BUG in subs()?

   subs('N*g', 'N', 'tg/u')
   subs('N*g', 'tg/u', 'N')

these two code lines give the same result:

   (g*tg)/u

but the first line should give back:

    (g*tg)/u

and the second result should be:

    N*g

since there is no 'tg/u' which could be substitued...

techdoc says:

   subs(S,old,new)

so i think it should only look for 'old' and change to 'new', not vice versa.

any idea how can i avoid that subs function changes 'new' to 'old' in the S expression?

Answer 1

[Sean de Wolski]

This is functionality available for backward compatibility. If the old does not occur, but the new does, it switches them.

doc subs

[Walter Roberson]

This is *documented* functionality.

http://www.mathworks.com/help/toolbox/symbolic/subs.html

If subs(s,old,new) does not change s, subs(s,new,old) is tried. This provides backwards compatibility with previous versions and eliminates the need to remember the order of the arguments. subs(s,old,new,0) does not switch the arguments if s does not change.

[Tamás Király]

thanks for pointing that out. I don't understand how did i missed that part of documentation...