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# list comprehension syntax..?

 P: n/a Sorry for a simple question- but I don't understand how to parse this use of a list comprehension. The "or" clauses are odd to me. It also seems like it is being overly clever (?) in using a lc expression as a for loop to drive the recursion. Thanks for any insight! Gregory ------------------------- # http://markbyers.com/moinmoin/moin.c...stSudokuSolver def solve(board): i=board.find('0') # find next open cell if i<0: # done if none... print board; exit("Done") [ m in [(i-j)%9*(i/9^j/9)*(i/27^j/27|i%9/3^j%9/3) or board[j] for j in range(81) ] or solve(board[:i]+m+board[i+1:]) for m in'%d'%5**18 ] ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- Aug 1 '06 #1
4 Replies

 P: n/a Gregory Guthrie wrote: Sorry for a simple question- but I don't understand how to parse this use of a list comprehension. The "or" clauses are odd to me. It also seems like it is being overly clever (?) in using a lc expression as a for loop to drive the recursion. You are spot on there. It is a list comprehension, but the resulting list is just thrown away, so using a list comprehension is a complete waste of time serving only to confuse the issue. Presumably it saved the author a character or two. [ exp for var in seq ] when the result isn't used can be rewritten as: for var in seq: exp and: exp1 or exp2 when the result is thrown away is just: if not exp1: exp2 So: [ m in [(i-j)%9*(i/9^j/9)*(i/27^j/27|i%9/3^j%9/3) or board[j] for j in range(81) ] or solve(board[:i]+m+board[i+1:]) for m in'%d'%5**18 ] is equivalent to: inner = [(i-j)%9*(i/9^j/9)*(i/27^j/27|i%9/3^j%9/3) or board[j] for j in range(81) ] for m in '3814697265625': if m not in inner: solve(board[:i]+m+board[i+1:]) (That inner list comprehension doesn't depend on m, so it doesn't need to be reevaluated each time round the loop except, again, to save a few characters.) The '%d'%5**18 looks to be a silly way to iterate through all possible digits for m even though it does some of them twice while saving one character over writing range(1,10). The strange expression I called 'inner' is a list containing the string value board[j] if j is in the same row, column or block as i, or an integer for any other cells. So 'm not in inner' is true only if the value for m is not already used in that row column or block and is therefore a possible candidate for that location in the board. Aug 1 '06 #2

 P: n/a Gregory Guthrie wrote: Sorry for a simple question- but I don't understand how to parse this use of a list comprehension. The "or" clauses are odd to me. It also seems like it is being overly clever (?) in using a lc expression as a for loop to drive the recursion. Thanks for any insight! Gregory ------------------------- # http://markbyers.com/moinmoin/moin.c...stSudokuSolver def solve(board): i=board.find('0') # find next open cell if i<0: # done if none... print board; exit("Done") [ m in [(i-j)%9*(i/9^j/9)*(i/27^j/27|i%9/3^j%9/3) or board[j] for j in range(81) ] or solve(board[:i]+m+board[i+1:]) for m in'%d'%5**18 ] The "or" clause (as you call it) has nothing to do with the list comprehension. The syntax being used here is [ big_expression for m in something] *and* the big_expression contains an "or" OPERATOR, with a complex_expression on one side and solve(...) on the other, like this complex_expression or solve(...) *and* the complex_expression contains a nested list comprehension like this m in nested_lc *and* nested_lc is [ugly_expression for j in range(81)] *and* ugly_expression contains another "or" OPERATOR with ... *and* sigh... Whoever put this expression together has made something that is completely unreadable, mostly unmaintainable, and not noticeably more efficient than code that is readable and maintainable. Moreover, all the work of creating the outer list seems to be wasted since that list is just thrown out. This is worse than "overly clever". Loops are for looping, list comprehension is for building lists. It is bad programming practice to use list comprehension for looping. Hope that helps you, Gary Herron > ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- Aug 1 '06 #3

 P: n/a Very helpful, thanks!! So I see that it parses as: m='1' a="asdf" b="1234" print [((m in a) or b) for m in '%d'%1234 ] I get it. Thanks, Greg "Duncan Booth" Sorry for a simple question- but I don't understand how to parse thisuse of a list comprehension.The "or" clauses are odd to me.It also seems like it is being overly clever (?) in using a lcexpression as a for loop to drive the recursion. You are spot on there. It is a list comprehension, but the resulting list is just thrown away, so using a list comprehension is a complete waste of time serving only to confuse the issue. Presumably it saved the author a character or two. [ exp for var in seq ] when the result isn't used can be rewritten as: for var in seq: exp and: exp1 or exp2 when the result is thrown away is just: if not exp1: exp2 So: [ m in [(i-j)%9*(i/9^j/9)*(i/27^j/27|i%9/3^j%9/3) or board[j] for j in range(81) ] or solve(board[:i]+m+board[i+1:]) for m in'%d'%5**18 ] is equivalent to: inner = [(i-j)%9*(i/9^j/9)*(i/27^j/27|i%9/3^j%9/3) or board[j] for j in range(81) ] for m in '3814697265625': if m not in inner: solve(board[:i]+m+board[i+1:]) (That inner list comprehension doesn't depend on m, so it doesn't need to be reevaluated each time round the loop except, again, to save a few characters.) The '%d'%5**18 looks to be a silly way to iterate through all possible digits for m even though it does some of them twice while saving one character over writing range(1,10). The strange expression I called 'inner' is a list containing the string value board[j] if j is in the same row, column or block as i, or an integer for any other cells. So 'm not in inner' is true only if the value for m is not already used in that row column or block and is therefore a possible candidate for that location in the board. ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- Aug 1 '06 #4

 P: n/a Gary Herron wrote: Gregory Guthrie wrote: >>Sorry for a simple question- but I don't understand how to parse this use ofa list comprehension.The "or" clauses are odd to me.It also seems like it is being overly clever (?) in using a lc expression asa for loop to drive the recursion.Thanks for any insight!Gregory-------------------------# http://markbyers.com/moinmoin/moin.c...stSudokuSolverdef solve(board):i=board.find('0') # find next open cellif i<0: # done if none... print board; exit("Done")[ m in [(i-j)%9*(i/9^j/9)*(i/27^j/27|i%9/3^j%9/3) or board[j] for j in range(81) ] or solve(board[:i]+m+board[i+1:]) for m in'%d'%5**18 ] The "or" clause (as you call it) has nothing to do with the list comprehension. The syntax being used here is [ big_expression for m in something] *and* the big_expression contains an "or" OPERATOR, with a complex_expression on one side and solve(...) on the other, like this complex_expression or solve(...) *and* the complex_expression contains a nested list comprehension like this m in nested_lc *and* nested_lc is [ugly_expression for j in range(81)] *and* ugly_expression contains another "or" OPERATOR with ... *and* sigh... Whoever put this expression together has made something that is completely unreadable, mostly unmaintainable, and not noticeably more efficient than code that is readable and maintainable. Moreover, all the work of creating the outer list seems to be wasted since that list is just thrown out. This is worse than "overly clever". Loops are for looping, list comprehension is for building lists. It is bad programming practice to use list comprehension for looping. Isn't it an advantage considering speed of the execution? I have just compared the speed of an explicit loop with the speed of a list comprehension doing the same thing in another context to see it is much better to use the latter. In board solvers speed is vital, so isn't it a good practice to use a list comprehension for looping in this context, anyway? Claudio > Hope that helps you, Gary Herron >>----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups----= East and West-Coast Server Farms - Total Privacy via Encryption =---- Aug 1 '06 #5

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