char array declaration help

[native moss]

I have this char array that will hold a string(name) but i want to make it where the size of that array is what the user inputs

    char src[MAX_LINE_LEN];
    printf("String Copy:\n");
     printf("Enter dest (string you want to copy to: ");
    scanf("%s", dest);
    int n = csc225_getline(dest, MAX_LINE_LEN);
    dest[] = dest[n]; ```
like for example this snippet of code
i have dest char array then i ask user to input a string for it

[fickle perch]

When your question is answered use !solved to mark the question as resolved.

Remember to ask specific questions, provide necessary details, and reduce your question to its simplest form. For more information use !howto ask.

[native moss]

then i use csc225_getLine which returns the number of characters written into dest including \0
and Im trying to declare the size of dest to the size of the string that the user inputted

[scenic jackal]

!vla

[fast trellis]

What Is a VLA And Why Is It 'Bad'?

A Variable Length Array (VLA) is an array where the size is not constant and depends on a variable.

VLAs have poor compiler support and can lead to inefficient code. The core issue with VLAs is that the compiler doesn't know the size of the stack frame. Without warning flags like -Wvla (turned on by -Wall) it can be easy to create a VLA by accident, even in C++ with some compilers.

Compiler Support

✅ available since C99
⛔ not available in C++ at all
⛔ was never supported by MSVC
⚠ optional feature since C11
⚠ supported as non-standard extension by GCC, clang

See Also

What technical disadvantages do C99-style VLAs have?
What's the point of VLA anyway?

[scenic jackal]

you should much rather use malloc/calloc

!man malloc

[fast trellis]

malloc(3)

malloc, free, calloc, realloc, reallocarray - allocate and free
dynamic memory

The malloc() function allocates size bytes and returns a pointer
to the allocated memory. The memory is not initialized. If size
is 0, then malloc() returns either NULL, or a unique pointer
value that can later be successfully passed to free().

Synopsis

#include <stdlib.h>

void *malloc(size_t size);
void free(void *ptr);
void *calloc(size_t nmemb, size_t size);
void *realloc(void *ptr, size_t size);
void *reallocarray(void *ptr, size_t nmemb, size_t size);

Also See

valgrind(1), brk(2), mmap(2), alloca(3), malloc_get_state(3), malloc_info(3), malloc_trim(3), malloc_usable_size(3), mallopt(3), mcheck(3), mtrace(3), posix_memalign(3)

[native moss]

?

im sry i dont comprehend this

[scenic jackal]

To make it short: There is such a thing as dynamically sized arrays, but it's bad, don't use it.

[native moss]

so. how can i make the char array the size of the string that the user inputs

for example i want char dest[] to be [5] if user inputs abby but if they put another name like tom i want it to be size 4

[scenic jackal]

Let n be the number of elements that the user wants:

char *dest = (char *) malloc(n * sizeof(char));  // sizeof(char) is always 1, but for other datatypes it's often not

The cast is also unnecessary, but where I learnt it, it was considered good practice

[native moss]

do you know why my printstatement for before= wont print? https://onlinegdb.com/AEwAsxKBB

[scenic jackal]

It's because of scanf. You can fix this by putting a space before the %s, but if all you're doing is reading a string I'd always use fgets. Also please have a look at this: http://sekrit.de/webdocs/c/beginners-guide-away-from-scanf.html

[native moss]

 printf("Enter src (string you want to copy from): ");
    //scanf("%s", src);
    fgets(src, MAX_LINE_LEN, stdin);
    printf("before= %s\n", dest);```

i fixed it but still get the same thing

[scenic jackal]

Please read the first sentence of my last message again.

[native moss]

im a little slow sorry

but what are u trying to say? I cant print because the scanf right? but i just removed it for the fgets liek you said

[scenic jackal]

Ah sry, I overlooked the comment

What's it doing now that you replaced it for fgets?

It might be that the first scanf still has something to do with this

so change that to an fgets, too

[native moss]

gives me the same thing

[scenic jackal]

Let me guess, you're using scanf before that already...

Just read the article: http://sekrit.de/webdocs/c/beginners-guide-away-from-scanf.html

[native moss]

ok

[scenic jackal]

it's really incredibly helpful, not only for user I/O, but also for general C understanding

[native moss]

yea i got it

[scenic jackal]

and it's not that long nor difficult

[native moss]

so scanf is a no go when taking user input?

yea thanks for the links

[scenic jackal]

If you just want a quick testing sesssion, scanf is fine, but it can lead to problems really fast, if you don't know exactly what you're doing

[native moss]

ill check em out im just on a time crunch w this project and prof didnt give us a long deadline

[scenic jackal]

Basically chapter 1 and 2 are just introducing all the problems that can arise, and chapter 3 and 4 show the actual implementations, so you could skip the first 2 chapters if you're in a rush.

[native moss]

ok im trying to run my main function in linux but im getting these errors do you know why that is?

because in the online compiler i had no errors in my program and it ran fine

[scenic jackal]

They're just warnings, but apparently you're not using the return value, which returns either the first parameter again or NULL in case of an error

I've never seen a compiler complain about that though

[native moss]

!solved

[fickle perch]

Thank you and let us know if you have any more questions!

[SOLVED] char array declaration help

[fickle perch]

char array declaration help