Pointer arrays

[Denis Palmeiro]

Hi, I've been reading K&R, and I'm a bit confused. In the book,
they have the following function:

void writelines(char *lineptr[], int nlines )
{
while( nlines-- > 0 )
printf("%s\n", *lineptr++);
}

Now when I initialize:

char *lines[] = {"Hello", "Hey"};

and pass it as the first arg in that function, it works exactly
like expected.

However, if I try to do the same thing in main:

int main(void) {

char *lineptr[] = {"Hello", "Hey"};
int i = 2;

while( i-- > 0 )
printf("%s\n", *lineptr++);

return 0;
}

I get the error:
test.c:9: error: wrong type argument to increment

Now, how come I can increment lineptr in writelines, but not in
main. If it's declared as an argument, is it another type?

Also, if this is an array, how come it's possible to increment it
in the writelines function. I thought you can't change what an
array points to?

Thanks in advance,
Denis Palmeiro

[Martin Ambuhl]

Re: Pointer arrays

Denis Palmeiro wrote:[color=blue]
> Hi, I've been reading K&R, and I'm a bit confused. In the book,
> they have the following function:
>
> void writelines(char *lineptr[], int nlines )
> {
> while( nlines-- > 0 )
> printf("%s\n", *lineptr++);
> }
>
> Now when I initialize:
> char *lines[] = {"Hello", "Hey"};
> and pass it as the first arg in that function, it works exactly
> like expected.
> However, if I try to do the same thing in main:[/color]
[color=blue]
> int main(void) {
>
> char *lineptr[] = {"Hello", "Hey"};
> int i = 2;[/color]
[color=blue]
> while( i-- > 0 )
> printf("%s\n", *lineptr++);
>
> return 0;
> }
>
> I get the error:
> test.c:9: error: wrong type argument to increment
>
> Now, how come I can increment lineptr in writelines, but not in
> main. If it's declared as an argument, is it another type?
>
> Also, if this is an array, how come it's possible to increment it
> in the writelines function. I thought you can't change what an
> array points to?[/color]

Please check the FAQ on the difference between a pointer and array.
Also check the FAQ for what happens when an array is used as an argument
in a function call.
It is considered polite to check the FAQ before posting. More
pointedly, it is considered rude not to have done so.

Compare your code to that below, which repeats your, in my view
ill-chosen, games with autodecrements and autoincrements:

#include <stdio.h>

void writelines(char *lineptr[], int nlines)
{
while (nlines-- > 0)
printf("%s\n", *lineptr++);
}

int main(void)
{

char *lines[] = { "Hello", "Hey" }, **decentargumen t;
int i = 2;

writelines(line s, 2);
decentargument = lines;
while (i-- > 0)
printf("%s\n", *decentargument ++);

return 0;
}

[Joe Wright]

Re: Pointer arrays

Denis Palmeiro wrote:[color=blue]
> Hi, I've been reading K&R, and I'm a bit confused. In the book,
> they have the following function:
>
> void writelines(char *lineptr[], int nlines )
> {
> while( nlines-- > 0 )
> printf("%s\n", *lineptr++);
> }
>
> Now when I initialize:
>
> char *lines[] = {"Hello", "Hey"};
>
> and pass it as the first arg in that function, it works exactly
> like expected.
>
> However, if I try to do the same thing in main:
>
> int main(void) {
>
> char *lineptr[] = {"Hello", "Hey"};
> int i = 2;
>
> while( i-- > 0 )
> printf("%s\n", *lineptr++);
>
> return 0;
> }
>
> I get the error:
> test.c:9: error: wrong type argument to increment
>
> Now, how come I can increment lineptr in writelines, but not in
> main. If it's declared as an argument, is it another type?
>
> Also, if this is an array, how come it's possible to increment it
> in the writelines function. I thought you can't change what an
> array points to?
>
> Thanks in advance,
> Denis Palmeiro
>[/color]

You can't pass an array to a function. Given..

void writelines(char *lineptr[], int nlines );

...the first argument is really 'char **lineptr', a pointer. In the
body of main()..

char *lineptr[] = {"Hello", "Hey"};

...lineptr is an array, not a pointer. You can't increment an array.

--
Joe Wright mailto:joewwrig ht@comcast.net
"Everything should be made as simple as possible, but not simpler."
--- Albert Einstein ---

[Jack Klein]

Re: Pointer arrays

On Wed, 28 Jul 2004 20:32:02 -0400, "Denis Palmeiro"
<entitledx@gta. igs.net> wrote in comp.lang.c:

As Martin already said, read the FAQ for this newsgroup, link in my
signature, which has a whole chapter on pointers and arrays.
[color=blue]
> Hi, I've been reading K&R, and I'm a bit confused. In the book,
> they have the following function:
>
> void writelines(char *lineptr[], int nlines )
> {
> while( nlines-- > 0 )
> printf("%s\n", *lineptr++);
> }
>
> Now when I initialize:
>
> char *lines[] = {"Hello", "Hey"};
>
> and pass it as the first arg in that function, it works exactly
> like expected.
>
> However, if I try to do the same thing in main:
>
> int main(void) {
>
> char *lineptr[] = {"Hello", "Hey"};
> int i = 2;
>
> while( i-- > 0 )
> printf("%s\n", *lineptr++);
>
> return 0;
> }
>
> I get the error:
> test.c:9: error: wrong type argument to increment
>
> Now, how come I can increment lineptr in writelines, but not in
> main. If it's declared as an argument, is it another type?[/color]

Again, the FAQ has an entire section on pointers and arrays, and the
relationship between them. Read it.
[color=blue]
> Also, if this is an array, how come it's possible to increment it
> in the writelines function. I thought you can't change what an
> array points to?[/color]

The last sentence above certainly indicates a serious misunderstandin g
of arrays in C, and their relationship with pointers.

An array is an object, not a pointer. It never points to anything.
Not at all, not ever. In many circumstances the name of an array is
automatically converted into a pointer to its first element, but the
array itself is not a pointer.
[color=blue]
> Thanks in advance,
> Denis Palmeiro[/color]

--
Jack Klein
Home: http://JK-Technology.Com
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