PARI/GP tips and tricks

[Dr Sardonicus]

Originally Posted by charybdis

divisors() can take a factorization matrix as input.

D'oh!

I should have just tried that! I should have realized it was very likely to work.

[tries an example, and it does indeed work]

Thank you!

[R. Gerbicz]

Once you factored n, you could add the primefactors, so then a new numdiv/divisors or any such functions when you know factor() would be called will be much faster.

? ?addprimes
addprimes({x=[]}): add primes in the vector x to the prime table to be used in trial 
division. x may also be a single integer. Composite "primes" are NOT allowed.

But if you would add too "many" factors, then it could actually slow down these calls, because the code goes through the added primes list (until n is not factorised). Really helpful, used it in real codes.

[paulunderwood]

How do I increase threadsizemax before running gp under Linux? I.e. what do I need to put in $HOME/.gprc?

[bur]

To have a progress bar, it would be useful to output a string to the same line, overwriting the previous output. Is that possible? Maybe by using "delete" characters as print() arguments?

[Neptune]

Originally Posted by bur

To have a progress bar, it would be useful to output a string to the same line, overwriting the previous output. Is that possible? Maybe by using "delete" characters as print() arguments?

This should work with printf():

printf(" ... %c", ... , 13)

with decimal '13' means the ASCII code for carriage return.


For example counting primes up to 10^9 and get progress information in a fixed line would look like this:

c=0;s=1e9;forprime(p=2,s,c++;if(c%10^5==0,printf(" %3.1f%%  %c",p/s*100,13)));c

Of course frequently calling printf() will slow down the loop, so in this example it is done only after every 100,000-th prime.

[Neptune]

A carriage return is also possible with:

print1(strchr(13))

[bur]

Thanks, that seems to be what I was looking for!

[a1call]

Very nice Neptune. Thank you. I too have been looking for a carriage return without the linefeed to no avail.
I have vague memory of cls in the old DOS having a similar functional use. I checked it just now and it works too:

\\ To Clear the Screen
system("cls")
\\

Note:
* Not sure if it would work on a Linux system.

Adding your code to my library. Thanks again.

[paulunderwood]

Originally Posted by a1call

Very nice Neptune. Thank you. I too have been looking for a carriage return without the linefeed to no avail.
I have vague memory of cls in the old DOS having a similar functional use. I checked it just now and it works too:

\\ To Clear the Screen
system("cls")
\\

Note:
* Not sure if it would work on a Linux system.

On Linux it is the clear command.

Cross platform: print("\e[H\e[2J")

[a1call]

Noted and filed to my library. Thank you.

[EdH]

I've been using printf "Text, text, text. \r" in my bash scripts in linux. Less for me to remember, although from my Windows days I can still remember cr = 13.