Hump-day puzzler.

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?

[Andrew Newell]

Extra challenge: I would love to see a solution for which BLANK returns TRUE.

[Kenneth Eaton]

@Andrew: Your challenge has been met. See below...

[Andrew Newell]

Nice going! How about one that exercises the entire if/then block? Can it be done?

[Jan]

@Matt: A too loose formulation. Please specify "print-out" and "exactly" exactly. And I assume you mean: "What can replace BLANK <in this code snippet> to get...". You like smilies? Here you have one :-)

[Matt Fig]

@Jan, not you too! (See Walter.) ;-)

[Andrew Newell]

Personally, I prefer the solutions in which all the code introduced goes in the spot where BLANK is and there are no external routines.

[Matt Fig]

So many good ideas, thanks to all who showed their ingenuity!

[Ned Gulley]

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]

Cool! Thanks for the heads up, Ned.

Answer 1

[Walter Roberson]

~fprintf('I Love ')

Solved in... a small number of seconds.

[Matt Fig]

Nice Walter. Of course you need to change the " to '. It took me about 1 minute to solve.

[Andrew Newell]

About 1 minute for me too. Went down a couple of blind alleys like [0 1] first.

[Andrew Newell]

Probably someone who has programmed in C would solve this quickly. With a FORTRAN background, I tend to avoid having side effects in my conditions.

[Walter Roberson]

fprintf(...) can be replaced with fwrite(1,...)

[Matt Fig]

A second solution! Walter, you should have put this as another answer. I only found the first solution.

[Oliver]

Where in the MatLab documentation can I read why this answer works?

[Jan]

@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]

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.

Answer 2

[Kenneth Eaton]

Here's a pretty goofy answer:

true) fprintf('I Love '); end; if (false

[Andrew Newell]

That's thinking outside the parentheses!

[Matt Fig]

My hat's off to you, Sir Eaton!

Answer 3

[Kenneth Eaton]

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. ;)

[Walter Roberson]

I like it!

Answer 4

[Jan]

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

[Andrew Newell]

Maybe this could be turned into a virus!

[Walter Roberson]

It doesn't satisfy the criteria that "I Love MATLAB" be the _only_ output.

Also in some versions there would be a loop of the prompt printing out over and over again in the spawned matlab, as you do not quit after the command.

[Jan]

@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]

Now that is dirty! Yet original. Nice work!

Answer 5

[David Young]

If you can read very quickly:

[fprintf('I love MATLAB') regexp('x', '(?@quit)')]

Answer 6

[Jonathan]

fprintf('I love MATLAB')) return%

[Walter Roberson]

Sneaky!

[Matt Fig]

Indeed!

Answer 7

[Andrew Newell]

This version outputs it in red!

~fprintf(2,'I love ')

[Matt Fig]

Now that is cool!

Answer 8

[Jan]

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.

[Matt Fig]

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]

The orphaned "I Love " can be avoided by putting:

& error

(or similar) after the call to TEXT.

Answer 9

[Walter Roberson]

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.

[Matt Fig]

I get an error: Too many output arguments.

See below for a fix.

[Walter Roberson]

Fixed, thanks Matt. This version is distinctly different than your variation. It works by quiting Matlab to avoid executing the rest of the "if" statement. This is within the boundaries of the puzzle conditions as they did not require that Matlab continue execution afterwards.

[Matt Fig]

LOL Walter (where's my emoticon), you are always one to know exactly what the rules are!

[David Young]

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).

Answer 10

[Matt Fig]

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

Answer 11

[Walter Roberson]

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.

[Jan]

if ({fprintf('I Love MATLAB\n'), evalc('keyboard')}), ...

Answer 12

[Andrew Newell]

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

Answer 13

[Walter Roberson]

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()

[Walter Roberson]

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]

@Walter: This does not "replace BLANK", but *defines* it.

Answer 14

[Walter Roberson]

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.

[Andrew Newell]

Whew! If we are allowed to call this code from outside, then a simpler approach could be used (see my separate post).

Answer 15

[Andrew Newell]

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])

[Walter Roberson]

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.

Answer 16

[David Young]

Assuming that execution time isn't a concern, and, please, no typing while the code is running:

[fprintf('I love MATLAB') input('')]

Answer 17

[Nikolay Chumerin]

My version#1 is:

true), [char(8*ones(1,8)) 'I Love MATLAB'], return%

[Walter Roberson]

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]

Hmm... on my system (Matlab 2009b 32bit, on Win7x86) it works.

Answer 18

[Nikolay Chumerin]

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.