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digitalmars.D - Understanding question about lvalues

reply jicman <jicman_member pathlink.com> writes:
Let us admire this partial code,

class User
{
char [] userID = null;
char [] fname = null;
char [] lname = null;
char [] NameOut  = null;
FCount forms[] = null;
}
class FCount
{
char[] FormApp = "Language";
int    FormCnt = 0;
}
User ProcessUserForms(char[] fxml, User u)
{
int i = 0;
bit found = false;
char[][] t;
t = std.string.split(fxml,"<Application name=\"");
t = std.string.split(t[1],".Applicat");
t = std.string.split(t[0],"applications.");
if(u.forms == null) //.length < 1)
{
FCount fc = new FCount();
u.forms.length = 1;
u.forms[0] = fc;
u.forms[0].FormApp = t[1];
u.forms[0].FormCnt = 1;
found = true;
}
else
{
for(i=0; i < u.forms.length; i++)
{
if(std.string.cmp(u.forms[i].FormApp,t[1]) == 0)
{
found = true;
u.forms[i].FormCnt++;
break;
}
}
if(!found)
{
FCount fc = new FCount();
u.forms.length = u.forms.length + 1; // observe this line
u.forms[length - 1] = fc;
u.forms[length - 1].FormApp = t[1];
u.forms[length - 1].FormCnt = 1;
}
}
return u;
}

This code compiles perfecty, but if I change this line 

u.forms.length = u.forms.length + 1; // observe this line

almost at the end, to

u.forms.length += 1; // observe this line

the compiler fails with

16:03:50.47>dmd DTSP1.d ws2_32.lib
DTSP1.d(740): u.forms.length is not an lvalue

Also, the same happens to,

u.forms.length++;

I just want to understand why.

thanks.

josť
Feb 22 2005
next sibling parent reply Dave <Dave_member pathlink.com> writes:
The array length is considered a property of an array and so is consistent with
properties as described under 'Class and Struct Properties' in
http://digitalmars.com/d/property.html

(See Note at section bottom).

- Dave

In article <cvg7i9$1cmt$1 digitaldaemon.com>, jicman says...
Let us admire this partial code,

class User
{
char [] userID = null;
char [] fname = null;
char [] lname = null;
char [] NameOut  = null;
FCount forms[] = null;
}
class FCount
{
char[] FormApp = "Language";
int    FormCnt = 0;
}
User ProcessUserForms(char[] fxml, User u)
{
int i = 0;
bit found = false;
char[][] t;
t = std.string.split(fxml,"<Application name=\"");
t = std.string.split(t[1],".Applicat");
t = std.string.split(t[0],"applications.");
if(u.forms == null) //.length < 1)
{
FCount fc = new FCount();
u.forms.length = 1;
u.forms[0] = fc;
u.forms[0].FormApp = t[1];
u.forms[0].FormCnt = 1;
found = true;
}
else
{
for(i=0; i < u.forms.length; i++)
{
if(std.string.cmp(u.forms[i].FormApp,t[1]) == 0)
{
found = true;
u.forms[i].FormCnt++;
break;
}
}
if(!found)
{
FCount fc = new FCount();
u.forms.length = u.forms.length + 1; // observe this line
u.forms[length - 1] = fc;
u.forms[length - 1].FormApp = t[1];
u.forms[length - 1].FormCnt = 1;
}
}
return u;
}

This code compiles perfecty, but if I change this line 

u.forms.length = u.forms.length + 1; // observe this line

almost at the end, to

u.forms.length += 1; // observe this line

the compiler fails with

16:03:50.47>dmd DTSP1.d ws2_32.lib
DTSP1.d(740): u.forms.length is not an lvalue

Also, the same happens to,

u.forms.length++;

I just want to understand why.

thanks.

josť

Feb 22 2005
parent jicman <jicman_member pathlink.com> writes:
Dave says...
The array length is considered a property of an array and so is consistent with
properties as described under 'Class and Struct Properties' in
http://digitalmars.com/d/property.html

(See Note at section bottom).

- Dave

thanks Dave.
Feb 22 2005
prev sibling parent Derek Parnell <derek psych.ward> writes:
On Tue, 22 Feb 2005 21:18:01 +0000 (UTC), jicman wrote:

 Let us admire this partial code,
 
 class User
 {
 char [] userID = null;
 char [] fname = null;
 char [] lname = null;
 char [] NameOut  = null;
 FCount forms[] = null;
 }
 class FCount
 {
 char[] FormApp = "Language";
 int    FormCnt = 0;
 }
 User ProcessUserForms(char[] fxml, User u)
 {
 int i = 0;
 bit found = false;
 char[][] t;
 t = std.string.split(fxml,"<Application name=\"");
 t = std.string.split(t[1],".Applicat");
 t = std.string.split(t[0],"applications.");
 if(u.forms == null) //.length < 1)
 {
 FCount fc = new FCount();
 u.forms.length = 1;
 u.forms[0] = fc;
 u.forms[0].FormApp = t[1];
 u.forms[0].FormCnt = 1;
 found = true;
 }
 else
 {
 for(i=0; i < u.forms.length; i++)
 {
 if(std.string.cmp(u.forms[i].FormApp,t[1]) == 0)
 {
 found = true;
 u.forms[i].FormCnt++;
 break;
 }
 }
 if(!found)
 {
 FCount fc = new FCount();
 u.forms.length = u.forms.length + 1; // observe this line
 u.forms[length - 1] = fc;
 u.forms[length - 1].FormApp = t[1];
 u.forms[length - 1].FormCnt = 1;
 }
 }
 return u;
 }
 
 This code compiles perfecty, but if I change this line 
 
 u.forms.length = u.forms.length + 1; // observe this line
 
 almost at the end, to
 
 u.forms.length += 1; // observe this line
 
 the compiler fails with
 
 16:03:50.47>dmd DTSP1.d ws2_32.lib
 DTSP1.d(740): u.forms.length is not an lvalue
 
 Also, the same happens to,
 
 u.forms.length++;
 
 I just want to understand why.
 
 thanks.
 
 josť

The 'length' is a property (thus it is really a function call) so it is like saying ... u.forms.length()++; which is not supported because a function call is not an lvalue. That's why 'u.forms.length += 1' also fails. But the approved method u.forms.length = u.form.length + 1; is like saying ... u.forms.length( u.forms.length() + 1 ); which is okay to use. -- Derek Melbourne, Australia 23/02/2005 9:31:49 AM
Feb 22 2005