How to find length of array without getting it from user?

Question

https://code.sololearn.com/cr7c1LvUmNni/?ref=app
Here I tried len=sizeof(a)/sizeof(*a) to get the length.
But everytime I try to do this, sizeof(a) just takes the total size of the declared array,
eg a[30] so sizeof(a) is becoming 30*4=120
So I tried declaring an empty array but it says it is invalid.
How can I solve this?

Answer 1

Jazz wow that was a very detailed explaination. Thanks a ton!

As you explained VLA, are they similar to dynamic memory allocation or are they completely different? Coz I never heard about it before.

Answer 2

Jazz then aren't all arrays VLAs? I mean reading the name,
I thought their size are modified at runtime,
like if I declare a[20]
then increasing its size, say a[25], are variable length arrays.

Isn't this the usual declaration, n for size of array and
int arr[n];
What makes it variable, just curious 🤔

Answer 3

Jazz Thanks a ton, this was an absolutely crystal clear explanation.

And No,no I knew before array sizes cannot be resized at compile and runtime (anyway I really didn't understand the difference between runtime and compile time before, but I learnt about it only after reading your explanation😅), I just thought, reading their name "variable length arrays", maybe there was a way to change them and that's why I asked.

You really made things a lot clearer anyway. Thanks and Cheers!