Why do so few people know the difference between arrays and pointers.

[Me]

Just a question/observation out of frustration.

I read in depth the book by Peter Van Der Linden entitled
"Expert C Programming" (Deep C Secrets). In particular the
chapters entitled:
4: The Shocking Truth: C Arrays and Pointers Are NOT the Same!
9: More about Arrays
10: More about Pointers

What blows me out of the water is the fact that 'every' programmer
comming out of college that I've interviewed thinks that pointers
and arrays are the same thing.

They go so far as to tell me that, in the following code, 'arr' is
a pointer to an array of integers. Or they tell me that 'arr' is a
pointer to an 'int'. When this is not the case at all.

int arr[100];

The variable 'arr' IS and array of 100 integers...THATS ITS TYPE MAN.
Just like the TYPE of 'i' in the following is 'int'.

int i;

and the type of 'ch' in the following is 'char *'.

char *ch = "string";

The TYPE of 'arr' above is 'an array of 100 integers'. That's WHY you
have to declare a pointer to it as follows:

int (*p)[100];

p = &arr; // Valid
p = arr; // Invalid (compiler will warn you) but works because the
address happens to be the same.

Note: When you use an array in an expression or as an R-Value, the result
of the
operation yields a pointer to the first element in the array. Thus:

int *ip = arr; // Valid: arr turns into an int pointer (a pointer to the
first element in the array)
// Not because ip is an 'int *' be because the array 'arr' is being
used in an expression
// as an R-Value.

Man the C teachers in college aren't doing their job!

 

[Malcolm]

" The Shocking Truth: C Arrays and Pointers Are NOT the
Same!

Array labels decay to pointers, so

void foo(int *ptr);

void bar(void)
{
int arr[100];

foo(arr);
}

calls foo with a pointer to an array of integers.

This ia all you need to know about C array pointer equivalence.

If you start using multi-dimensional arrays and fancy declarations you
deserve all that is coming to you.

 

[E. Robert Tisdale]

Just a question/observation out of frustration.

I read in depth the book by Peter Van Der Linden entitled
"Expert C Programming" (Deep C Secrets).
In particular the chapters entitled:
4: The Shocking Truth: C Arrays and Pointers Are NOT the Same!
9: More about Arrays
10: More about Pointers

Obvious hyperbole.

What blows me out of the water is the fact that
'every' programmer comming out of college that I've interviewed
thinks that pointers and arrays are the same thing.

They go so far as to tell me that, in the following code,
'arr' is a pointer to an array of integers.
Or they tell me that 'arr' is a pointer to an 'int'.
When this is not the case at all.

int arr[100];

The variable 'arr' IS and array of 100 integers...THATS ITS TYPE MAN.
Just like the TYPE of 'i' in the following is 'int'.

int i;

and the type of 'ch' in the following is 'char*'.

char* ch = "string";

The TYPE of 'arr' above is 'an array of 100 integers'.
That's WHY you have to declare a pointer to it as follows:

int (*p)[100];

p = &arr; // Valid
p = arr; // Invalid (compiler will warn you)
// but works because the address happens to be the same.

Note: When you use an array in an expression or as an R-Value,
the result of the operation
yields a pointer to the first element in the array. Thus:

int* ip = arr; // Valid: arr turns into an int pointer
// (a pointer to the first element in the array)
// Not because ip is an 'int *'
// but because the array 'arr' is being used
// in an expression as an R-Value.

Man the C teachers in college aren't doing their job!

Do you feel better now that you got that off your chest?

 

[Me]

" The Shocking Truth: C Arrays and Pointers Are NOT the
Same!

Array labels decay to pointers, so

void foo(int *ptr);

void bar(void)
{
int arr[100];

foo(arr);
}

calls foo with a pointer to an array of integers.

Actually you are mistaken...probably you just mis-typed.

It calls foo with a pointer to an int.... not a pointer to an array of
ints.
It just so happens that what the pointer is pointing to is the first int in
the allocated array of 100 ints, but 'foo' doesn't know that. foo() just
thinks
it is an 'int' pointer.

 

[E. Robert Tisdale]

Array labels decay to pointers, so

void foo(int *ptr);

void bar(void) {
int arr[100];
foo(arr);
}

calls foo with a pointer to an array of integers.

Actually you are mistaken...probably you just mis-typed.

It calls foo with a pointer to an int....
not a pointer to an array of ints.
Correct.

It just so happens that what the pointer is pointing to
is the first int in the allocated array of 100 ints,

I'm pretty sure that you don't mean to imply that
it is just an accident that arr is converted
into a pointer to the first element of arr.

but 'foo' doesn't know that.
foo() thinks [that] it is just an 'int' pointer.

 

[CBFalconer]

.... snip ...

What blows me out of the water is the fact that 'every' programmer
comming out of college that I've interviewed thinks that pointers
and arrays are the same thing.

When I got out of college in nineteen-<mumble> I certainly didn't
think so.

 

[Kieran Simkin]

Just a question/observation out of frustration.

I read in depth the book by Peter Van Der Linden entitled
"Expert C Programming" (Deep C Secrets). In particular the
chapters entitled:
4: The Shocking Truth: C Arrays and Pointers Are NOT the Same!
9: More about Arrays
10: More about Pointers

What blows me out of the water is the fact that 'every' programmer
comming out of college that I've interviewed thinks that pointers
and arrays are the same thing.

They go so far as to tell me that, in the following code, 'arr' is
a pointer to an array of integers. Or they tell me that 'arr' is a
pointer to an 'int'. When this is not the case at all.

int arr[100];

The variable 'arr' IS and array of 100 integers...THATS ITS TYPE MAN.
Just like the TYPE of 'i' in the following is 'int'.

int i;

and the type of 'ch' in the following is 'char *'.

char *ch = "string";

The TYPE of 'arr' above is 'an array of 100 integers'. That's WHY you
have to declare a pointer to it as follows:

int (*p)[100];

p = &arr; // Valid
p = arr; // Invalid (compiler will warn you) but works because the
address happens to be the same.

Note: When you use an array in an expression or as an R-Value, the result
of the
operation yields a pointer to the first element in the array. Thus:

int *ip = arr; // Valid: arr turns into an int pointer (a pointer to the
first element in the array)
// Not because ip is an 'int *' be because the array 'arr' is being
used in an expression
// as an R-Value.

Man the C teachers in college aren't doing their job!

I'm currently teaching my self C from books and the internet and when I
first started learning, constant mentions of the "equivalence" of arrays and
pointers confused the hell out of me because based on what I understood
(correctly) about pointers and arrays, there is nothing equivalent about
them, they're completely different although related things. What would have
saved me a bunch of headscratching would be definitions of an array and a
pointer and then statements of the following facts:

An array, when referenced without an element number decays into a pointer to
that array's first element (without needing to use the & operator)

#include <stdio.h>
int main (void) {
int foo[10];
int *bar;
bar = foo;
printf("%i",bar[1]); /* is equivalent to the following */
printf("%i",*(bar+1)); /* both will output the contents of foo[1] */
return 0;
}

Based on a understanding of pointers and arrays, this explains all that
needs to be explained without using the words "arrays and pointers are
equivalent" or similar which I've seen in several places and are just
blatantly wrong.

Excuse me and please do correct me if I've used any incorrect terminology
here, as I said, I'm still learning.

 

[Daniel Rudy]

And somewhere around the time of 06/08/2004 14:10, the world stopped and
listened as Me contributed the following to humanity:

Just a question/observation out of frustration.

I read in depth the book by Peter Van Der Linden entitled
"Expert C Programming" (Deep C Secrets). In particular the
chapters entitled:
4: The Shocking Truth: C Arrays and Pointers Are NOT the Same!
9: More about Arrays
10: More about Pointers

What blows me out of the water is the fact that 'every' programmer
comming out of college that I've interviewed thinks that pointers
and arrays are the same thing.

Yes and no. An array is a list of data of some type. A pointer is a
reference that points to some data in memory.

Internally, the array identifier acting as the base address, coupled
with the index * data size, generates a pointer to that data element in
memory. All of this is internal to the compiler/linker and is usually
transparent to the programmer, at least that's what my expeiriance has been.

 

[Mike Wahler]

" The Shocking Truth: C Arrays and Pointers Are NOT the
Same!

Array labels decay to pointers, so

void foo(int *ptr);

void bar(void)
{
int arr[100];

foo(arr);
}

calls foo with a pointer to an array of integers.

Wrong. Calls 'foo()' with a pointer to an 'int'.
(type 'int*'). Pointer-to-int and pointer-to-array-of-ints
are not the same type.

int *p; /* pointer to int */
int (*pa)[100]; /* pointer to array of 100 ints */

This ia all you need to know about C array pointer equivalence.

I don't think one should 'know' incorrect information.

If you start using multi-dimensional arrays and fancy declarations you
deserve all that is coming to you.

What's 'wrong' with using multi-dimensional arrays,
and what will come to me if I use one?

I won't ask what you consider a 'fancy' declaration.

-Mike

 

[*a]

hi all; i'm a self-taught C-hobbyst; i want to sing k&r's praises: their
book was somewhat hard for me, i proceeded slowly, sometimes at the "speed"
of few pages/week, but it's impossible to confuse pointer with arrays then;
the problem is C needs a non-superficial study

 

[Keith Thompson]

An array, when referenced without an element number decays into a pointer to
that array's first element (without needing to use the & operator)

An array name (an identifer that refers to an array object) decays
into a pointer to the first element whenever it appears in an
expression, unless it's the operand of a sizeof or unary "&" operator.

Note that the decay even occurs in an indexing expression:

int array_obj[10];
int x;
...
x = array_obj[5];

In the assignment, the name "array_obj" decays to a pointer to the
first element of the array object. The indexing operator takes two
operands, a pointer and an integer.

Read section 6 of the C FAQ.

 

[Michael Mair]

Cheerio,

What blows me out of the water is the fact that 'every' programmer
comming out of college that I've interviewed thinks that pointers
and arrays are the same thing.

If it helps you feel better: We have a finite element code we are
working with, written in C, largely undocumented, many thousand lines,
and are looking for students to work for us.
We get many applications from so-called software engineering students
in their final year. Every single interview came to the point that
they did not "know" any programming language at all while boasting
knowledge of a multitude. We never ever did even get to the point
where such "fine details" like the differences between arrays and
pointers would have played a role. Same thing when we were looking
for someone doing some things in Matlab for us.
So, do not tell me about frustration ;-)

Man the C teachers in college aren't doing their job!

Based on my sad experiences, I am at the moment teaching C to
beginners in order to get someone able to work with us.
I did never tell anyone that pointers and arrays were equivalent,
in fact, I only started with "&array[0]" and came up with the "idea"
of using "array" later on, explained exactly what pointers are,
what arrays are, how they relate -- nonetheless, "pointer equals array"
is exactly the sort of idea I see them using in their assignments.
I try to exorcise it in the next lesson but somehow it sticks. I
suggested a couple of C books to the students but somehow I never
caught anyone reading one of them. They just get the cheapest C books
off Amazon thinking this will do... :-/
Seeing my efforts having no effect for at least ten percent of my
students, I can only say: Maybe, the fault is not only in the teachers.

Another thing is that you have to learn some things entirely by
yourself. Only after you made a serious mistake or saw the outcome
of some mistake you will remember it by heart. A friend of mine
for example needed to find out by himself that a quadratic time
complexity is _much_ worse than linear time complexity of an
algorithm - even though he theoretically knew it beforehand.
Looking at myself and at my colleagues as well, I'd say that every
year I am still learning quite a lot of things in all the programming
languages I am using.



Just my two cents
Michael

 

[Vijay Kumar R Zanvar]

An array name (an identifer that refers to an array object) decays
into a pointer to the first element whenever it appears in an
expression, unless it's the operand of a sizeof or unary "&" operator.

.... and when a character string literal is used to initialize an array.

Note that the decay even occurs in an indexing expression:

int array_obj[10];
int x;
...
x = array_obj[5];

In the assignment, the name "array_obj" decays to a pointer to the
first element of the array object. The indexing operator takes two
operands, a pointer and an integer.

Read section 6 of the C FAQ.

 

[Christian Bau]

Another thing is that you have to learn some things entirely by
yourself. Only after you made a serious mistake or saw the outcome
of some mistake you will remember it by heart. A friend of mine
for example needed to find out by himself that a quadratic time
complexity is _much_ worse than linear time complexity of an
algorithm - even though he theoretically knew it beforehand.
Looking at myself and at my colleagues as well, I'd say that every
year I am still learning quite a lot of things in all the programming
languages I am using.

In my experience, different people learn in different ways. Some prefer
to actually try out things because it helps them learning. However,
trying out things is also useful to check if the theory is really
correct (I once saw a C++ programmer using the quicksort algorithm, but
for sorting an array of variable sized items, and the underlying methods
for changing an item took time O (total size of all items) when changing
the size of an item. The code was not O (N log N), and it was not fast
enough (noticable delay in the user interface in non-trivial cases)).

Also, an algorithm with higher time complexity may be faster than one of
lower complexity up to a certain size. Consider method A taking (N^(2/3)
log^2 (N)) nanoseconds, and method B taking N^0.7 nanoseconds. For large
N, method A is faster. For any problem that can be solved in a billion
years, method A is slower.

 

[Giorgos Keramidas]

Man the C teachers in college aren't doing their job!

Based on my sad experiences, I am at the moment teaching C [...]
I suggested a couple of C books to the students but somehow I never
caught anyone reading one of them. They just get the cheapest C books
off Amazon thinking this will do... :-/ Seeing my efforts having no
effect for at least ten percent of my students, I can only say: Maybe,
the fault is not only in the teachers.

I don't think it is only the teachers' fault.

While being a student at the Computer Engineering & Informatics
Department here in Patras, Greece, I used to say that about 80% of the
students who enter my department every year are not interested (and,
quite probably, they never will be) in learning anything beyond the
absolutely necessary stuff they have to "put up with" in order to get a
degree. Only a small number of the remaining 20% has the patience and
perseverance it takes to learn more about the fine details of
programming--regardless of the language in question.

Another thing is that you have to learn some things entirely by
yourself.

So very true.

- Giorgos

 

[CBFalconer]

If it helps you feel better: We have a finite element code we are
working with, written in C, largely undocumented, many thousand lines,
and are looking for students to work for us.
We get many applications from so-called software engineering students
in their final year. Every single interview came to the point that
they did not "know" any programming language at all while boasting
knowledge of a multitude. We never ever did even get to the point
where such "fine details" like the differences between arrays and
pointers would have played a role. Same thing when we were looking
for someone doing some things in Matlab for us.
So, do not tell me about frustration ;-)


Based on my sad experiences, I am at the moment teaching C to
beginners in order to get someone able to work with us.

Maybe you are looking at the wrong end of the age spectrum?

 

[Michael Mair]

Hi CBFalconer,

Maybe you are looking at the wrong end of the age spectrum?

Well, that might be true - however, I have only money for
students at my hands, and they do not come in arbitrary ages.
When I learned C, I took to it quick enough -- of course,
I needed another five years to get a deeper understanding
but the basics were clear rather quickly.

However, I am confident that about a third of my students will
be able to work with our code after a short introduction to the
general layout, concepts and ideas.


Cheers,
Michael

 

[Michael Mair]

Hello Christian,

In my experience, different people learn in different ways. Some prefer
to actually try out things because it helps them learning. However,
trying out things is also useful to check if the theory is really
correct (I once saw a C++ programmer using the quicksort algorithm, but
for sorting an array of variable sized items, and the underlying methods
for changing an item took time O (total size of all items) when changing
the size of an item. The code was not O (N log N), and it was not fast
enough (noticable delay in the user interface in non-trivial cases)).

True enough - especially when creating your own algorithmss you cannot
do without
(As to the complexity: Can happen. Some things you have to write down on
a sheet of paper first in order to have the full overview of what
happens when, how often and with what relation to N...)

Also, an algorithm with higher time complexity may be faster than one of
lower complexity up to a certain size. Consider method A taking (N^(2/3)
log^2 (N)) nanoseconds, and method B taking N^0.7 nanoseconds. For large
N, method A is faster. For any problem that can be solved in a billion
years, method A is slower.

Nice example, thanks
However, I was talking about linear and quadratic complexity with the
highest leading constant for the linear algorithm being about two to
four times higher than for the quadratic one. Here, you get a noticeable
difference for several thousands of objects...


Cheers,
Michael

 

[JV]

What's 'wrong' with using multi-dimensional arrays,
and what will come to me if I use one?

I won't ask what you consider a 'fancy' declaration.

-Mike

I think what confuses people learning C is that if you declare two arrays:

int a[10]
int b[10][10]

then a[0] is integer and b[0] is a pointer to integer.
-Jyrki

 

[Joona I Palaste]

I think what confuses people learning C is that if you declare two arrays:

int a[10]
int b[10][10]

then a[0] is integer and b[0] is a pointer to integer.

It would confuse them, yes. Fortunately it's not true. b[0] is an
array of ten integers.

--
/-- Joona Palaste ([email protected]) ------------- Finland --------\
\-- http://www.helsinki.fi/~palaste --------------------- rules! --------/
"The obvious mathematical breakthrough would be development of an easy way to
factor large prime numbers."
- Bill Gates