Thread: Need help with altering info on a txt file

That's why we need user-editable FAQs!

Indeed, it doesn't work. I was thinking of printf(), in which an asterisk indicates the width or whatever of a field.

You should (I know you can't edit it, but keep it in mind for the Wiki) add a note about error checking. Number of format specifiers for scanf(), NULL for fgets().

I'm thinking of adding/changing into this:

MSDN has an article on scanf and how you specify the size: http://msdn2.microsoft.com/en-us/lib...x1(VS.80).aspx
Remember that scanf also stops reading at the first blank space ( ), so if you type something like "Hello World" - it will only read "Hello". Both of these problems can be fixed using fgets.
But wait! There's a third problem with scanf! If scanf encounters some characters it doesn't want to read, it simply leaves them in the buffer - so in the example above with "Hello World", there will still be " World" left in the buffer after scanf returns! That means the next time you call scanf to read something from the user, it will read this " World" and not prompt the user for anything.
Further, even if the user does enter something and scanf reads it, it will usually leave the newline ('\n') in the buffer so the next call to scanf will return immediately just like the above example!
To fix this, usually you would do a loop with getchar until get clear the input buffer of all remaining characters (see examples). Then scanf will behave as expected again.
But you can also just use fgets, which takes an argument for buffer size. Here's documentation for fgets on MSDN: http://msdn2.microsoft.com/en-us/lib...kf(VS.71).aspx
Remember that fgets reads an entire line and also stores the '\n' (the newline) in your buffer.

And lastly, I will differentiate between '\0', 0 and NULL.
Essentially, they all translate to the same thing to the compiler - 0. However, this does NOT mean that you should assign NULL to a character index.

Code:

char buffer[10];
buffer[0] = NULL; /* Bad! */

NULL is used to set pointers to point at 0 only, and with pointers only. Usually, 0 is an guaranteed to be invalid address (at least in Windows), so we typically set pointers to NULL when we don't need them so as to flag them, "these are not valid pointers anymore, don't use them."
With strings, you should preferably use '\0' since it's the actual character for terminating strings. 0 is the same, so it's fine to assign 0, too, but it is recommended to use '\0'.

Code:

char buffer[] = "Hello World!";
buffer[5] = 0; /* Buffer will now be "Hello" */
buffer[5] = ' '; /* Buffer will now be "Hello World!" */
buffer[5] = '\0'; /* Buffer will now be "Hello" */

Take heed also that NULL is defined as ((void*)0) in C since it's supposed to be used with pointers only (it's quite valid in C). Although due to C's poor type safety, it's possible to assign NULL to a character array, though you may get a warning. Right, so the idea is to avoid compiler warnings, so don't do it. The idea with 0, '\0' and NULL are all to avoid confusion. Use them where appropriate or you might confuse someone else how reads your code.
As a last note, NULL is defined as 0 in C++, so it's easier to get away with setting a character array to NULL, but again, don't do it.

...And...

And lastly... an example of how to strip trailing newlines from your buffer:

Code:

char buffer[100];
int size;
fgets(buffer, sizeof(buffer), stdin);
size_t size = strlen(buffer);
if (buffer[size - 1] == '\n') buffer[size - 1] = '\0';

This code will basically take the length of the string and loop so long as the last element in the array is '\n' (newline). If it is a newline, it will truncate the value with '\0' (which essentially sets this as the new end of the string).

Please correct any mistakes

Originally Posted by Elysia

Please correct any mistakes

Why a for loop to iterate the whole string when the only newline we care about is the last one before the null terminator?
Why the calling of strlen() so many times?

Code:

   fgets( user_input, sizeof user_input, stdin );
    
    len = strlen( user_input ) - 1; /* get the lenght of string before '\0' */
    if ( user_input[len] == '\n' )/* if there's a new line before '\0' */
    {
         user_input[len] = '\0'; /* eliminate the '\n' */
    }

I like assigning the return value of strlen() to variables of type size_t, because that's strictly what it should be -- although many people just use ints. size_t is unsigned, so assigning it to an int gives you some warnings when you enable them. In addition, with REALLY long strings, an int might not have enough range.

NULL is used to truncate pointers, and with pointers only.

"Truncate"? Not exactly. More like, "assigning NULL to a pointer makes the pointer point at nothing". Also consider linking to http://faq.cprogramming.com/cgi-bin/...&id=1043284376

Code:

buffer[5] = '\0'; /* Buffer will not be "Hello" */

You mean "now", right?

I'm telling you, we need a Wiki FAQ . . . .

Originally Posted by Aia

Why a for loop to iterate the whole string when the only newline we care about is the last one before the null terminator?
Why the calling of strlen() so many times?

Code:

   fgets( user_input, sizeof user_input, stdin );
    
    len = strlen( user_input ) - 1; /* get the lenght of string before '\0' */
    if ( user_input[len] == '\n' )/* if there's a new line before '\0' */
    {
         user_input[len] = '\0'; /* eliminate the '\n' */
    }

Because of dwks samples! Blame dwks, not me!
So I changed it to remove a single newline instead.

Originally Posted by dwks

I like assigning the return value of strlen() to variables of type size_t, because that's strictly what it should be -- although many people just use ints. size_t is unsigned, so assigning it to an int gives you some warnings when you enable them. In addition, with REALLY long strings, an int might not have enough range.

You're right. I don't really remember what all these functions return, so I just check compiler warnings about truncating and change variable type appropriately. Anyway, it's size_t now.

"Truncate"? Not exactly. More like, "assigning NULL to a pointer makes the pointer point at nothing". Also consider linking to http://faq.cprogramming.com/cgi-bin/...&id=1043284376

Bad wording, perhaps? I changed the explanation a little - so maybe it's better this time?

Code:

buffer[5] = '\0'; /* Buffer will not be "Hello" */

You mean "now", right?

I'm telling you, we need a Wiki FAQ . . . .

I do! Mistake!

Because why is a place

It will eventually be pushed down by the flood of all other threads.
Yes, we wrote that so you (and all other newbies making the same mistake!) can understand the dangers of doing it!

You're not supposed to bump threads older than two weeks.

@Elysia: now that there really is a Wiki FAQ, why not make this into a page for it?

Indeed. I dunno where my draft I had stored somewhere disappeared to, however.

If only we could destroy all the C-book parts that introduce scanf() to the C-student...
It actually is more straightforward to use the other input functions.

And also typically more error prone, sadly.