Emacs — ispell — No word lists (Ubuntu)

In Ubuntu, if you get:

Error: No word lists can be found for the language “en_US”.

when trying to start ispell (e.g., M-x ispell-buffer) from Emacs, then you need to install the aspell-en package:

sudo apt-get install aspell-en

Emacs — Using rectangles

In Emacs, you can not only select by lines, but also by columns. If you select by columns, the selection is called a rectangle. You can use rectangles with the following shortcuts:

• C-@ or C-Space — start selection (as usual)
• M-x delete-rectangle or C-x r d — delete the rectangle delimited by the start selection and the current cursor position
• M-x kill-rectangle or C-x r k — then you can use — M-x yank-rectangle or C-x r y
• M-x clear-rectangle or C-x r c — replace the contents of the rectangle with spaces
• M-x open-rectangle or C-x r o — insert spaces in the area and push the original contents to the right

C/C++ — Insert sequential values into container

To insert sequential values into a container, you could use the iota function available in ext/numeric header. For example:

#include <ext/numeric>
#include <vector>
…

std::vector<int> v(5);
__gnu_cxx::iota(v.begin(), v.end(), 0);

v will contain {0, 1, 2, 3, 4}.

For more information see sgi documentation

Python — Convert string to HTML/XML entities

In order to convert a string to HTML/XML entities you can use:

”.join(['&#%d;' % ord(ch) for ch in text])

For example if text=’mailto’ the output would be:

‘& #109;& #97;& #105;& #108;& #116;& #111;’

This might be useful in hiding email addresses in web pages.

Unix/Linux — Swap Ctrl and Caps Lock in X11

In order to swap the Ctrl and Caps Lock keys in X11, you can add the following line in Section “InputDevice” of file /etc/X11/xorg.conf:

Option “XkbOptions” “ctrl:swapcaps”

If you woud like to keep the Ctrl where it is and make Caps Lock act as a Ctrl also, you can add:

Option “XkbOptions” “ctrl:nocaps”

Unix/Linux — generate large file

In order to generate a large file of a set size you can use:

dd if=/dev/zero of=test100M.bin bs=100000000 count=1

where of is the output file and bs is its size in bytes.

C/C++ — Friends and Templates

In C++, if you declare a template class and you would like to have a friend function that has an argument of the current class type, you need to be careful when specifying the name of the template types. For example:

template <class T>
class A {
…
template <class T>
friend void b(A<T> a);
} ;

Will confuse the compiler, as you use the same template name, T, for the class template and the function template type. What you need to do, it to use a different name for the function template type:

template <class T>
class A {
…
template <class V>
friend void b(A<V> a);
} ;

Notice that I used T for the class template type and V for the function template type.

Unix/Linux — Redirect output with sudo

Sometimes you might need to redirect the output with sudo privileges. For example:

echo myhost.com > /etc/hostname

will give you the following error:

-bash: /etc/hostname: Permission denied

Unfortunately, using:

sudo echo myhost.com > /etc/hostname

will give the same error.

The solution is to start bash as sudo and then give the entire command to bash as input:

sudo bash -c “echo myhost.com > /etc/hostname”

C++ — Convert string to upper/lower case

To convert an std::string to upper case you can use the following:

#include <algorithm>
#include <string>
…
std::string data = “Abc”;
std::transform(data.begin(), data.end(),
data.begin(), ::toupper);

For converting to lower case, you need to replace ::toupper with ::tolower.

JavaScript — Trim white space

Here is a way to trim the white space from a string in Javascript:

replace(/^\s+|\s+$/g, ”)

By removing |\s+$ it will do a left trim only, and by removing ^\s+| it will do a right trim only (g is no longer necessary).