Reversing a string

SystemVerilog
1

Hi,

Someone help me with the below code.

module tb;
string s1 = "string";
string s2;
  
  initial begin
    foreach(s1[i]) begin
      s2[(s1.len()) - 1 + i] = s1[i];
    end
    $display("reversed string=%s", s2);
    end
endmodule

The above code which iam trying to reverse a string just gives me output as shown below
reversed string=

Can someone tell me why? But it works fine when I initialize s2 with some string.

2

In reply to janudeep3:

Per the LRM, the length of a string is the size of the string assigned to it. Since you don’t assign anything to s2 when it is declared, it has a size of 0. Trying to store characters to a null string should be an error.

You should first assign s1 to s2 to create a string of the correct size, then do the reverse function.

3

In reply to cgales:

Hi cgales,

Do you mean to say that string declaration and assignment should be done at a time?
Like
string s2 = “string”;

4

In reply to janudeep3:


module tb;
string s1 = "string";
string s2;
 
  initial begin
    s2 = s1;
    foreach(s1[i]) begin
      s2[(s1.len()) - 1 + i] = s1[i];
    end
    $display("reversed string=%s", s2);
    end
endmodule
5

In reply to janudeep3:
A much simpler solution is to use the streaming operator ( IEEE 1800-2012 section 11.4.14 Streaming operators (pack/unpack) )

module tb;
string s1 = "string";
string s2;
 
  initial begin
    s2 = {<<8{s1}};
    $display("reversed string=%s", s2);
    end
endmodule
6

In reply to dave_59:

Hi Dave,

In this case also, s2 was not assigned ant default value. It was an empty string. But how did it accept?

7

In reply to janudeep3:
You can always make an assignment to a string variable as a whole, and the target variable gets sized to hold the string (e.g.
s2=s1; or
s2=“Hello”;). The problem already mentioned with your original code is that you were trying to write to select individual characters of a string that had not been sized yet. So it was treated as an out of bounds select and nothing happens.

8

In reply to dave_59:

Hi Dave ,

What is the significance of having >>8 , I tried your code with >>4 which is not working correctly.
I also increased the string : stringisgood and tried with the same code >>8 in that case also it worked fine.

Can you please give your comment ?

Thanks ,
Sarang

9

In reply to sarang:

Hi Sarang,

Each character in a string is stored as 8 bits. So for reversing we need to use 8. If you use 4, then half of the bits gets reversed which does not give correct result. Lets take an example of a string with two characters and you can find the solution

module tb;
  string s1 = "is";
  string s2;
  initial
    begin
      $display("string s1 in binary format = %b ", s1);
      s2 = {<<8{s1}};
      $display("reversed string=%b and string is %s", s2,s2);
      s2 = {<<4{s1}};
      $display("reversed string=%b and string is %s", s2,s2);
    end
endmodule

Output will be
string s1 in binary format = 01101001 01110011
reversed string=01110011 01101001 and string is si
reversed string=0011 0111 1001 0110 and string is 7?

ASCII VALUE OF i is 01101001
ASCII VALUE OF s is 01110011
ASCII VALUE OF 7 is 00110111
ASCII VALUE OF ? is 10010110

10

In reply to janudeep3:

Hi Janudeep ,

Thanks for the update , I tried with <<9 , which was also giving incorrect answer. Now my understanding got clear. Irrespective of string size since each character in string gets stored by 8 bits need to use <<8 for reversing the string.

Once again thanks for clearing my doubt.

Thanks,
Sarang