Last updated on: 10 November 2008 06:01:33 PM

This page contains miscellaneous information that can't be fitted to any other categories. It is mostly related to programming language tips & tricks. And this may contain junks also... (things that are not related with programming)

Let's say "Hello World"

"Hello World" is a well known programming term, widely used when teaching programming lessons to a novice. It is a name of your first program created with a programming language that is new to you. Hello World programs simply print out the sentence "Hello World" to the screen/stdout, and here are some of them:

In C:

#include <stdio.h>

void main() {
   prinft("Hello World\n");
}

In C++:

#include <iostream.h>

void main() {
  cout << "Hello World" << endl;
}

In Pascal:

program HelloWorld;
begin
   writeln('Hello World');
end.

In Java:

class HelloWorld {
   public static void main(String[] args) throws IOException {
      System.out.println("Hello World");
   }
}

In Visual Basic:

Sub Form_Load()
   msgbox "Hello World"
End Sub

In Scheme:

(write-line "Hello World")

Approximation

This is the way computer calculate complicated mathematical functions such as Sqrt, Sine, Cosine, Tangent, etc.

Approximation will cause a loss in precision and this can cause some problem.
Example:
if (sqrt(4) == 2) break; /* what happens if sqrt(4) returns 2.00000001 ??? */

However, you can fix this problem using epsilon
Example:
if (sqrt(4)-2 < 0.001) break;

C Language Shortcuts

These shortcuts are mainly available for C/C++ language.

i = i+1;
is equivalent to
i++; /* other example of this shortcut: i--; for i=i-1; */

i = i+2;
is equivalent to
i += 2; /* other example of this shortcut: -=, *=, /=, %=, etc */

index++;
if (index>=n) index=0;
is equivalent to
index=(index+1) mod n;

index--;
if (index<0) index=n-1;
is equivalent to
index=(index+n-1) mod n;

if (a<b) min = a; else min = b;
is equivalent to
min = a<b ? a : b; /* the_statement ? if_true : if_false, is the shortcut for single if-else */

#include <math.h>
int a=7;
int a_div_two_round_up = (int) ceil(a/2); /* result: 4 */
is equivalent to
int a=7;
int a_div_two_round_up = (a+1)/2; /* result: 4, you don't have to use floating point */

#include <math.h>
int a=7;
int a_div_two_round_down = (int) floor(a/2); /* result: 3 */
is equivalent to
int a=7;
int a_div_two_round_down = a/2; /* result: 3, operator / by default is an integer division */

Swapping two integer values

Swapping two integer values is important, especially in a sorting algorithm that involves integer numbers. There are several ways to do it. Let's see the methods one by one starting from the most obvious one.

1. Use a temporary variable, the most straightforward implementation.

#include <stdio.h>

void swap(int *a,int *b) {
  int temp;
  temp = *a; // a = 7 ; b = 49; temp = 7
  *a = *b;   // a = 49; b = 49; temp = 7
  *b = temp; // a = 49; b = 7 ; temp = 7
}

void main() {
  int num1 = 7,num2 = 49;
  printf("%d %d\n",num1,num2);
  swap(&num1,&num2);
  printf("%d %d\n",num1,num2);
}

2. Only use 2 variable, a bit slower than first method because this method involves computation, but use less memory.

#include <stdio.h>

void swap(int *a,int *b) {
  // initially     a =  7; b = 49
  *a += *b;     // a = 56; b = 49
  *b = *a - *b; // a = 56; b =  7
  *a -= *b;     // a = 49; b =  7
}

void main() {
  int num1 = 7,num2 = 49;
  printf("%d %d\n",num1,num2);
  swap(&num1,&num2);
  printf("%d %d\n",num1,num2);
}

3. Finally, we arrive at the best method so far, using XoR. This is the most efficient because it doesn't use additional variable as temporary storage and it is the fastest because all swaps occurs in-situ, you only change 'bits' instead of changing the whole variable. XoR operations is faster than additions/substractions. See the example below. Btw you know what XoR does, doesn't you?

#include <stdio.h>

void swap(int *a,int *b) {
  // initially a = 000111 ( 7) and b = 110001 (49)
  *a ^= *b; // a = 110110 (54) and b = 110001 (49)
  *b ^= *a; // a = 110110 (54) and b = 000111 ( 7)
  *a ^= *b; // a = 110001 (49) and b = 000111 ( 7)
}

void main() {
  int num1 = 7,num2 = 49;
  printf("%d %d\n",num1,num2);
  swap(&num1,&num2);
  printf("%d %d\n",num1,num2);
}

Btw, this trick is only applicable for integers. If you want to swap floating point numbers or strings, please revert back to the first (direct) method.

Discrete Mathematics

Discrete means separated from the others.

The name Discrete Mathematics comes from the distinction between continuous and discrete mathematical objects. Discrete Mathematics concerns processes that consist of a sequence of individual steps. The ideas of Discrete Mathematics underlie the science and technology specific to the computer age.

What you'll learn if you take Discrete Mathematics are: Proof Technique, Sets, Relations, Functions, Graphs, Trees, Pigeon Hole Principle, Counting & Combinatorics, Probability and many more which will be useful for any other areas in Computer Science.