On Feb 17, 7:05 am, Wolfgang Draxinger <[EMAIL PROTECTED]> wrote: > nexes wrote: > > there is more data that needed to be assigned(i.e. a couple > > megs of data) it would be simpler (and more efficient) to > > do a compare rather then assigning all that data to an array, > > since you are only going to be using 1 value and the rest > > of the data in the array is useless. > > Ouch, you're hurting my brain... > > Okay, let's state this first: A lookup table IS the more > efficient solution. > > Somehow you seem to think, that a lookup table will require more > resources (memory I guess you thought) than a sequence of > comparisons. However you didn't take into account, that the > program code itself requires memory, too (for the operation > codes). For the sake of simplicity let's assume, that every > operation code and every variable takes exactly one "blurp" of > memory (what a blurp is exactly we leave undefined here, it's > just the smallest unit of memory our hypothetically machine can > process). > > Let's also assume, that you can pack everything, that happens in > a operation into a single opcode, which by definition fits into > a blurp: In case of a comparision this would include the value, > to operand to compare to and where to jump if the comparision > satisfies. Assignment is a own operation, thus it doesn't go > into the compare operation (it would be rather inefficient to > try to fit all possible things you do after a comparision into a > huge variety of comparision opcodes). > > In the following example each line should be considered as a > single operation. And let's assume that every operation takes one > tick to execute. Labels take no memory. A table is a label with > a number of values following. The label of the table itself > doesn't consume memory either, however a compiler/assembler > might decide to insert a relative jump just before the table so > that the program doesn't run into it. > > The operations are like this: > > compare $variable $value $jump_label > > assign $variable $value | $table[$index] > > jump $jump_label > > $jump_label: > > $table $number_of values: > $value > $value > ... > > The shortest compare/assign program for the month problem would > look like this > > compare month 1 month_31 > compare month 2 month_28 > compare month 3 month_31 > compare month 4 month_30 > ... > compare month 11 month_30 > compare month 12 month_31 > month_30: assign $days 30 > jump end > month_31: assign $days 31 > jump end > month_28: assign $days 28 > end: > > This program consists of 12+5 = 18 operations. There are no > tables, to the program consumes only those 18 blurps. Running > time is between 3 and 14 ticks. > > Now let's have a look on the table based variant > > days_in_month 12: > 31 > 30 > 28 > 31 > ... > 30 > 31 > assign $days days_in_month[$month] > > This program consists of 2 operations (table jump and assignment) > and 12 values. This makes a memory consumption of 12+2 = 14 > blurps. Running time is always 2 ticks. You see that the table > based approach both takes less memory (14 blurbs vs. 18 blurps) > and always runs faster (2 ticks vs. 3...14 ticks) than the > comparison/jump/assignment based solution. > > And that was just this artificial example. In most real world > applications the operands to a comparision would take additional > memory, at it's best the opcode would also address some > register(s), but not more, so this would also add the > instructions to fill the registers with the values. > > Now you might want to state the above examples are indeed > artificial. But OTOH they're not so artificial then again. Most > assembler code works that way and if you take the AVR > architecture it almost looks that way there. And if you're > thinking of Python you can bet, that an opcode will not contain > all information, but have the operands encoded in additional > values, consuming memory. > > Look up Table is more efficient: Debate solved. > > Wolfgang Draxinger > -- > E-Mail address works, Jabber: [EMAIL PROTECTED], ICQ: 134682867
m - 2 and 30 + bool(1 << m & 5546) or 28 sub x' x 2 cjump x' 0 $but_28 shift x'' 1 x and x'' 5546 cjump x'' 0 $not_31 add x'' 1 $not_31: add x'' 30 cjump x'' 0 $but_28 assign $days x'' jump $end $but_28: assign $days 28 jump $end $end: 22 blups, 4, 9, 10, 12, or 13 ticks. -- http://mail.python.org/mailman/listinfo/python-list
