Category Archives: C/C++

How to Insert 1 Bit Into An Integer in C/C++

Inserting a bit into an integer, means to insert a bit into a specific position of an integer, with the highest bit removed. For example, for a one byte integer 11001010, if I insert a 1 at digit position 4, which is marked here 1100|1010 as |, the most significant bit, which is the left most 1, would be eliminated and the rest of the 3 bits on the left side of | would be shifted to the left for 1 bit.

This is not hard to implement, but surprisingly I cannot find any existing code snippet for such a simple job.

Here is the code snippet I wrote for C++ (which should also work for C with slight modification), and it can be easily translated into most other languages. Here, T is any integer type.

#include <stddef.h> // for size_t
template <typename T> // if used in C, remove this line and replace T with an integer type
T insert_bit(T n,   // The integer we are going to insert into
             size_t position, // position is the position of the new bit to be inserted
             bool new_bit) // whether the newly inserted bit is true or false
    T x = n;
    T y = x;
    x <<= 1;
    if (new_bit)
        x |= (((T) 1) << position);
        x &= ~(((T) 1) << position);
    x &= ((~((T) 0)) << position);
    y &= ~((~((T) 0)) << position);
    x |= y;
    return x;

Don’t hesitate to comment if you think there is a better (more elegant or faster) way.

Generate Ctags Files for C/C++ Source Files and All of Their Included Header Files

This post is for those people who use Exuberant Ctags. If you are using other versions of ctags, this post may not be useful.

When using ctags to generate the tags file for C/C++ projects, usually we use the following command:

ctags -R .

For some users that need more info of the symbols, they may use this command instead:

ctags -R --c++-kinds=+p --fields=+iaS --extra=+q .

No matter which one you use, the generated tags file only contains the symbols in the files in your project source tree, but not any external file, such as standard header files (e.g. stdio.h, stdlib.h), etc. thus editors or IDEs that use tags files, such as Vim, are not able to locate symbols in external header files. There was a solution: generate a tags file for any external header files first, and let the editor or IDE read both the generated tags file and the tags file for the project source tree. For example, the following command will generate a tags file for all your system header files on UNIX/Linux:

ctags -R --c++-kinds=+p --fields=+iaS --extra=+q /usr/include

This command usually takes a very long time to finish, and finally it gives a quite large tags file, which causes the editor or IDE a long time to search this tags file for symbols. To solve this problem, I came up with another idea.

Why must we generate a tags file containing all the symbols in the system header? If we only generate the tags file only for the header files that are related to our projects, would it be faster? That’s the point of this idea. We could first search for the header files that are included in our projects, and then we use ctags to generate a tags file for these files and our source files, in this way, a much smaller tags file that containing all the symbols that maybe useful for the project is generated.

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