simple pointers question

[ryanselk]

I am new with C.

I have these declarations:
char *temp;
int samples;
//temp is not null and is pointing where I want it.
//I need a simple IF statement like this:

if (temp == "samples")
{
samples = temp;
}

How would I make this statement?

How can I get samples to take the value that temp is pointing to? I
need to de-reference, I think but im unsure how to do it. Do I need to
cast it somehow? How would I go about doing this? Any help would be
excellent!

[Pietro Cerutti]

ry******@gmail.com wrote:

I am new with C.

I have these declarations:
char *temp;
int samples;
//temp is not null and is pointing where I want it.
//I need a simple IF statement like this:

if (temp == "samples")
{
samples = temp;
}

How would I make this statement?

strncmp and google are your friends...
--
Pietro Cerutti

PGP Public Key:
http://gahr.ch/pgp

[ryanselk]

On Jun 25, 11:22 am, Pietro Cerutti <[e-mail]:

[e-mail] wrote:

I am new with C.

I have these declarations:

char *temp;
int samples;

//temp is not null and is pointing where I want it.

//I need a simple IF statement like this:

if (temp == "samples")
{
samples = temp;
}

How would I make this statement?

strncmp and google are your friends...

--
Pietro Cerutti

PGP Public Key:http://gahr.ch/pgp

thanks,

But I still dont get it. It seems this function is for comparing the
size of strings/arrays.

How can i use this to initialize 'samples' from what the 'temp'
pointer is poiting to?

[Pietro Cerutti]

ry******@gmail.com wrote:

On Jun 25, 11:22 am, Pietro Cerutti <[e-mail]:

>[e-mail] wrote:

>>I am new with C.
I have these declarations:
char *temp;
int samples;
//temp is not null and is pointing where I want it.
//I need a simple IF statement like this:
if (temp == "samples")
{
samples = temp;
}
How would I make this statement?

strncmp and google are your friends...

--
Pietro Cerutti

PGP Public Key:http://gahr.ch/pgp

thanks,

But I still dont get it. It seems this function is for comparing the
size of strings/arrays.

From the man pages:
The stpcpy() and strcpy() functions copy the string src to dst
(including the terminating `\0' character.)

>
How can i use this to initialize 'samples' from what the 'temp'
pointer is poiting to?

using strncpy..
--
Pietro Cerutti

PGP Public Key:
http://gahr.ch/pgp

[Pietro Cerutti]

Pietro Cerutti wrote:

ry******@gmail.com wrote:

>On Jun 25, 11:22 am, Pietro Cerutti <[e-mail]:

>>[e-mail] wrote:
I am new with C.
I have these declarations:
char *temp;
int samples;
//temp is not null and is pointing where I want it.
//I need a simple IF statement like this:
if (temp == "samples")
{
samples = temp;
}
How would I make this statement?
strncmp and google are your friends...

--
Pietro Cerutti

PGP Public Key:http://gahr.ch/pgp

thanks,

But I still dont get it. It seems this function is for comparing the
size of strings/arrays.

From the man pages:
The stpcpy() and strcpy() functions copy the string src to dst
(including the terminating `\0' character.)

Ups, I was supposed to write this:

The strcmp() and strncmp() functions lexicographically compare the null-
terminated strings s1 and s2.

Opened the wrong man page...

>

>How can i use this to initialize 'samples' from what the 'temp'
pointer is poiting to?

using strncpy..

--
Pietro Cerutti

PGP Public Key:
http://gahr.ch/pgp

[Richard Tobin]

In article <f1***************************@news.hispeed.ch>,
Pietro Cerutti <ga**@gahr.chwrote:

>//temp is not null and is pointing where I want it.

//I need a simple IF statement like this:

if (temp == "samples")
{
samples = temp;
}

How would I make this statement?

>strncmp and google are your friends...

Why strncmp()? Did you think he said "not null-terminated" rather than
"not null"? strcmp() is the answer.

-- Richard
--
"Consideration shall be given to the need for as many as 32 characters
in some alphabets" - X3.4, 1963.

[Richard Heathfield]

Pietro Cerutti said:

ry******@gmail.com wrote:

>I am new with C.

<snip>

>>
if (temp == "samples")
{
samples = temp;
}

How would I make this statement?

strncmp and google are your friends...

Why strncmp? It looks to me like he wanted strcmp.

--
Richard Heathfield <http://www.cpax.org.uk>
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Pietro Cerutti]

Richard Tobin wrote:

In article <f1***************************@news.hispeed.ch>,
Pietro Cerutti <ga**@gahr.chwrote:

>>//temp is not null and is pointing where I want it.

//I need a simple IF statement like this:

if (temp == "samples")
{
samples = temp;
}

How would I make this statement?

>strncmp and google are your friends...

Why strncmp()? Did you think he said "not null-terminated" rather than
"not null"? strcmp() is the answer.

Because I prefer to always impose bounds when dealing with strings...

What's wrong with

strncmp(temp, "samples", sizeof("samples"));

>
-- Richard

--
Pietro Cerutti

PGP Public Key:
http://gahr.ch/pgp

[Richard Heathfield]

Pietro Cerutti said:

ry******@gmail.com wrote:

<snip>

>But I still dont get it. It seems this function is for comparing the
size of strings/arrays.

From the man pages:

The man pages are not normative, and in this case they were misleading.

<snip>

>How can i use this to initialize 'samples' from what the 'temp'
pointer is poiting to?

using strncpy..

Wrong.

--
Richard Heathfield <http://www.cpax.org.uk>
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Richard Heathfield]

ry******@gmail.com said:

I am new with C.

I have these declarations:
char *temp;
int samples;
//temp is not null and is pointing where I want it.
//I need a simple IF statement like this:

if (temp == "samples")

You will need strcmp, which is prototyped in <string.hto do this
comparison. if(strcmp(temp, "samples") == 0) /* the strings compare
equal */

{
samples = temp;
}

How would I make this statement?

You can't. samples has type int, whereas temp has type char *. The
assignment doesn't make sense.

What you need to do is take a step back from this plan you have for a
solution, and tell us what the wider *problem* is. You're so close to
the trees that we can't see which wood you're in.

--
Richard Heathfield <http://www.cpax.org.uk>
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Pietro Cerutti]

Richard Heathfield wrote:

Pietro Cerutti said:

>ry******@gmail.com wrote:

<snip>

>>But I still dont get it. It seems this function is for comparing the
size of strings/arrays.

From the man pages:

The man pages are not normative, and in this case they were misleading.

<snip>

>>How can i use this to initialize 'samples' from what the 'temp'
pointer is poiting to?

using strncpy..

Wrong.

It depends on how you interpret the sencente:
"How can I get samples to take the value that temp is pointing to?"

English is not my mother language, so please correct me if I'm actually
wrong, but in "take the value" I read "copy the value" not "point to the
same value"...
--
Pietro Cerutti

PGP Public Key:
http://gahr.ch/pgp

[Richard Heathfield]

Pietro Cerutti said:

Richard Tobin wrote:

>In article <f1***************************@news.hispeed.ch>,
Pietro Cerutti <ga**@gahr.chwrote:

<snip>

>>strncmp and google are your friends...

Why strncmp()? Did you think he said "not null-terminated" rather
than
"not null"? strcmp() is the answer.

Because I prefer to always impose bounds when dealing with strings...

That's what the null terminator is for - imposing a bound.

What's wrong with

strncmp(temp, "samples", sizeof("samples"));

Nothing, particularly, except that it is identical in meaning to
strcmp(temp, "samples") - am I to take it that you enjoy typing?

--
Richard Heathfield <http://www.cpax.org.uk>
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Pietro Cerutti]

Richard Heathfield wrote:

Pietro Cerutti said:

>Richard Tobin wrote:

>>In article <f1***************************@news.hispeed.ch>,
Pietro Cerutti <ga**@gahr.chwrote:

<snip>

>>>strncmp and google are your friends...
Why strncmp()? Did you think he said "not null-terminated" rather
than
"not null"? strcmp() is the answer.

Because I prefer to always impose bounds when dealing with strings...

That's what the null terminator is for - imposing a bound.

>What's wrong with

strncmp(temp, "samples", sizeof("samples"));

Nothing, particularly, except that it is identical in meaning to
strcmp(temp, "samples") - am I to take it that you enjoy typing?

Yes I do, otherwise I wouldn't be into computers ;-)

Anyway, you both Richards are right.

--
Pietro Cerutti

PGP Public Key:
http://gahr.ch/pgp

[Default User]

ry******@gmail.com wrote:

On Jun 25, 11:22 am, Pietro Cerutti <[e-mail]:

[e-mail] wrote:

I am new with C.

I have these declarations:

char *temp;
int samples;

//temp is not null and is pointing where I want it.

//I need a simple IF statement like this:

if (temp == "samples")
{
samples = temp;
}

How would I make this statement?

strncmp and google are your friends...

--
Pietro Cerutti

Please trim signatures the parts after -- in posts. Many newsreaders do
that automatically, Google Groups does not.

thanks,

But I still dont get it. It seems this function is for comparing the
size of strings/arrays.

No, it is not. That would be strlen(). The given function, strcmp() is
for comparing strings. Generally, == will not give you a useful answer,
as it will compare the pointers.

How can i use this to initialize 'samples' from what the 'temp'
pointer is poiting to?

Your question doesn't make any sense. The only way for the conversion
to take place would be for temp to contain the string "samples".
However, that can't be converted to an int, unless you mean converting
the pointer to an int. However, that's an implementation-defined
operation and almost never useful in a real program.

Go back and THOROUGHLY explain what you want, including sample data.
Then we can figure out how to help.


Brian

[Richard Tobin]

In article <2e***************************@news.hispeed.ch>,
Pietro Cerutti <ga**@gahr.chwrote:

>Because I prefer to always impose bounds when dealing with strings...

Why?

>What's wrong with

strncmp(temp, "samples", sizeof("samples"));

It's more verbose, confusing to the reader, and quite likely slower.

-- Richard
--
"Consideration shall be given to the need for as many as 32 characters
in some alphabets" - X3.4, 1963.

[Richard Heathfield]

Pietro Cerutti said:

Richard Heathfield wrote:

>Pietro Cerutti said:

>>ry******@gmail.com wrote:

<snip>

>>>But I still dont get it. It seems this function is for comparing
the size of strings/arrays.
From the man pages:

The man pages are not normative, and in this case they were
misleading.

<snip>

>>>How can i use this to initialize 'samples' from what the 'temp'
pointer is poiting to?

using strncpy..

Wrong.

It depends on how you interpret the sencente:

No, it doesn't. The OP's requirements are not yet clear, but there are
lots of things I can identify out of hand as being *not* what he needs,
and this list certainly includes a katharometer, a half-submerged
patamar, a relief map of the Metropolitan District of South Humberside,
a light green balmacaan with extra pockets, and strncpy.

<snip>

--
Richard Heathfield <http://www.cpax.org.uk>
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[ryanselk]

On Jun 25, 12:00 pm, Richard Heathfield <[e-mail]:

Pietro Cerutti said:

Richard Heathfield wrote:

Pietro Cerutti said:

>[e-mail] wrote:

<snip>

>>But I still dont get it. It seems this function is for comparing
the size of strings/arrays.
From the man pages:

The man pages are not normative, and in this case they were
misleading.

<snip>

>>How can i use this to initialize 'samples' from what the 'temp'
pointer is poiting to?

>using strncpy..

Wrong.

It depends on how you interpret the sencente:

No, it doesn't. The OP's requirements are not yet clear, but there are
lots of things I can identify out of hand as being *not* what he needs,
and this list certainly includes a katharometer, a half-submerged
patamar, a relief map of the Metropolitan District of South Humberside,
a light green balmacaan with extra pockets, and strncpy.

<snip>

--
Richard Heathfield <http://www.cpax.org.uk>
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

Im sorry guys I was not clean and now I realize it.
I now see what you guys where talking about, and you are correct that
I was way off.
I have:

char *temp;
//Lets say that this is pointing to the word "Samples"

char *temp2
//Lets say that this is pointing to the number '32'

int samples;
//null

I need a statement that does this:

If (temp1 is equal to the word 'samples')
set samples to the value of temp2 (32).
So far I got (with your help)
if (strcmp(temp, "samples") == 0) //if temp == samples
{
//makes samples = temp2

}

but im unsure of how to get the middle statement. Thanks again guys I
hope this is clearer. I reduced my code down to what i thought was
minimal but looking back it wasn't enough code to work off of
initially. Thanks!

[Default User]

ry******@gmail.com wrote:

On Jun 25, 12:00 pm, Richard Heathfield <[e-mail]:

--
Richard Heathfield <http://www.cpax.org.uk>
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

PLEASE TRIM SIGNATURES.

I have:

char *temp;
//Lets say that this is pointing to the word "Samples"

char *temp2
//Lets say that this is pointing to the number '32'

Why do you have string? Why not an integer in the first place? Is this
user input or something?

int samples;
//null

I need a statement that does this:

If (temp1 is equal to the word 'samples')
set samples to the value of temp2 (32).
So far I got (with your help)
if (strcmp(temp, "samples") == 0) //if temp == samples
{
//makes samples = temp2

}

Please fix your indents. Your code is hard to read.

but im unsure of how to get the middle statement. Thanks again guys I
hope this is clearer. I reduced my code down to what i thought was
minimal but looking back it wasn't enough code to work off of
initially. Thanks!

The easiest way, but arguably the worst, is to use the atoi() function.
That is not a robust function, there's no way to tell whether a string
contained "0" or if there was an error. You also can't tell if there
was a partial conversion ("123skiddoo").

Better, but more trouble, is strtol(). You can read up on it here:

<http://www.mkssoftware.com/docs/man3/strtol.3.asp>

You'll need to pass an endptr address, check that, then check errno.
That's some things for you to look at. Let us know how it's going.

Brian

[ryanselk]

On Jun 25, 12:29 pm, "Default User" <[e-mail]:

[e-mail] wrote:

On Jun 25, 12:00 pm, Richard Heathfield <[e-mail]:

--
Richard Heathfield <http://www.cpax.org.uk>
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

PLEASE TRIM SIGNATURES.

I have:

char *temp;
//Lets say that this is pointing to the word "Samples"

char *temp2
//Lets say that this is pointing to the number '32'

Why do you have string? Why not an integer in the first place? Is this
user input or something?

int samples;
//null

I need a statement that does this:

If (temp1 is equal to the word 'samples')
set samples to the value of temp2 (32).

So far I got (with your help)

if (strcmp(temp, "samples") == 0) //if temp == samples
{
//makes samples = temp2

}

Please fix your indents. Your code is hard to read.

but im unsure of how to get the middle statement. Thanks again guys I
hope this is clearer. I reduced my code down to what i thought was
minimal but looking back it wasn't enough code to work off of
initially. Thanks!

The easiest way, but arguably the worst, is to use the atoi() function.
That is not a robust function, there's no way to tell whether a string
contained "0" or if there was an error. You also can't tell if there
was a partial conversion ("123skiddoo").

Better, but more trouble, is strtol(). You can read up on it here:

<http://www.mkssoftware.com/docs/man3/strtol.3.asp>

You'll need to pass an endptr address, check that, then check errno.
That's some things for you to look at. Let us know how it's going.

Brian

Thanks guys! I got it working fine using the atoi function,

But I will be looking into the strtol one as well as I suppose a
partial conversion is likely with my program. (its a program that
splits strings and reads certain parts of a big header file, kind of a
pain in the ass haha).

[Richard Heathfield]

Pietro Cerutti said:

<snip>

If temp2 points to an array of chars where the first is '3', the
second is '2' and the third is '\0', then the function atoi() from
stdlib.h may help.

samples = atoi(temp2);

Whilst that's fine provided temp2 does indeed point to "32", it is
nevertheless a bad idea, in general, to advocate atoi, because its
behaviour on overflow is undefined. It would be far better to use
strtol.

--
Richard Heathfield <http://www.cpax.org.uk>
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Pietro Cerutti]

Richard Heathfield wrote:

Pietro Cerutti said:

<snip>

>If temp2 points to an array of chars where the first is '3', the
second is '2' and the third is '\0', then the function atoi() from
stdlib.h may help.

samples = atoi(temp2);

Whilst that's fine provided temp2 does indeed point to "32", it is
nevertheless a bad idea, in general, to advocate atoi, because its
behaviour on overflow is undefined. It would be far better to use
strtol.

Thanx for pointing it out.

--
Pietro Cerutti

PGP Public Key:
http://gahr.ch/pgp

[Keith Thompson]

ry******@gmail.com writes:
[...]

I have:

char *temp;
//Lets say that this is pointing to the word "Samples"

char *temp2
//Lets say that this is pointing to the number '32'

int samples;
//null

I need a statement that does this:

If (temp1 is equal to the word 'samples')
set samples to the value of temp2 (32).
So far I got (with your help)
if (strcmp(temp, "samples") == 0) //if temp == samples
{
//makes samples = temp2

}

but im unsure of how to get the middle statement. Thanks again guys I
hope this is clearer. I reduced my code down to what i thought was
minimal but looking back it wasn't enough code to work off of
initially. Thanks!

One more thing I forgot to mention. I see that you have a variable
named samples, and a string with the value "samples". You should be
aware that there's no way to get from one to the other, unless you
explicitly code it yourself; variable names exist only in your C
source file, not in the program itself. I think you already know, but
I thought I should mention it.

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[tom lewton]

On 25 Jun 2007 at 17:49, Richard Heathfield wrote:

>What's wrong with

strncmp(temp, "samples", sizeof("samples"));

Nothing, particularly, except that it is identical in meaning to
strcmp(temp, "samples") - am I to take it that you enjoy typing?

you seem to enjoy typing yourself, if the code you post here is anything
to go on: frequent verbosity along the lines of

if(p!=NULL) {
singlestatement();
}

instead of

if(p)
singlestatement();

[Richard Heathfield]

tom lewton said:

On 25 Jun 2007 at 17:49, Richard Heathfield wrote:

>>What's wrong with

strncmp(temp, "samples", sizeof("samples"));

Nothing, particularly, except that it is identical in meaning to
strcmp(temp, "samples") - am I to take it that you enjoy typing?

you seem to enjoy typing yourself, if the code you post here is
anything to go on: frequent verbosity along the lines of

if(p!=NULL) {
singlestatement();
}

instead of

if(p)
singlestatement();

Yes, it does seem like that, doesn't it? But in my defence I offer the
following:

1) I reserve my use of if(foo) to situations where foo is clearly
supposed to represent a Boolean concept - for example, I'll happily
write if(isprint((unsigned char)ch) and be glad of the saving. But
since pointers are not an on/off thing, I prefer not to pretend they
are, when using "if". I do this for readability reasons; I do find that
it makes the code quicker to read and understand.

2) It is not uncommon for people to maintain this:

if(condition)
singlestatement();

into this:

if(condition)
singlestatement();
otherstatement();

when what they intended was:

if(condition)
{
singlestatement();
otherstatement();
}

I have found it easier to put the braces in right at the start, as it
saves me debugging time later.

So whilst I agree that my style does cost extra typing time, I am
content that it saves me time overall.

In the OP's case, however, we had a construct that was in fact
equivalent to a strcmp, but this was not self-evident - it took longer
to write *and* longer to read, and I suspect it would take longer to
maintain, too.

--
Richard Heathfield <http://www.cpax.org.uk>
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999