03
Mar
14

Functional Data Structures in C++: Lists

David Corne:

Great article by Bartosz Milewski. I’m always interested in applying different paradigms to C++. Here is my attempt at this.

Originally posted on   Bartosz Milewski's Programming Cafe:

“Data structures in functional languages are immutable.”

What?! How can you write programs if you can’t mutate data? To an imperative programmer this sounds like anathema. “Are you telling me that I can’t change a value stored in a vector, delete a node in a tree, or push an element on a stack?” Well, yes and no. It’s all a matter of interpretation. When you give me a list and I give you back the same list with one more element, have I modified it or have I constructed a brand new list with the same elements plus one more?

Why would you care? Actually, you might care if you are still holding on to your original list. Has that list changed? In a functional language, the original data structure will remain unmodified! The version from before the modification persists — hence such data structures are called persistent (it has…

View original 4,199 more words

27
Nov
13

Using ediff with Version Control

This is just a quick note about a nice emacs feature.

If you develop with emacs, you may have heard of/used ediff. This is a very handy diff program which runs inside emacs. We use this with a custom hook at work to work with our version control system. I found out that it works nicely with more standard version control systems as well.

The main way to use ediff is the command

  ediff-buffers

This lets you choose two buffers to diff. To do this for different revisions of a file you can use the command

  ediff-revision

This will ask for which file you want to view revisions of (default, the current buffer), and the two revisions to compare (default latest revision and the current state). The you will have the file you asked for loaded in the two revisions you asked for.

Using mercurial personally I find this more informative than the output from hg diff in complicated cases. I believe this works for any version control system recognised by emacs, e.g. mercurial git and subversion.

08
Oct
13

C++ Digraphs

This is not as you might think, an article about implementing directed graphs in C++. The digraphs I am writing about are sequences of characters which act as a stand in for other characters. Digraphs and trigraphs exist in many languages, but I will be focusing on C++. The difference between digraphs and trigraphs is simple the number of characters, a digraph is 2 characters and a trigraph is 3 characters.

Digraphs are part of the language because in the past special characters were hard to input. This was generally because they were not on the keyboard, but in a few cases, they were not even in the code page!

So what use is this now, when modern keyboards have all the symbols we could want and we can use different encodings in source files?

Here is a screenshot of the C++ standard where it defines the allowed digraphs.

Digraphs in C++ Standard

C++ standard digraphs

The first column are backwards compatible digraphs for C. The next two columns are rather interesting.

For example. If you write the word or in a C++ source file, the compiler will replace that with ||. This means that the following code compiles and works.

//=============================================================================
int main() {
  bool a = false;
  bool b = true;
  std::cout << "a or b == " << (a or b) << std::endl;
  return 0;
}

Not only will this work for built in types, this works for user defined operators too. This is because the compiler simply substitutes the symbols.

Here is an example class and calling code which uses all the C++ digraphs.

Class

This is just a stub class which defines a lot of operators which do nothing.

//=============================================================================
class Example {
public:

  //===========================================================================
  // logic operators
  bool operator&&(const Example& t) {
    return true;
  }
  
  bool operator||(const Example& t) {
    return true;
  }
  
  bool operator!() {
    return true;
  }
  
  //===========================================================================
  // bitwise operators
  bool operator&(const Example& t) {
    return true;
  }
  
  bool operator|(const Example& t) {
    return true;
  }
  
  bool operator^(const Example& t) {
    return true;
  }
  
  bool operator~() {
    return true;
  }
  
  //===========================================================================
  // logic equals operators
  Example operator&=(const Example& t) {
    return t;
  }
  
  Example operator|=(const Example& t) {
    return t;
  }
  
  Example operator^=(const Example& t) {
    return t;
  }
  
  Example operator!=(const Example& t) {
    return t;
  }
  
};

Calling Code

//=============================================================================
int main() {
  bool a = true;
  bool b = true;
  if (a and b) {
    cout << "\"and\" is an operator." << endl;
  }
  if (false or b) {
    cout << "\"or\" is also an operator." << endl;
  }
  if (not false) {
    cout << "\"not\" is also an operator." << endl;
  }

  Example t_1, t_2;
  if (t_1 and t_2) {
    cout << "\"and\" even works for classes" << endl;
  }
  
  if (t_1 or t_2) {
    cout << "\"or\" even works for classes" << endl;
  }
  
  if (not t_1) {
    cout << "\"and\" even works for classes" << endl;
  }
  
  if (t_1 bitand t_2) {
    cout << "\"bitand\" also works for classes" << endl;
  }
  
  if (t_1 bitor t_2) {
    cout << "\"bitor\" even works for classes" << endl;
  }
  
  if (compl t_1) {
    cout << "\"compl\" even works for classes" << endl;
  }
  
  if (t_1 xor t_2) {
    cout << "\"xor\" even works for classes" << endl;
  }
  
  t_1 and_eq t_2;
  cout << "\"and_eq\" even works for classes" << endl;
  
  t_1 or_eq t_2;
  cout << "\"or_eq\" even works for classes" << endl;
  
  t_1 xor_eq t_2;
  cout << "\"xor_eq\" even works for classes" << endl;

  return 0;
}

This gives the output;

"and" is an operator.
"or" is also an operator.
"not" is also an operator.
"and" even works for classes
"or" even works for classes
"and" even works for classes
"bitand" also works for classes
"bitor" even works for classes
"compl" even works for classes
"xor" even works for classes
"and_eq" even works for classes
"or_eq" even works for classes
"xor_eq" even works for classes

This file can be found here.

This is an interesting feature, but is it useful?

I think it is, I think that the code;

if (not fail and ok) {
  // ...
}

Is more readable than;

if (!fail && ok) {
  // ...
}

However just because these are defined in the standard does not mean they can be used. These are largely considered a legacy part of the standard and there is limited support for it.

In particular Microsoft’s C++ compiler will not compile it. Both clang and g++ will compile it however, so if you are compiling using either of these you can use this nice feature.

Thanks for reading.

20
May
13

Keeping History Using the cd Command

If you use shell scripting a lot from the command line, this will probably improve your experience of it. This is for users of linux/mac command line or of cygwin on windows. Anywhere you can use the bash cd command.

Firstly (if you didn’t know) cd remembers the last directory you went to. To access this use;

cd -

This will change to the previous directory you were in. If you want to supercharge this you can add the following to your .bashrc/.profile/… customization file.

# petar marinov, http:/geocities.com/h2428, this is public domain
cd_func ()
{
  local x2 the_new_dir adir index
  local -i cnt

  if [[ $1 ==  "--" ]]; then
    dirs -v
    return 0
  fi

  the_new_dir=$1
  [[ -z $1 ]] && the_new_dir=$HOME

  if [[ ${the_new_dir:0:1} == '-' ]]; then
    #
    # Extract dir N from dirs
    index=${the_new_dir:1}
    [[ -z $index ]] && index=1
    adir=$(dirs +$index)
    [[ -z $adir ]] && return 1
    the_new_dir=$adir
  fi

  #
  # '~' has to be substituted by ${HOME}
  [[ ${the_new_dir:0:1} == '~' ]] && the_new_dir="${HOME}${the_new_dir:1}"

  #
  # Now change to the new dir and add to the top of the stack
  pushd "${the_new_dir}" > /dev/null
  [[ $? -ne 0 ]] && return 1
  the_new_dir=$(pwd)

  #
  # Trim down everything beyond 11th entry
  popd -n +11 2>/dev/null 1>/dev/null

  #
  # Remove any other occurence of this dir, skipping the top of the stack
  for ((cnt=1; cnt <= 10; cnt++)); do
    x2=$(dirs +${cnt} 2>/dev/null)
    [[ $? -ne 0 ]] && return 0
    [[ ${x2:0:1} == '~' ]] && x2="${HOME}${x2:1}"
    if [[ "${x2}" == "${the_new_dir}" ]]; then
      popd -n +$cnt 2>/dev/null 1>/dev/null
      cnt=cnt-1
    fi
  done

  return 0
}

alias cd=cd_func

This means that you can keep a history of the past 10 directories which you have visited and change between them.

To access this list type

cd --

Which will print a list like.

0  /e/projects/dgc/or14126h_3
1  /e/projects/dgc/or14126h_3/orthotics.dev
2  /c/Users/dgc/Dropbox/Coding
3  /e/projects/dgc
4  /e/projects/dgc/or14126h_2
5  /e/devdisk/dcm/configs
6  /e/devdisk/dcm
7  /e/devdisk
8  /e

And then

cd -5

will take you to entry 5 in the list.

To be clear I did not write this, I found it in the default cygwin .bashrc. The author of this was Petar Marinov.

I have found this very helpful, particularly when using a new python module. This is the process;

  • I would be in my working area
  • download it from pypi
  • cd to my download area
  • untar it
  • cd into it
  • run python setup.py install
  • then want to go back but cd – would get me to my download area.

This function removes that minor inconvenience and I have found it very useful.

Thanks for reading.

29
Apr
13

Unusual C++ use of #include

Originally posted on Muhamad Hesham's T-Blog:

"The

The usual use is to write it at the very top of your .h/.cpp files to include other header files like (Windows.h, iostream, cstdio, etc..) to use the what is defined inside in your .h/.cpp file.

The unusual use is to use it to initialize a data-structure like arrays by including a text file that contains the array initialization data between the array initializer list parentheses – e.g XX XXX[] = { <HERE> }. This is is illustrated in the sample program below. The same concept can be used to initialize an array of any dimension.

// main.cpp
#include <cstdio>
#include <Windows.h>

struct Person
{
    char        *pName;
    unsigned    Age;
    unsigned    Height;
    char        Gender;
};

// Declare and initialize a 1D array of Persons structs using PersonTableData text file
// #include "PersonsTableData" will be expanded at COMPILE TIME to the content of PersonsTableData file
Person PersonsTable[] = {
    #include "PersonsTableData"

View original 73 more words

18
Apr
13

Resource Acquisition is Initialization (RAII)

This post will be about the Resource Acquisition is Initialization (RAII) pattern which is a creational pattern. This is going to be the first non Gang of Four pattern I will write about.

The Purpose

The idea behind this pattern is to correctly dispose of all the resources that you acquire. This pattern was first written about by Bjarne Stroustrup, the creator of C++. The most common examples of this pattern are in opening and closing files and web sockets. It is also important in controlling mutexes so you can write tread safe code.

Continue reading ‘Resource Acquisition is Initialization (RAII)’

08
Apr
13

Comparisons in C++

This is about the comparison operators in C++ and making them easy to implement. There are 6 comparison operators in C++; ==, !=, <, <=, >, and >=. If you want to be able to apply all of these to a class, and you have the right type of order, you only need to implement one function which will determines them all. Incidentally this is called a total order, but I won’t go into what that means here.

The tl;dr of this is look at this repository and use a class from there to easily implement custom comparison operators.

This idea is to map the order onto the real numbers (doubles). So you have a function which takes two instances of a class and returns a double. If this function returns a negative number then the first instance is less than the second, if it returns 0 they are equal and if it returns a positive number then the first instance is greater than the second.

This will be easier to see with an example, so here is a class which is just data storage for an int.

Continue reading ‘Comparisons in C++’




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