Why does this crash?

[pkirk25]

Hi all,

I'm learning C and trying to find characters in a string array. All my
efforts result in crashing programs. Can anyone see why?

#include <stdio.h>
int main()
{

char strArray[10] = {"abcdefg"};

int i;

int j = strlen(strArray );

for (i = 0; i < j; ++i)
{
char a = strArray[i];

if (a = 'c')
{

printf("%s\n", a);
return 0;
}
}

return 0;
}

[Tak-Shing Chan]

On Sun, 24 Sep 2006, Richard Heathfield wrote:

pkirk25 said:

<snip>

>char a = strArray[i];

if (a = 'c')

You mean:

if(a == 'c')

>{

printf("%s\n ", a);

You probably mean:

printf("%c\n", a);

It's crashing because you're treating a, a single char, as if it were a
pointer to the first in a whole bunch of char.

... or printf("%s\n", strArray). Also, the OP was calling
strlen without a prototype (undefined behaviour).

Tak-Shing

[Eric Sosman]

pkirk25 wrote:

Hi all,

I'm learning C and trying to find characters in a string array. All my
efforts result in crashing programs. Can anyone see why?

#include <stdio.h>

int main()
{
char strArray[10] = {"abcdefg"};
int i;
int j = strlen(strArray );

for (i = 0; i < j; ++i)
{
char a = strArray[i];
if (a = 'c')
{
printf("%s\n", a);

It crashes here because the "%s" code requires that you
provide a matching string argument, but `a' is just a `char'
and not a string. A string is not a single `char', but an
array of `char', one of which has the value zero to mark the
string's end.

Two other points: The `if' statement doesn't do what you
probably intend (go back to your textbook and read about the
difference between the `=' and `==' operators), and you need
to #include the <string.hhead er if you want to use strlen().

--
Eric Sosman
es*****@acm-dot-org.invalid

[CBFalconer]

Richard Heathfield wrote:

pkirk25 said:

<snip>

>char a = strArray[i];

if (a = 'c')

You mean:

if(a == 'c')

You can avoid these problems by adapting the habit of putting the
constant first, i.e.:

if ('c' == a) ...

and the compiler will complain if you have only a single '='.

--
Some informative links:
<news:news.anno unce.newusers
<http://www.geocities.c om/nnqweb/>
<http://www.catb.org/~esr/faqs/smart-questions.html>
<http://www.caliburn.nl/topposting.html >
<http://www.netmeister. org/news/learn2quote.htm l>
<http://cfaj.freeshell. org/google/>

[pkirk25]

Many thanks all. Its allowed me to proceed ot the point where Ic an
find what I want in a string but now it crashes when I try to extract
it to another string.

I've tried several approaches here and all bomb out. Where am I going
wrong please?

#include <stdio.h>
#include <string.h>

int main(void)
{
char strLong[100] = {"Pointer to a confused guy"};
char strDst[100];

int a = 0;

for (a = 0; a < 5; ++a)
{
strcat(strDst, strLong[a]);
}
return 0;
}

[Joe Wright]

pkirk25 wrote:

Hi all,

I'm learning C and trying to find characters in a string array. All my
efforts result in crashing programs. Can anyone see why?

#include <stdio.h>

#include <string.hwill be needed below.

>

int main()

int main(void) is preferred.

{

char strArray[10] = {"abcdefg"};

int i;

int j = strlen(strArray );

for (i = 0; i < j; ++i)
{
char a = strArray[i];

if (a = 'c')

Surely not an assignment. You mean 'if (a == 'c')' don't you?

{

printf("%s\n", a);

I assume you want to print the string beginning with the 'c'.
See below.

return 0;
}
}

return 0;
}

Maybe this is what you mean?

#include <stdio.h>
#include <string.h>

int main(void)
{
char strArray[10] = {"abcdefg"};
int i;
int j = strlen(strArray );
for (i = 0; i < j; ++i) {
char a = strArray[i];
if (a == 'c') {
printf("%s\n", strArray+i);
return 0;
}
}
return 0;
}

Maybe not.

--
Joe Wright
"Everything should be made as simple as possible, but not simpler."
--- Albert Einstein ---

[trm]

pkirk25 schrieb:

I've tried several approaches here and all bomb out. Where
am I going wrong please?

#include <stdio.h>
#include <string.h>

int main(void)
{
char strLong[100] = {"Pointer to a confused guy"};

strLong contains a string of length 25.

char strDst[100];

strDst isn't a string. Exercise: why not? How would you
make it into a string? Trick question: what does it actually
contain? Why?

>
int a = 0;

for (a = 0; a < 5; ++a)
{
strcat(strDst, strLong[a]);

This line has three problems. First, strLong[a] is not of the
appropriate type for strcat(), so this shouldn't even compile
as it is written here. Second, as mentioned above, strDst
isn't a string; using C's string functions on things that aren't
strings generally means anything at all can (and eventually
will) happen. Third, assuming the first two errors are fixed,
you appear to be trying to stuff 116 chars into a 100 char
array. I trust it is obvious why that's bad.

}
return 0;
}

[CBFalconer]

pkirk25 wrote:

>
Many thanks all. Its allowed me to proceed ot the point where Ic
an find what I want in a string but now it crashes when I try to
extract it to another string.

I've tried several approaches here and all bomb out. Where am I
going wrong please?

#include <stdio.h>
#include <string.h>

int main(void)
{
char strLong[100] = {"Pointer to a confused guy"};
char strDst[100];

int a = 0;
for (a = 0; a < 5; ++a)
{
strcat(strDst, strLong[a]);
}
return 0;
}

The following fixes to avoid undefined behaviour may help.

#include <stdio.h>
#include <string.h>

int main(void)
{
char strLong[100] = {"Pointer to a confused guy"};
char strDst[100] = ""; /* <-- Nota bene */
int a = 0;

while (strlen(strDst) + strlen(&strLong[++a]) + 1 < 100) {
putchar(a + '0');
strcat(strDst, &strLong[a]);
}
puts(strDst);
return 0;
}

--
Some informative links:
<news:news.anno unce.newusers
<http://www.geocities.c om/nnqweb/>
<http://www.catb.org/~esr/faqs/smart-questions.html>
<http://www.caliburn.nl/topposting.html >
<http://www.netmeister. org/news/learn2quote.htm l>
<http://cfaj.freeshell. org/google/>

[Keith Thompson]

CBFalconer <cb********@yah oo.comwrites:

Richard Heathfield wrote:

>pkirk25 said:

<snip>

>>char a = strArray[i];

if (a = 'c')

You mean:

if(a == 'c')

You can avoid these problems by adapting the habit of putting the
constant first, i.e.:

if ('c' == a) ...

and the compiler will complain if you have only a single '='.

[...]

Yes, you can, but many people, including me, find that style almost
unbearably ugly.

Train yourself to read "=" as "assign" and "==" as "is equal to".

--
Keith Thompson (The_Other_Keit h) 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.

[Richard Heathfield]

Keith Thompson said:

CBFalconer <cb********@yah oo.comwrites:

>Richard Heathfield wrote:

>>pkirk25 said:

<snip>

char a = strArray[i];

if (a = 'c')

You mean:

if(a == 'c')

You can avoid these problems by adapting the habit of putting the
constant first, i.e.:

if ('c' == a) ...

and the compiler will complain if you have only a single '='.

[...]

Yes, you can, but many people, including me, find that style almost
unbearably ugly.

Yes, they do, but many people, including me, don't. :-)

Train yourself to read "=" as "assign" and "==" as "is equal to".

That's an excellent idea, although personally I prefer "becomes" over
"assign". Once you've done that, though, that doesn't mean you'll always
type what you meant to type. (And if you've *really* trained yourself to
read "==" as "is equal to", then surely it doesn't matter from a
readability perspective which way round they go.)

Incidentally, around 15 years ago I was debugging a production code bug
report. I spotted the following line merely in passing, as it were - it
wasn't the problem I was looking for, but it was nevertheless most
assuredly a problem:

k == 5;

(That was the exact code. I can't remember what k stood for, let alone 5.)

Unfortunately, no parallel trick of the language exists for forcing the
compiler to detect this kind of error.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at above domain (but drop the www, obviously)