evaluate a string expression in if statement Linux C/C++ - C

"parul.prasad@wipro.com" wrote:
>
> How can we evaluate a string expression in if statement Linux C/C++

By passing it to a function designed to evaluate it. What that
does will depend on the definition of 'evaluate'.

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"parul.prasad@wipro.com" <parul.prasad@wipro.com> writes:
> How can we evaluate a string expression in if statement Linux C/C++

If your question were specific to Linux, it would be off-topic here.

There is no such language as "C/C++". C and C++ are two separate (but
related) languages. C is the only one we discuss here.

Here's an example of evaluating a string expression in an if statement
in C:

#include <stdio.h>
int main(void)
{
if ("string expression") {
printf("ok\n");
}
return 0;
}

That answers the question you asked. I doubt that it answers the
question that you *meant* to ask.

You can expect slow feedback in this newsgroup, because articles have
to be approved by the moderator before they can appear. You might
have better luck in comp.lang.c, since I expect we'll need some
back-and-forth discussion before we (a) figure out what you're asking,
and (b) come up with a good answer.

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parul.prasad@wipro.com wrote:
> How can we evaluate a string expression in if statement Linux C/C++

It's not clear what you're trying to do. What do you mean by "evaluate
a string expression?" If you're comparing two strings, for example,
you'll want something like:

if(strcmp(s1, s2) == 0) {
puts("The strings are equivalent.");
}
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"parul.prasad@wipro.com" wrote:
> How can we evaluate a string expression in if statement Linux C/C++

It's not clear what you mean by that. C doesn't have "string expressions".
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On 15 Nov 2006 05:19:29 GMT, "parul.prasad@wipro.com"
<parul.prasad@wipro.com> wrote in comp.lang.c.moderated:

> How can we evaluate a string expression in if statement Linux C/C++

First, there is no language C/C++. There is C and there is C++, and
we only discuss the former here.

Second, before anyone can even begin to answer your question, you have
to tell us what you mean by "evaluate a string expression". C doesn't
actually have anything called a "string expression".

The following code evaluates the length of a string in an if
expression, but is probably not what you want:

#include <string.h>

int is_long_enough(const char *s)
{
if (strlen(s) > 10)
return 1;
else return 0;
}

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parul.prasad@wipro.com wrote:
> How can we evaluate a string expression in if statement Linux C/C++

Without custom code, you probably can't.
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On 15 Nov 2006 05:19:29 GMT, "parul.prasad@wipro.com"
<parul.prasad@wipro.com> wrote:

>How can we evaluate a string expression in if statement Linux C/C++

First decide if you are going to code in C or C++. The two languages
are not the same. If you want to use C++, this is not the correct
newsgroup.

Then explain what you mean by evaluate. There are numerous C
functions which can process a string as part of an if expression.


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parul.prasad@wipro.com wrote:
> How can we evaluate a string expression in if statement Linux C/C++

OK. So it's hard to understand what you mean - maybe a more elaborate
question would have been clearer. However, as this is
comp.lang.c.moderated, I'm going to ignore any c++ features and just
talk about C strings.

A C string is a bit wierd in that it is actually a pointer to some
memory - all this "sometext" stuff is just to make initializing the
memory easier if you need to say text.

Thus if I go:

char str[16];

Then I've allocated a 16 char (probably 1 byte length each) long array
on the stack. Saying

str[7]

is identical to

*(str+7)

Either will give us the 8th character in the array. The first is
"special" syntax used by C for arrays it knows lots about. The second
just treats str as a pointer to a char array, which is what it really
is.

Anyway, you want to "evaluate a string expression" "in an if". Well,
that doesn't really make sense, as a string expression isn't really
very boolean, but the meanings I can think of are:

1) Test to see whether str is "true" or "false".
How about:

if( strcmp( str, "true" ) == 0 ) { return 1 }
else if( strcmp( str, "false" ) == 0 ) { return 0 }
else return -1;

2) Test to see whether a string pointer is null ( you should do this
before any of the other tests too, probably).

if( str == NULL ) return -1;

3) Test to see whether a string is "":

if( str[0] == 0 ) return 0;
OR
if( strlen(str) == 0 ) return 0;

4) Test to see whether two pointers are to the same string in memory
(ie both at same buffer)

if( str1 == str2 ) return 1

5) Test to see whether the string arrays referred to by str1 and str2
are the same

if( ! strcmp( str1, str2 ) ) return 1;

As you can see, you asked a really vague question. It's unlikely that
people will always take the time to give you an answer to a question
like this, and besides I might not be telling you what you need to know
- I'm having to guess.

In future, I'd suggest a longer question. I realise that English
probably isn't your first language, so I understand that it costs more
effort and might require 20 minutes with a dictionary. For that I
apologise, but it seems that mailing lists are dominated by English
speakers who can't cope with anything else. Like me

I hope this reply is of some use,

Rupert
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On 15 Nov 2006 05:19:29 GMT in comp.lang.c.moderated,
"parul.prasad@wipro.com" <parul.prasad@wipro.com> wrote:

>How can we evaluate a string expression in if statement Linux C/C++

Use a string function?

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