
elseif(var=2)
print "var=2"

Is ther no easier way??

one way is using dicts:

swoosh = {1: "var = 1", 2: "var = 2"}
print swoosh[var]
--
Marius Bernklev

[Alex Martelli]

Rudi Hansen <rs************ **@pobox.dk> wrote:

I dont seem to be able to find the switch statement in Python.
Right, there isn't one.

I would like to be able to do

switch(var)
case 1 :
print "var = 1"
case 2:
print "var = 2"

But it seems that i have to do.

if(var=1)
print "var =1"
elseif(var=2)
print "var=2"

Is ther no easier way??

several, starting with

if var in (1,2): print 'var = %s' % var

The most Pythonic idiom, when you have to do something substantial in
each branch of the switch, is a dictionary of callables -- in this toy
case it might be:

switch = {1: lambda: sys.stdout.writ e('var=1\n'),
2: lambda: sys.stdout.writ e('var=2\n'), }
switch.get(var, lambda: '')()

An if/elif tree is also fine, though the syntax is not as you think...:

if var == 1:
print 'var is one'
elif var == 2:
print 'var is two'

Alex

[Sion Arrowsmith]

In article <CR************ ********@news00 0.worldonline.d k>,
Rudi Hansen <rs************ **@pobox.dk> wrote:

I dont seem to be able to find the switch statement in Python.

I would like to be able to do

switch(var)
case 1 :
print "var = 1"
case 2:
print "var = 2"
5 lines, 55 characters including the print statements.

But it seems that i have to do.

if(var=1)
print "var =1"
elseif(var=2 )
print "var=2"
4 lines, 52 characters (once corrected) including the print
statements (as written in the first example).
Is ther no easier way??

In what way do you want it to be "easier"?

--
\S -- si***@chiark.gr eenend.org.uk -- http://www.chaos.org.uk/~sion/
___ | "Frankly I have no feelings towards penguins one way or the other"
\X/ | -- Arthur C. Clarke
her nu becomeþ se bera eadward ofdun hlæddre heafdes bæce bump bump bump

[Roy Smith]

In article <1Z*******@news .chiark.greenen d.org.uk>,
Sion Arrowsmith <si***@chiark.g reenend.org.uk> wrote:

In article <CR************ ********@news00 0.worldonline.d k>,
Rudi Hansen <rs************ **@pobox.dk> wrote:

I dont seem to be able to find the switch statement in Python.

I would like to be able to do

switch(var)
case 1 :
print "var = 1"
case 2:
print "var = 2"

5 lines, 55 characters including the print statements.

It's even longer if you include the required "break" statement at the
end of case 1 (assuming you're talking about a C-style switch statement
with fall-through cases).

That being said, there are times when I miss switch. For some kinds of
multi-branch logic, I think it expresses the meaning better than a
string of if's. But it's hardly necessary.

The real reason, IMHO, switch existed in C was because it would let the
compiler build very efficient jump tables for switches with many cases.
Imagine switching on the ascii value of a character and having a 128
different cases. The compiler could build a 128 entry jump table and
the switch statement would compile down to a single machine instruction.

[Isaac To]

>>>>> "Roy" == Roy Smith <ro*@panix.co m> writes:

Roy> The real reason, IMHO, switch existed in C was because it
Roy> would let the compiler build very efficient jump tables for
Roy> switches with many cases. Imagine switching on the ascii
Roy> value of a character and having a 128 different cases. The
Roy> compiler could build a 128 entry jump table and the switch
Roy> statement would compile down to a single machine instruction.

C switch is much more flexible than one would expect. It is not just
a cheap replacement of a sequence of if-then-else. E.g., in an
exercise of "The C++ Programming Language" of Bjarne Stroustrup, you
can see the following example code:

void send(int *to, int *from, int count) {
int n = (count + 7) / 8;
switch (count % 8) {
case 0: do { *to++ = *from++;
case 7: *to++ = *from++;
case 6: *to++ = *from++;
case 5: *to++ = *from++;
case 4: *to++ = *from++;
case 3: *to++ = *from++;
case 2: *to++ = *from++;
case 1: *to++ = *from++;
} while (--n > 0);
}
}

Note that this reduces the number of branch statements to execute
8-folds for any compiler but the smartest (as compared to the simplest
code "do { *to++ = *from++ } while (--count > 0);").

I'd instead guess that C has switch because at the times before C,
people code in assembly, and those tricks are popular. The design of
most programming language represents the norm of coding at that time.

Regards,
Isaac.

[Cameron Laird]

In article <ro************ ***********@rea der1.panix.com> ,
Roy Smith <ro*@panix.co m> wrote:

[Roy Smith]

Isaac To <ik****@netscap e.net> wrote:

C switch is much more flexible than one would expect. It is not just
a cheap replacement of a sequence of if-then-else. E.g., in an
exercise of "The C++ Programming Language" of Bjarne Stroustrup, you
can see the following example code:

void send(int *to, int *from, int count) {
int n = (count + 7) / 8;
switch (count % 8) {
case 0: do { *to++ = *from++;
case 7: *to++ = *from++;
case 6: *to++ = *from++;
case 5: *to++ = *from++;
case 4: *to++ = *from++;
case 3: *to++ = *from++;
case 2: *to++ = *from++;
case 1: *to++ = *from++;
} while (--n > 0);
}
}

La-de-dah, Bjarne discovers loop unrolling. You don't need a switch
statement to unroll loops.

Hold on a second. Cases 1 through 7 jump into the middle of the do
loop!? Pardon me while I barf. I didn't even know that was legal.
That's the kind of code that gives C++ a bad name.

[Jaime Wyant]

Don't blame c++. This is a relic of 'C'.

http://catb.org/~esr/jargon/html/D/Duffs-device.html

On Fri, 10 Sep 2004 11:16:12 -0400, Roy Smith <ro*@panix.co m> wrote:

Isaac To <ik****@netscap e.net> wrote:

C switch is much more flexible than one would expect. It is not just
a cheap replacement of a sequence of if-then-else. E.g., in an
exercise of "The C++ Programming Language" of Bjarne Stroustrup, you
can see the following example code:

void send(int *to, int *from, int count) {
int n = (count + 7) / 8;
switch (count % 8) {
case 0: do { *to++ = *from++;
case 7: *to++ = *from++;
case 6: *to++ = *from++;
case 5: *to++ = *from++;
case 4: *to++ = *from++;
case 3: *to++ = *from++;
case 2: *to++ = *from++;
case 1: *to++ = *from++;
} while (--n > 0);
}
}

La-de-dah, Bjarne discovers loop unrolling. You don't need a switch
statement to unroll loops.

Hold on a second. Cases 1 through 7 jump into the middle of the do
loop!? Pardon me while I barf. I didn't even know that was legal.
That's the kind of code that gives C++ a bad name.
--
http://mail.python.org/mailman/listinfo/python-list

[Peter Otten]

Isaac To wrote:

a cheap replacement of a sequence of if-then-else. E.g., in an
exercise of "The C++ Programming Language" of Bjarne Stroustrup, you
can see the following example code:

void send(int *to, int *from, int count) {
int n = (count + 7) / 8;
switch (count % 8) {
case 0: do { *to++ = *from++;
case 7: *to++ = *from++;
case 6: *to++ = *from++;
case 5: *to++ = *from++;
case 4: *to++ = *from++;
case 3: *to++ = *from++;
case 2: *to++ = *from++;
case 1: *to++ = *from++;
} while (--n > 0);
}
}

This is otherwise known a Duff's device:

http://catb.org/~esr/jargon/html/D/Duffs-device.html

Peter