Hump-day puzzler.

Here's a pretty goofy answer:

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

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. ;)

If you can read very quickly:

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

This version outputs it in red!

~fprintf(2,'I love ')

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

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.

fprintf('I love MATLAB')) return%

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.

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

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.

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

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()

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.

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])

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

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

My version#1 is:

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

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.