making c codes faster

some basic tricks for speeding up c codes:

• division is much slower than multiplication
• log10(), sqrt() are also very slow -- should avoid using these unless absolutely necessary
• inlines can be of tremendous help, as it may be easier for the compiler to optimize
  
• a for loop like for(i=1;i<=10;i++) can be re-written as for(i=10;i--;), which may save a little bit of time

more tricks can be found here