How to check if a function exists in PHP?
Answer:
You can use the function_exists to check if a function exists in PHP before calling it.
E.g.
<?php
if ( function_exists("my_function") ) {
my_function();
}
May 272010
May 222010
How to debug in Python?
Answer:
Assume you have a simple python script like:
a = 1
b = 2
c = a + b
print c
To enable debugging, add the following line at the top of the program
import pdb
and in the line you want to break, add
pdb.set_trace()
So the whole program become:
import pdb
a = 1
b = 2
pdb.set_trace()
c = a + b
print c
When you execute the script by python test.py, you will in the debug mode.
Some useful commands:
1. Print variables: p a
2. Step over: n
3. Continue: c
4. Quit: q
Apr 302010
Character classes not working in grep command
Seems grep / egrep does not support character classes, e.g.
# find -type f | grep -e '[\d]'
It finds all the file contains the character d, instead of decimal number(s).
Answer:
By default, egrep only understand the POSIX Basic Regular Expressions (BRE) standard, so what would like to use is:
# find -type f | grep -e '[[:digit:]]'
or you can force to interpret as a Perl regular expression
# find -type f | grep -P -e '[\d]'
Apr 292010
Strict comparison in PHP
Answer:
PHP has two ways to compare two variables, type-safe and type-less, e.g.
Type-safe
print ( "123" === "123" ); // print 1
print ( 123 === 123 ); // print 1
print ( 123 === "123" ); // print nothing
Type-less
print ( "123" == "123" ); // print 1
print ( 123 == 123 ); // print 1
print ( 123 == "123" ); // print 1
Reference: http://php.net/manual/en/language.operators.comparison.php
Apr 282010
Floating point precision in PHP
Answer:
It is typical that simple decimal fractions like 0.1 or 0.7 cannot be converted into their internal binary counterparts without a small loss of precision. This can lead to confusing results: for example, floor((0.1+0.7)*10) will usually return 7 instead of the expected 8, since the internal representation will be something like 7.9.
E.g.
$a = 0.1;
$b = 0.7;
print $a + $b;
print (( $a + $b ) * 10);
Reference: http://www.php.net/manual/en/language.types.float.php
So, never compare two floating numbers in your program.
