Hump-day puzzler.

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Matt Fig
Matt Fig 2011-2-16
If you have seen this before, please let others figure it out!
if (BLANK)
disp('I Love ')
else
disp('MATLAB')
end
What can replace BLANK to get the print-out (exactly): I Love MATLAB
How many solutions are there? For extra pride (or pain), how long did it take you to get it?
  9 个评论
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Ned Gulley
Ned Gulley 2011-2-18
Matt, your puzzler got a mention on Loren's blog. http://blogs.mathworks.com/loren/2011/02/18/hump-day-puzzler-on-matlab-answers/
Matt Fig
Matt Fig 2011-2-18
Cool! Thanks for the heads up, Ned.

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采纳的回答

Walter Roberson
Walter Roberson 2011-2-16
~fprintf('I Love ')
Solved in... a small number of seconds.
  8 个评论
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Jan
Jan 2011-2-23
@Oliver: "help fprintf" tells, that FPRINTF replies the number of printed characters. "help not" explains, that 0 is replied if the argument is not zero. Finally "help if" states, that the ELSE branch is executed, if the argument of IF is 0.
Andrew Newell
Andrew Newell 2011-2-24
In case that is not clear: >>n = fprintf('I Love ') prints 'I Love ' to the standard output and returns a value of 7 for n. In the command "if (~fprintf('I Love '))" the if statement sees ~n (which is zero, or false) and your command window sees 'I Love ' because it is by default the standard output.

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更多回答（17 个）

Kenneth Eaton
Kenneth Eaton 2011-2-16
Here's a pretty goofy answer:
true) fprintf('I Love '); end; if (false
  2 个评论
Andrew Newell
Andrew Newell 2011-2-16
That's thinking outside the parentheses!
Matt Fig
Matt Fig 2011-2-16
My hat's off to you, Sir Eaton!

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Kenneth Eaton
Kenneth Eaton 2011-2-16
In response to Andrew's extra challenge for a solution in which BLANK returns true, here's an absolutely insane one:
function output = BLANK
disp('I Love MATLAB'); % Display the output
assignin('caller','disp',@shadow_disp); % Shadow the DISP function in
% the caller workspace
output = true; % Return true
end
function shadow_disp(~) % This will be immediately invoked by the
evalin('caller','clear disp'); % next call to DISP in the caller
% workspace. It displays nothing, but it
end % unshadows DISP in the caller workspace
It will also work the same way if it returns false. ;)
Jan
Jan 2011-2-16
This prints the wanted string, but not in this Matlab:
if (system('matlab -r "disp(''I Love MATLAB'')" &'))
disp('I Love ')
else
disp('MATLAB')
end
  4 个评论
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Jan
Jan 2011-2-17
@Walter: Sorry, I cannot find an "only" in the question. There is an "exactly".
The DISP command should run once only. Which version creates a loop and print the string over and over again?
Matt Fig
Matt Fig 2011-2-17
Now that is dirty! Yet original. Nice work!

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David Young
David Young 2011-2-17
If you can read very quickly:
[fprintf('I love MATLAB') regexp('x', '(?@quit)')]
Jonathan
Jonathan 2011-2-18
fprintf('I love MATLAB')) return%
Andrew Newell
Andrew Newell 2011-2-16
This version outputs it in red!
~fprintf(2,'I love ')
Jan
Jan 2011-2-17
With a free interpretation of "print-out":
if (text(0.5, 0.5, 'I Love MATLAB'))
disp('I Love ')
else
disp('MATLAB')
end
I simply ignore the orphaned "I Love " - who cares about junk in the command window, if there is a fancy GUI.
Ah, "print-out" means most likely a print-out:
if ({axes('Visible', 'off'); text(0.5, 0.5, 'I Love MATLAB'); print})
disp('I Love ')
else
disp('MATLAB')
end
I admit, Matlab complains about too many output arguments for PRINT in Matlab 6.5 and about a not assigned VARARGOUT in Matrlab 2009a. But the actual print-out is clean.
  2 个评论
Matt Fig
Matt Fig 2011-2-17
Also dirty! I thought about including the phrase, "to the command line" in the problem description, but decided I'd omit it and see where it led.
Matt Fig
Matt Fig 2011-2-17
The orphaned "I Love " can be avoided by putting:
& error
(or similar) after the call to TEXT.

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Walter Roberson
Walter Roberson 2011-2-16
Corrected as per Matt's note about output arguments:
function TF = BLANK
disp('I Love MATLAB');
quit
end
Major time waste: trying to find a way to execute return or quit or break or exit or dbstop in an expression context to avoid having to use a named function.
  4 个评论
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Matt Fig
Matt Fig 2011-2-16
LOL Walter (where's my emoticon), you are always one to know exactly what the rules are!
David Young
David Young 2011-2-17
Time has been wasted: you can use regexp to execute quit in an expression. See my answer below (or above, as the case may be).

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Matt Fig
Matt Fig 2011-2-16
A variation on Walter's theme. Though I am not sure how different this is from just calling FPRINT in the conditional, and it is not really used to replace BLANK....
function TF = BLANK
fprintf('%s','I Love ');
TF = false;
end
Walter Roberson
Walter Roberson 2011-2-16
Perhaps someone might be able to get this approach to work properly:
evalc('fwrite(1,''I Love MATLAB''),quit')
The evalc() works, the quit happens, but the text is not displayed. Adding in an fseek(1,0,0) should in theory force a flush but it doesn't, not even if you add a pause() statement to give time for execution. Though now that I think of it, that might be because the output is being captured by the evalc().
eval() alone cannot process the "quit" portion: it complains about unexpected matlab expression.
  1 个评论
Jan
Jan 2011-2-16
if ({fprintf('I Love MATLAB\n'), evalc('keyboard')}), ...

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Andrew Newell
Andrew Newell 2011-2-16
A variation on Kenneth's answer that prints the message to the left of the prompt:
true) fprintf('I Love MATLAB'); end; return; if (false
Walter Roberson
Walter Roberson 2011-2-16
A variation on Kenneth's shadowing:
function output = BLANK
assignin('caller','disp',@shadow_disp);
output = true;
end
function shadow_disp(S)
disp([S 'MATLAB']);
evalin('caller','clear disp');
end
This has the difference of using what is passed to the disp()
  2 个评论
Walter Roberson
Walter Roberson 2011-2-16
Note that the shadowing solutions do not work if the test is rewritten on a single line, as
if (BLANK); disp('I Love '); else; disp('MATLAB'); end
as in these cases, the value for disp() is taken at parsing time. Also, these shadowing solutions might perhaps not work in 2011b scripts as the JIT is now applied to scripts.
Jan
Jan 2011-2-17
@Walter: This does not "replace BLANK", but *defines* it.

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Walter Roberson
Walter Roberson 2011-2-16
Andrew:
To exercise the entire if/else block, put the test in named file and execute it, with BLANK set to BLANK(mfilename) . BLANK.m would have a persistent variable; if the persistent variable is empty, then set the variable to something, evalc() the mfile whose name was passed in, fwrite(1) the string returned by evalc, and return false . If the persistent variable is not empty, then set it empty and return true .
The mfile would start executing, would call BLANK, which would set its internal flag and recurse the mfile. The second call to BLANK would detect the flag being set and would return true (no recursion), so that recursed call would display the "I Love " and then exit the recursion. Now back at the first level, BLANK has captured the "I Love " and displays it suppressing the newline, and returns false, so the non-recursed mfile executes the else, printing out the "MATLAB" and exiting.
  1 个评论
Andrew Newell
Andrew Newell 2011-2-16
Whew! If we are allowed to call this code from outside, then a simpler approach could be used (see my separate post).

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Andrew Newell
Andrew Newell 2011-2-16
If we are allowed to save the if/else block to a file (say LoveMatlab.m), then this code could exercise both parts of the block:
BLANK=true;
S = evalc('LoveMatlab');
BLANK=false;
T = evalc('LoveMatlab');
disp([S(1:end-1),T])
  1 个评论
Walter Roberson
Walter Roberson 2011-2-16
I don't feel that this fits within the spirit of the question, that the code structure shown should be what is executed and somehow that causes the desired action.

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David Young
David Young 2011-2-17
Assuming that execution time isn't a concern, and, please, no typing while the code is running:
[fprintf('I love MATLAB') input('')]
Nikolay Chumerin
Nikolay Chumerin 2011-2-19
My version#1 is:
true), [char(8*ones(1,8)) 'I Love MATLAB'], return%
  2 个评论
Walter Roberson
Walter Roberson 2011-2-19
Unfortunately backspace, char(8), does not back up past the carriage return and linefeed that would be output after the 'ans = ' that is emitted for the expression.
Nikolay Chumerin
Nikolay Chumerin 2011-2-19
Hmm... on my system (Matlab 2009b 32bit, on Win7x86) it works.

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Nikolay Chumerin
Nikolay Chumerin 2011-2-19
My version#2 is:
isunix), a=12; else a=8*ones(1,8); end; [char(a) 'I Love MATLAB'], return %
works on Matlab 2007b, 2009b, Linux x64 as well as on Matlab 2009b on Win7x86 and Matlab 2010b on WinXPx86.

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