Adjusting data points that fit a criteria

Basically for some points say

x = 1:1:100; y = x;

plot(x,y);

how can i for example add 1 for x values between 10 and 20.

i tried

if x >10 & x <20;
x1 = x+1;
end

plot(x1,y)

but it doesnt seem to make a variable, but i dont get an error either

Answer 1

[Wayne King]

   x = 1:1:100; y = x;
   x(x>10 & x<20) = x(x>10 & x<20)+1;
   plot(x,y)

If you want a new variable

     x1 = x;
     x1(x1>10 & x1<20) = x1(x1>10 & x1<20)+1;
     plot(x1,y)