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digitalmars.D.learn - Modify char in string

reply "Tim" <nrgyzer gmail.com> writes:
Hi everyone,

is there any chance to modify a char in a string like:

void main()
{
    string sMyText = "Replace the last char_";
    sMyText[$ - 1] = '.';
}

But when I execute the code above I'm always getting "cannot 
modify immutable expression at sMyText[__dollar -1LU]". I though 
D supported such modifications anytime. How can I do that now...?
May 18 2014
next sibling parent "bearophile" <bearophileHUGS lycos.com> writes:
Tim:


 is there any chance to modify a char in a string like:

 void main()
 {
    string sMyText = "Replace the last char_";
    sMyText[$ - 1] = '.';
 }

 But when I execute the code above I'm always getting "cannot 
 modify immutable expression at sMyText[__dollar -1LU]". I 
 though D supported such modifications anytime. How can I do 
 that now...?


D strings are immutable. And mutating immutable variables is a 
bug in D. So you can't do that. You have to work around the 
problem. One solution is to not have a string, but something more 
like a char[] in the first place, and mutate it.

If you have a string, you can do (this works with the current GIT 
DMD compiler):



/// Not Unicode-safe.
string dotLast(in string s) pure nothrow {
     auto ds = s.dup;
     ds[$ - 1] = '.';
     return ds;
}

void main() {
     import std.stdio;

     immutable input = "Replace the last char_";
     immutable result = input.dotLast;
     result.writeln;
}


That code doesn't work if your text contains more than the Ascii 
chars.

Bye,
bearophile
May 18 2014
prev sibling parent reply "Chris Cain" <zshazz gmail.com> writes:
On Sunday, 18 May 2014 at 18:55:59 UTC, Tim wrote:

 Hi everyone,

 is there any chance to modify a char in a string like:


As you've seen, you cannot modify immutables (string is an 
immutable(char)[]). If you actually do want the string to be 
modifiable, you should define it as char[] instead.

Then your example will work:

     void main()
     {
        char[] sMyText = "Replace the last char_";
        sMyText[$ - 1] = '.';
     }

If you actually want it to be immutable, you can still do it, but 
you can't modify in-place, you must create a new string that 
looks like what you want:

     void main()
     {
        string sMyText = "Replace the last char_";
        sMyText = sMyText[0 .. $-1] ~ ".";
        // you would do
        //sMyText[0 .. 5] ~ "." ~ sMyText[6..$];
        // to "replace" something in the 5th position
     }

Note that the second method allocates and uses the GC more (which 
is perfectly fine, but not something you want to do in a tight 
loop). For most circumstances, the second method is good.
May 18 2014
parent reply "Tim" <nrgyzer gmail.com> writes:
On Sunday, 18 May 2014 at 19:09:52 UTC, Chris Cain wrote:

 On Sunday, 18 May 2014 at 18:55:59 UTC, Tim wrote:

 Hi everyone,

 is there any chance to modify a char in a string like:


 As you've seen, you cannot modify immutables (string is an 
 immutable(char)[]). If you actually do want the string to be 
 modifiable, you should define it as char[] instead.

 Then your example will work:

     void main()
     {
        char[] sMyText = "Replace the last char_";
        sMyText[$ - 1] = '.';
     }

 If you actually want it to be immutable, you can still do it, 
 but you can't modify in-place, you must create a new string 
 that looks like what you want:

     void main()
     {
        string sMyText = "Replace the last char_";
        sMyText = sMyText[0 .. $-1] ~ ".";
        // you would do
        //sMyText[0 .. 5] ~ "." ~ sMyText[6..$];
        // to "replace" something in the 5th position
     }

 Note that the second method allocates and uses the GC more 
 (which is perfectly fine, but not something you want to do in a 
 tight loop). For most circumstances, the second method is good.


Thanks - I already tried:

      void main()
      {
         char[] sMyText = "Replace the last char_";
         sMyText[$ - 1] = '.';
      }

but I always getting "Error: cannot implicitly convert expression 
("Replace the last char_") of type string to char[]". I know, I 
can use cast(char[]) but I don't like casts for such simple 
things...
May 19 2014
parent =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 05/19/2014 10:07 AM, Tim wrote:


 I already tried:

       void main()
       {
          char[] sMyText = "Replace the last char_";
          sMyText[$ - 1] = '.';
       }

 but I always getting "Error: cannot implicitly convert expression
 ("Replace the last char_") of type string to char[]".


That's a good thing because that string literal is immutable. If that 
code compiled you would get undefined behavior.


 I know, I can use cast(char[])


Unfortunately, not in this case. That undefined behavior would manifest 
itself as a "Segmentation fault" on many systems. :)


 but I don't like casts for such simple things...


What you want to do makes sense only if you have a mutable ASCII string. 
Such strings are generated at run time so the problem is usually a non 
issue:

import std.stdio;

void main()
{
     foreach (line; stdin.byLine) {
         char[] s = line.dup;    // (or .idup if you want immutable)
         s[$-1] = '.';
         writefln("Input : %s", line);
         writefln("Output: %s", s);
     }
}

Ali
May 19 2014