@AmusingYeti  Thanks but in my Maple, nothing is printed until it reaches the end. 

@Markiyan Hirnyk 

Thank you and vv. Your answers are really helpful.
It seems that there is again a problem. Let us try this similar code

restart; f := proc (x) if is(x < 0) then x else x^2 end if end proc; plot(f(x), x = -2 .. 2); f(-1) 

It plots only x^2 on the whole domain! that is not correct but gives a correct answer for the evaluation.

Please help. Thank you

@vv It is great. I appreciate your help.

Dear @Preben Alsholm 

I got it. Your answer is concise and really helpful.

Thank you so much

@Markiyan Hirnyk 

I have some big matrices with unknown entries. The output for the determinant is very long and it seems that all the coefficients are positive. I used Ctrl+F to see whether there is a minus sign or not but I found that it works for some other symbols and letters even in 2-D math but not for "-".

Thank you for the great help. Conversion to expression works fine. 

Thank you @Markiyan Hirnyk 

I always use Fine/Replace (Ctrl+F). It always work properly but for finding minus sign, it does not work. It shows the message"No Matches Found". I even tried a-b+c-d!