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when i got a normal struct variable code:
but when my object is a part of an array and i take just one objectCode:struct human {
char name[50];
int age;
float height;
};
struct human TR = {"Tony", 12, 1.5}; \\this works
the same data input method doesnt work
why is that?
Code:#include <stdio.h>
typedef struct robot ROBOT;
struct robot {
char *name;
int energy;
};
int main() {
int i;
ROBOT robots[3];
robots[0] = {"Lunar Lee", 50}; //this dont work
But this does:
You can only initialize a variable when it is declared, never afterward.Code:ROBOT robots[3] = {{"Lunar Lee", 50}, {"Terran Tim", 75}, {"Martian Marsha", 30}};
but i did declared it first
ROBOT robots[3];
Once you hit that semicolon, your chance at initializing the variable is over -- do it then, or forever hold your peace. (After that, you'll have to assign things normally without benefit of curly braces.)
when i do
ROBOT robots[3];
every cell of robots without a value
you said that after this i can only assign values like:
robots[0].energy=5
etc..
why i cant using curly braces
whats so special about it?
It's initialization, simply put.
After you've defined something, you cannot initialize it. You can only assign it.
That's the way it works.
initialization is when i create a new variable of some data type
so is define
what is the difference between two of those terms?
No, it is not.
Definition is creating something, in simple words.
Initialization is when you assign a value when creating it.
Assigning is when you assign something after it has been created.
Code:int X = 5; // Definition of variable, initializing it with value 5.
int x2; // Defining variable x2
x2 = 5; // Assigning 5 to x2
Using the term 'assign' in the definition while trying to differentiate the terms 'assign' and 'initialize' seems like a really bad idea.Quote:
Originally Posted by Elysia
How about this:
Initialize: To specify the very first value that an object will have after it is created.
Assign: To set the value of the object on the left-hand-side equal to the value of the object on the right-hand-side. The left-hand-side must evaluate to a modifiable lvalue (meaning you can't write "5 = x + 1;" since 5 isn't a modifiable lvalue). The right-hand-side must evaluate to a legal rvalue.
Furthermore, to help clarify things, think of any value or list of values in curly braces as an "initializer list".
Maybe this will help you sort it out in your head. It helps me at least.Code:int A[5];
A = {1, 2, 3, 4, 5}; // Illegal, you can't assign an initializer list to an object
// Initializer lists can only be used to initialize objects
I don't know, but your explanation seems more detailed and better, though.Quote:
Originally Posted by arpsmack
ok so in my first example
i am doing initialization
in the second example
first i am defining a variable robots[3] (all of it cells are without data)
then i am assigning values to one of it cells
they both changing the values of cell 0Code:why it ok to do
robot[0].name="Lunar Lee";
robots[0].energy=5;
but not
robots[0] = {"Lunar Lee", 50};
??
ahhhh curly brACES CAN BE USED ONLY ones at the declaration of the variable
so if it true then
why we can do this after we initialize robots[3]
this line comes against this rule in which we cant initialize more then onesCode:ROBOT robots[3] = {{"Lunar Lee", 50}, {"Terran Tim", 75}, {"Martian Marsha", 30}};
??
First is assigning, second is initialization (which can be done only at the point when you create the variable).Quote:
Originally Posted by transgalactic2
And the second post works because you are creating a new robot array and initializing it.
Also, string literals should be const: http://cpwiki.sourceforge.net/Common...kes_and_errors
so when i do
the second line replaces the old robots[3] variablesCode:ROBOT robots[3] ;
then
ROBOT robots[3] = {{"Lunar Lee", 50}, {"Terran Tim", 75}, {"Martian Marsha", 30}};
and builds a new robots[3] ??
Nope, it will give a compile error (robots already defined).
This is the same as doingQuote:
Originally Posted by transgalactic2
I have to do the whole array right here, before the semicolon, or I lose my chance. Here each struct would be initialized with a bunch of things in curly braces; to initialize the array I take all of them and surround them in an extra pair of curly braces.Code:int two_d_array[2][4] = {{1,2,3,4},{5,6,7,8}};
in this example
we do create the same variable twice
but here on the second time we are creating robots[3] using ROBOT (which is the same type as robot type)
why is it possible??
we are creating the same variable twice
Code:#include <stdio.h>
typedef struct robot ROBOT;
struct robot {
char *name;
int energy;
};
int main() {
int i;
ROBOT robots[3]; //1st time
robots[0].name = "Lunar Lee";
robots[0].energy = 50;
robots[1].name = "Planetary Pete";
robots[1].energy = 20;
robots[2].name = "Martian Matt";
robots[2].energy = 30;
ROBOT robots[3] = { {"Lunar Lee", 50},{"Planetary Pete", 20},{"Martian Matt", 30} }; //2nd time
It is not possible.
And for goodness sake, change that struct of yours!
Error 1 error C2086: 'ROBOT robots[3]' : redefinition g:\w00t\visual studio 2008\projects\temp\temp3.cpp 13Code:struct robot {
const char* name;
int energy;
};
i took this example from this tutorial:
http://irc.essex.ac.uk/www.iota-six...._to_struct.asp
its the last topic on the page
called
"Arrays of Structure Variables"
where is my mistake in understanding this example?
They do not redefine the variable. It is enclosed in comments.
And despite comments, the code inside the comments actually compiled file (remember to remove the first defining of the variable).
But the example does have one thing wrong, however.
If you're going to assign string literals to variables, then use const char*, not char*.
Oh and I found this:
I just want to point out that this is wrong, WRONG, WRONG!Quote:
Instinctively, I first declared my array of robots, then I tried initializing each element one by one like this:
robots[0] = {"Lunar Lee", 50};
Unfortunately, MSVC++ complained, and so I initialized each name and energy field separately for each robot, as shown above.
MSVC is right to complain here, because it is not possible according to the standard.
Better find yourself a better tutorial... Try this site's tutorials, instead.
thanks
so curly braces can go ones only at the initialization of the variable
why do i need to change the char pointer into constant?
Only at the definition or declaration of a variable, as pointed out.Quote:
Originally Posted by transgalactic2
I already mentioned that. Try following links.Quote:
why do you want to change the char pointer into constant?
http://cpwiki.sourceforge.net/Common...kes_and_errors
It should be a pointer to constant characters if you intend to always use string literals for the names, since string literals are constant characters (as opposed to just plain characters).Quote:
Originally Posted by transgalactic2
if i get it correctly
string literal is a char pointer to which we initialized it using curly braces
string literals are on read only memory
and there are no way to change them after initialization
No, a string literals is something like this:
"Hello World!"
String literals are read-only, yes.
They cannot ever be changed, since they are written correctly from the start (know what I mean?).
A string literals is typically stored in a read-only place and the compiler then assigns a const char* pointer to wherever you assign or initialize it.
so its not only about char pointers
str also a string literalCode:char str[]="hello world";
do i need to keep it as a constant too?
When initializing an array with a string literal, the string literal is copied into the buffer, so you don't need to make it constant.
Code:char* str = "Hellq World!";
str[4] = 'o'; // Oops. Error at runtime.
char str2[] = "Hellq World!";
str2[4] = 'o'; // OK
const char* str3 = "Hellq World!";
str3[4] = 'o'; // Oops. Compile error.
ok so this constant read only memory thing only goes
for char pointers string literals?
When assigning to (or initializing) pointers, yes, then the pointer should be const (pointer to const char).
thanks