Archive for the ‘Code’ Category

QgsExpression with Python

In QGIS, there is this cool feature called by Expression. I use it mostly on Field Calculator in attribute table. There are some places that you can find also. You can read more about this Expression here.

This week, I tried to use it in Python. It turned out very simple (of course, I need to Google it here and there first how to use it). The class name is QgsExpression. You can import it from qgis.core. Below a sample code how to use it:

from qgis.core import QgsField, QgsExpression
def sum_fields(layer, output_field_name, input_fields):
    """Sum the value of input_fields and put it as output_field.

    :param layer: The vector layer.
    :type layer: QgsVectorLayer

    :param output_field_name: The output field name.
    :type output_field_name: str

    :param input_fields: List of input fields' name.
    :type input_fields: list
    # Creating expression
    string_expression = ' + '.join(input_fields)
    sum_expression = QgsExpression(string_expression)

    # Get the output field index
    output_idx = layer.fieldNameIndex(output_field_name)
    # Output index is not found
    if output_idx == -1:
        output_field = QgsField(output_field_name, QVariant.Double)
        output_idx = layer.fieldNameIndex(output_field_name)

    # Iterate to all features
    for feature in layer.getFeatures():
        feature[output_idx] = sum_expression.evaluate(feature)


The code above is a function to sum the value in list of fields (input_fields) into an output field (output_field_name) in a layer.

Basically, you just need to create the expression, and evaluate it to get the value from the expression. In this doc, I found out that using prepare will make the evaluation faster for many features.

static const in C++

When you want to use static const in your C++ code, you need to know whether the variable is integral or not. If it’s integral, you can initialize it in the header file (.h). Otherwise, you need to initialize it in the souce file (.cpp). Here some example:

// myclass.h
class MyClass {
  static const int myInt = 0; // Can be initialized here
  static const string myString;  // Need to be initialized in the source file


// myclass.cpp
const string MyClass::myString = 'My String';

If you are not familiar with integral type, you can read here:

What’s the different between static and const?

static in class member means, you do not need to hava the class instance to access it.

const to make a variable constant (not mutable).

They can be used together as const static or static const. Both are the same.

Kategori:CPP Tag:, ,

Explicit Keyword in C++

I am trying to do more C++ lately (after 7-8 years). Ouch, I am feeling old now. I am finding something new (or perhaps refinding). I will try post about it regulary here. So, here we go my first post:

It’s about explicit keyword. This keywords is used to prevent an implicit convertion when creating an object. See this example:

class String {
    String(int n); // allocate n bytes to the String object
    String(const char *p); // initializes object with char *p

Then if you call

String a = '3';

It will implicitly converted ‘3’ to integer, and create String object with 3 bytes allocation. Here, we need the explicit keywords. Like this:

class String {
    explicit String(int n); // allocate n bytes to the String object
    String(const char *p); // initializes object with char *p

By doing this, we prevent the ‘3’ to be converted to int, rather it will create String object that has char *p = ‘3’.



Kategori:CPP Tag:

[Python] Comparing NaN, Inf, None, 0, 1,…

What is the result of this code snippet:

my_list = [1, 2, 3, 9, 1]

Yes, you are right, the result will be like this:

[1, 1, 2, 3, 9]

Hmm, that’s easy. What’s about if you’re asked which is bigger of None and NaN? Or True and NotImplemented? Or Ellipsis (What the hell is this Ellipsis?).
Just try it:

my_weird_list = [True, False, 0, -1, 1, 100, float('nan'), float('inf'), -float('inf'), NotImplemented, Ellipsis, None]
print my_weird_list.sort()

What will be the output? If your answer is None, you are right because sort() method return None #Kidding 🙂

Let’s print the my_weird_list. And, here we go :

[None, -inf, -1, False, 0, True, 1, 100, nan, inf, NotImplemented, Ellipsis]

For your note, int(True) == 1, so 1 and True is the same value when you compared them.

The good thing (for me at least), None is less than anything, even -inf. And, there is bigger value than inf. 🙂

The other thing is, the position of nan in the sorted list. If you compare nan with anything it always give False. Even when nan == nan. It will return True if you use != operator.


So, it’s true that False is bigger (better?) than None or nothing.

Kategori:Python Tag:,

[Python] Return a or b or c

Apakah yang akan dilakukan fungsi ini:

def foo(a, b, c):
    return a or b or c
def bar(a, b, c):
    return a and b and c

Jika diberi input seperti ini:

foo(1, 2, 3)  # 1
bar(1, 2, 3)  # 2
foo('', None, 'pret')  # 3
bar('', None, 'pret')  # 4

Yak benar, si python bakal memberikan hasil sebagai berikut:

1 # 1
3 # 2
'pret' # 3
''  # 4

Mudah dipahami bahwa fungsi foo akan mengembalikan nilai pertama yang bernilai True dan fungsi bar akan mengembalikan nilai terakhir yang bernilai True sebelum ketemu nilai False.
Atau lebih mudahnya, akan mengembalikan nilai terakhir ketika ekspresi bernilai True
Menarik bukan? Minimal menurut saya (soalnya baru nemu).

Kategori:Python Tag:, , ,

InaSAFE 1.0

1 Desember 2012 1 komentar

After I graduated, I work for AIFDR to develop InaSAFE. What is it? InaSAFE is free software that produces realistic natural hazard impact scenarios for better planning, preparedness and response activities. This is an open source software, developed by developers around the world. From all (five) continents actually.

The latest InaSAFE’s version is 1.0. It’s launched in the 5th AMCDRR  (Asian Ministerial Conference on Disaster Risk Reduction) in Yogyakarta. InaSAFE was presented by Trevor Dhu from AIFDR. You can watch it here. And the other good news, our Indonesian President, SBY, came to our booth and got explanation about InaSAFE by Trevor Dhu also. You can watch it here.

Can I use InaSAFE? Of course. You can go to the website, and read the instruction. Or basically, just install QGIS (it’s free also) and fetch the plugin to get InaSAFE.

The other thing, if you want to look at the code, you can go to the github. And may be you can help us 🙂

My personal opinion. This is my first real job. And this is amazing. A big project (awesome developers, I learnt many things from them, I’m very lemcups, I’m new to everything in the project except the python thing and the programming) and I like it.

But of course, there are something I couldn’t get. Yeah, life is about choosing your path. As long as you can choose, be graceful.

PS : I write this post in English because I will take iBT next Saturday, and I haven’t studied.

PS II : Sorry, I can’t keep my word to post one post in a month. I got sick twice this semester. And another excuses.

Kategori:Project, Python Tag:, ,

Python : List, Tuple, Set, dan Dictionary (1)

31 Maret 2012 1 komentar

Salah satu bagian penting dari pemrograman adalah struktur data. Kali ini, saya ingin berbagi hal yang saya tahu mengenai struktur data di Python. Ada empat yakni List, Tuple, Set, dan Dictionary. Akan saya jelaskan secara singkat masing-masing jenis struktur data.

  1. List
    Sama seperti pada bahasa lain, list di Python merupakan sekumpulan elemen atau objek. Tapi, kita bisa memiliki bermacam-macam jenis elemen dalam satu list. Misal, string, integer, objek, list lain, maupun struktur data lain. Karakteristik utama dari list ini adalah, memiliki index dan mutable (bisa di-otak-atik). List di python menggunan zeroth based index. Contoh :

    list1 = [] # deklarasi list
    list1.append('elemen_0') #menambah 1 buah elemen di ujung list
    print list1[0] # mencetak element pertama di list1

    Hal penting lain yang menarik adalah, kita tidak perlu mendeklarasikan ukuran list di awal ataupun tipe isi list. Cukup deklarasikan, dan pakai.

  2. Tuple
    Berbeda dengan list, Tuple merupakan struktur data yang immutable, artinya tidak bisa diotak-atik setelah dideklarasikan. Tapi, masih sama seperti list, isi tuple bisa macam-macam, tidak harus satu jenis. Dan, bisa diakses dengan menggunakan indexnya. Contoh penggunaan :

    tuple0 = () # deklarasi tuple kosong
    tuple0 = tuple() # deklarasi tuple kosong
    tuple1 = ('bakso',) # deklarasi tuple dengan 1 elemen, perhatikan ada tanda koma setelah 'bakso'
    tuple2 = ('soto', 'mie ayam') # deklarasi tuple dengan 2 elemen, tanpa tanda koma
    print tuple2[0] # menampilkan element pertama
  3. Set
    Sama seperti set di matematika, set di python merupakan sebuah himpunan. Tanpa ada index. Yang paling saya suka adalah adanya fungsi seperti irisan dan gabungan. Contoh :

    set1 = set() # deklarasi set kosong
    set1.add(1) # menambahkan isi set1
    set2 = set()
    print set1 | set2 # menampilkan union dari set1 dan set2
  4. Dictionary
    Terakhir adalah dictionary. Layaknya kamus, kita menyimpan suatu value dengan key tertentu. Semua bisa jadi key, asal dia immutable. Sama seperti yang lainnya, kita bisa mengisi semau kita, tipe data semau kita juga. Contoh :

    dict1 = {} # deklarasi dictionary kosong
    dict['key'] = 'value' # mengisi dengan kunci 'key' dengan 'value'
    print dict['key'] # menampilkan isi dict['key']

Itu adalah perkenalan struktur data di Python. Oh iya, string itu termasuk list of char namun immutable lho. Hmm, sedikit trik-trik yang saya gunakan. Misalnya, mengurutkan isi dictionary terhadap value, menghapus semua element list dengan nilai tertentu, atau membuat dictionary dari list. Sampai ketemu di tulisan selanjutnya. CMIIW lho…

Kategori:Code, Python Tag:
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