Creating a user length defined array in C

Question

I'm trying to make an array with variable starting length to get a string. The code should count the words and adjust the size of the array, but this is only a test and I expose it here because I want to know if it's a good practice or one error. And if there is something I should know about, or I must have in mind.

Note, I talk about C, not C++

#include <stdio.h>
int main()
{       int c,b,count;
        scanf("%d",&c);
        count=c+1;
        getchar();
        char a[count];
        for ( c=b=0 ; c!=count && b!='\n' ; c++ )
        {
                b=getchar();
                a[c]=b;
        }
        a[c]='\0';
        printf("%s",a); printf("%d",c-1);
}

I don't need change the size of the array at the execution time.

I was testing and I don't remember well why I'm using the c variable at first time instead of count directly, but I remember the first getchar was to flush the buffer, because it didn't work without the getchar.

I don't know why I need to put getchar. If I delete the getchar the program fails.

Anyway the program works fine. The first time you run, it expects a number with scanf and then expects the text.

If the text is larger than the size of the array the program will ignore it.

The number is the size of the array.

My questions are:

It is a good practice do a[variable] to do this job?

Why I need the getchar?

It will be portable? I mean, I don't know if some systems or standards don't accept this like some old C compilers or somewhat.

There are better methods?

Answer 1

It is a good practice do a[variable] to do this job?

It depends on someone's compiler configuration. It has been supported since C99. However since there's not a good reason to use it in such a simple program, use the standard malloc instead. Here's an in-depth discussion of the topic.

Why I need the getchar?

There's likely some input still buffered up in your terminal, and that first character is discarding it. Try printing the value out to the screen to see what it is, that might help as figure it out.

It will be portable?

See my answer to your first question. It will probably work on modern versions of gcc, but for example it doesn't work in Windows C (which is still basically on C89).

Answer 2

It is a good practice do a[variable] to do this job?

Where the size is determined by arbitrary user input without imposed limits, it is not good practice. A user could easily enter a very large value and overrun the stack.

Use either dynamic allocation, or check and coerce the input value to some sensible limit.

Also worth noting that VLAs are not supported in C++ or some C compilers, so the code lacks portability.

Why I need the getchar?

The user has to enter at least a newline for scanf() to return, but the %d format specifier does not consume non-digit characters, so it remains buffered. However your code is easily broken by entering additional non-digit characters for example "16a<newline>" will assign 16 to c, and the a will be discarded leaving the newline buffered as before. A better solution is:

while( getchar() != `\n` ) {}

It will be portable? I mean, I don't know if some systems or standards don't accept this like some old C compilers or somewhat.

Adoption of C99 VLAs is variable, and in C11 they are optional in any case.

There are better methods?

I hesitate to say "better", but safer and more flexible and portable ways sure. With respect to the array allocation, you could use malloc().

Answer 3

Using malloc or calloc would be a better choice in C

https://www.tutorialspoint.com/c_standard_library/c_function_malloc.htm