Hello. Just a quick question: What are people's practices when you have to define the (arbitrary) maximum that some array can take in C. So, some people just choose a round number hoping it will be big enough, others the prime number closer to the round number (!), etc., other some more esoteric number, like the prime number closer to... and so on.
I'm wondering, then, what are some best practices for deciding such values?
Thanks.
There is no general rule. Powers of twos work for buffers, I use 1024 quite often for string buffers in C but any other number would work. Prime numbers are useful for hash tables where simple modulo-hashing works well with prime-number sizes. Of course you define the size as a symbolic constant so that you can change it later.
If I need to do this I usually go with either a power of two, or for larger data sets, the number of pages required to hold the data. Most of the time though I prefer to allocate a chunk of memory on the heap and then realloc if the buffer size is insufficient later.
If I can't pin down a reasonable maximum I tend to use malloc and realloc to grow the array as needed. Using a fixed size array when you can't gurantee that it is large enough for the intended purpose is hazardous.
Best practice is to avoid arbitrary limits whenever possible.
It's not always possible, so second-best practice is to take an educated estimate of the largest thing that the array is ever likely to need to hold, and then round up by a healthy margin, at least 25%. I tend to prefer powers of ten when I do this, because it makes it obvious on inspection that the number is an arbitrary limit. (Powers of two also often signify that, but only if the reader recognizes the number as a power of two, and most readers-of-code don't have that table memorized much past 216. If there's a good reason to use a power of two and it needs to be bigger than that, write it in hex. End of digression.) Always document the reasoning behind your estimate of the largest thing the array needs to hold, even if it's as simple as "anyone with a single source file bigger than 2GB needs to rethink their coding style" (actual example)
Don't use a prime number unless you specifically need the properties of a prime number (e.g. as Juho mentions, for hash tables -- but you only need that there if your hash function isn't very good -- but often it is, unfortunately.) When you do, document that you are intentionally using prime numbers and why, because most people do not recognize prime numbers on sight or know why they might be necessary in a particular situation.
I only define a maximum when I have a strong reason for a particular number to be the maximum. Otherwise, I size it dynamically, perhaps with a sanity-check maximum (e.g. a person's name should not be several megabytes long).
Round numbers (powers of 2) are used because they are often easy for things like malloc to use (many implementations keep up with memory in blocks of various power of two sizes), easier for linkers to use (in the case of static or global arrays), and also because you can use bitwise operations to test for limits of them, which are often faster than < and >.
Prime numbers are used because using prime number sized hash tables is supposed to avoid collision.
Many people likely use both prime number and power of two sizes for things in cases where they don't actually provide any benefit, though.
It really isn't possible to predict at the outset what the maximum size could be.
For example, I coded a small cmdline interpreter, where each line of output produced was stored in a char array of size 200. Sufficient for all possible outputs, don't you think?
That was until I issued the env command which had a line with ~ 400 characters(!).
LS_COLORS='no=00:fi=00:di=01;34:ln=01;36:pi=40;33:so=01;35:bd=40;33;01:cd=40;33;01:or=01;
05;37;41:mi=01;05;37;41:ex=01;32:*.cmd=01;32:*.exe=01;32:*.com=01;32:*.btm=01;32:*.bat=01;32:*.sh=01;
32:*.csh=01;32:*.tar=01;31:*.tgz=01;31:*.arj=01;31:*.taz=01;31:*.lzh=01;31:*.zip=01;31:*.z=01;31:*.Z=01;
31:*.gz=01;31:*.bz2=01;31:*.bz=01;31:*.tz=01;31:*.rpm=01;31:*.cpio=01;31:*.jpg=01;35:*.gif=01;35:*.bmp=01;
35:*.xbm=01;35:*.xpm=01;35:*.png=01;35:*.tif=01;35:';
Moral of the story: Try to use dynamic allocation as far as possible.