incompatible types in assignment

[v4vijayakumar]

Whats wrong with the code in line no. 7?!

#cat test3.c
#include <stdio.h>

int main(int argc, char *argv[])
{
int ia1[10] = {0, 1, 2};
int ia2[10];
ia2 = ia1;

printf("%d\n", ia2[2]);

return 0;
}
#gcc test3.c
test3.c: In function `main':
test3.c:7: incompatible types in assignment
#

 

[Richard Heathfield]

v4vijayakumar said:

Whats wrong with the code in line no. 7?!

#cat test3.c
#include <stdio.h>

int main(int argc, char *argv[])
{
int ia1[10] = {0, 1, 2};
int ia2[10];
ia2 = ia1;

printf("%d\n", ia2[2]);

return 0;
}
#gcc test3.c
test3.c: In function `main':
test3.c:7: incompatible types in assignment

You can't assign to arrays in C. They are lvalues, but not "modifiable
lvalues".

Fix:

#include <string.h>

and then, provided the two arrays are of the same size and type, you can
replace your assignment attempt with this::

memcpy(ia2, ia1, sizeof ia2);

 

[maurice.gray]

Also.

int main(int argc, char *argv[])
{
int ia1[10] = {0, 1, 2};
int *ia2;
ia2 = ia1;

printf("%d\n",*ia2+2);

return 0;

}

 

[maurice.gray]

#include <stdio.h>

int main(int argc, char *argv[])
{
int ia1[10] = {0, 1, 2};
int *ia2;
ia2 = ia1;

printf("%d\n",*(ia2+2));

return 0;

}

 

[Haider]

Whats wrong with the code in line no. 7?!

#cat test3.c
#include <stdio.h>

int main(int argc, char *argv[])
{
int ia1[10] = {0, 1, 2};
int ia2[10];
ia2 = ia1;

printf("%d\n", ia2[2]);

return 0;
}
#gcc test3.c
test3.c: In function `main':
test3.c:7: incompatible types in assignment
#

array name is not a varaiable so you can't assign i.e. ia2=ia1; or
perform ia2++. refer to K&R (2nd) section 5.3.
above code will work with following fix
in place of int ia2[10]; use int *ia2.
rest is fine.

 

[CBFalconer]

Whats wrong with the code in line no. 7?!

#cat test3.c
#include <stdio.h>

int main(int argc, char *argv[])
{
int ia1[10] = {0, 1, 2};
int ia2[10];
ia2 = ia1;

printf("%d\n", ia2[2]);

return 0;
}
#gcc test3.c
test3.c: In function `main':
test3.c:7: incompatible types in assignment
#

Arrays are not fundamental objects, and cannot be bodily assigned
in C. You could write:

for (i = 0; i < 10; i++) ia1 = ia2;

after declaring i, or use such routines as memcpy. These copy the
elements one by one.

--
"Our enemies are innovative and resourceful, and so are we.
They never stop thinking about new ways to harm our country
and our people, and neither do we." -- G. W. Bush.
"The people can always be brought to the bidding of the
leaders. All you have to do is tell them they are being
attacked and denounce the pacifists for lack of patriotism
and exposing the country to danger. It works the same way
in any country." --Hermann Goering.

 

[Nelu]

<snip>
int ia1[10] = {0, 1, 2};
int ia2[10];
ia2 = ia1;
<snip>

in place of int ia2[10]; use int *ia2.
rest is fine.

It depends what he wants to do.
This doesn't copy the elements so, a change to ia2
will reflect in ia1. If he wants to change values in
ia2 and keep the ia1 unchanged he should either follow
Richard Heathfield's example, or copy the values one by
one in a for loop.

 

[whyglinux]

Whats wrong with the code in line no. 7?!

#cat test3.c
#include <stdio.h>

int main(int argc, char *argv[])
{
int ia1[10] = {0, 1, 2};
int ia2[10];
ia2 = ia1;

printf("%d\n", ia2[2]);

return 0;
}
#gcc test3.c
test3.c: In function `main':
test3.c:7: incompatible types in assignment
#

ia1, which is used as an rvalue, is converted to type int*. On the
other hand, ia2, as an lvalue, is of type int[10]. Since int* can never
be converted to int[10] (while int[10] to int* conversion is defined),
the compiler complains.

 

[whyglinux]

Arrays are not fundamental objects, and cannot be bodily assigned
in C.

Pointers, structs and unions, as well as arrays, are all derived (not
fundamental) types, but they are assignable while arrays are not.

 

[whyglinux]

array name is not a varaiable so you can't assign i.e. ia2=ia1; or
perform ia2++. refer to K&R (2nd) section 5.3.
above code will work with following fix
in place of int ia2[10]; use int *ia2.
rest is fine.

The defination of variable is not given and seldom used in the
standard because of its ambiguity. At least, variable can refer to one
of the following 3 meanings:

1. Named objects that can only be modifiable.
2. Named objects (modifiable and non-modifiable)
3. Objects (both named and unamed).

So I can not say you and K&R are wrong when saying "array name is not a
variable" as it can be interpreted as "array name is not modifiable".
However, I prefer that array names are variables (if we do not care
whether they can be changed or not) as that also said in K&R (2nd).
Yes, I am sure you can find such descriptions in the book.

These indicate that the meaning of variable used in K&R (2nd) is
inconsistent.

As to why ia2=ia1 is invalid, follow what Richard Heathfield have said.

 

[CBFalconer]

Pointers, structs and unions, as well as arrays, are all derived (not
fundamental) types, but they are assignable while arrays are not.

Alright, if you insist, go ahead and make assignments to arrays. I
predict they won't work in C. Note that a struct or union actually
defines a new type, an array only defines an aggregate of an old
type.

--
"Our enemies are innovative and resourceful, and so are we.
They never stop thinking about new ways to harm our country
and our people, and neither do we." -- G. W. Bush.
"The people can always be brought to the bidding of the
leaders. All you have to do is tell them they are being
attacked and denounce the pacifists for lack of patriotism
and exposing the country to danger. It works the same way
in any country." --Hermann Goering.

 

[William J. Leary Jr.]

Alright, if you insist, go ahead and make assignments to arrays. I
predict they won't work in C.

I think you misread "...while arrays are not."

- Bill

 

[Keith Thompson]

Alright, if you insist, go ahead and make assignments to arrays. I
predict they won't work in C.

He knows that; he specifically acknowledged that arrays are not
assignable.

Note that a struct or union actually
defines a new type, an array only defines an aggregate of an old
type.

Structs, unions, and arrays are all aggregates defined in terms of
existing types. The standard referss to struct, union, array, and
function types as "derived types" in C99 6.2.5.

An array type is certainly a type. It's probably not a "first-class"
or "fundamental" type, because, among other things, it doesn't support
assignment or comparison, but C doesn't talk about "first-class" or
"fundamental" types. Structures can be assigned and arrays can't, but
C doesn't make any strong fundamental distinction between them on that
basis; they just happen to support different sets of operations.

C99 6.2.5p21:

Arithmetic types and pointer types are collectively called _scalar
types_. Array and structure types are collectively called
_aggregate types_.

and a footnote:

Note that aggregate type does not include union type because an
object with union type can only contain one member at a time.

(So can a single-element structure, but that's not a big deal; most
references in the standard to "aggregate types" talk about "aggregate
or union" types.)

 

[Jack Klein]

Pointers, structs and unions, as well as arrays, are all derived (not
fundamental) types, but they are assignable while arrays are not.

You are completely incorrect about pointers. They are not derived
types at all, they are fundamental scalar types in C. If you believe
differently, kindly cite the section from the C standard that supports
your position.

 

[pete]

You are completely incorrect about pointers. They are not derived
types at all, they are fundamental scalar types in C. If you believe
differently, kindly cite the section from the C standard that supports
your position.

N869
6.2.5 Types

[#19] Any number of derived types can be constructed from
the object, function, and incomplete types, as follows:

-- A pointer type may be derived from a function type, an
object type, or an incomplete type, called the
referenced type.

 

[Suman]

You are completely incorrect about pointers. They are not derived
types at all, they are fundamental scalar types in C. If you believe
differently, kindly cite the section from the C standard that supports
your position.

N869
6.2.5 Types

[#19] Any number of derived types can be constructed from
the object, function, and incomplete types, as follows:

-- A pointer type may be derived from a function type, an
object type, or an incomplete type, called the
referenced type.

N869
[#20]These methods of constructing *derived types*[1] can be applied
recursively.

[1]Emphasis mine. That's it -- I'd think.

 

[Keith Thompson]

You are completely incorrect about pointers. They are not derived
types at all, they are fundamental scalar types in C. If you believe
differently, kindly cite the section from the C standard that supports
your position.

They're scalar types *and* derived types. (pete already posted the
relevant citation.)

Note that the standard never uses the term "fundamental" in this
context.