if and operators

• Conditional

  if (condition)
    {
      statement;
    }
  else if (condition2)
    {
      statement2;
    }
  else
    {
      statement3;
    }
  
  nextStatement;
  

  • Example:
    

    if (randNum < 0.2)
      x = 1;
    else if (randNum < 0.5)
      x = 2;
    else
      x = 3;
    

    Note that you can omit { } if there is only 1 statement for a condition. But if you need to do a couple things if a codition is met, you need { } to make a ``block'' of statements.

    if (randNum < 0.2)
     {
       x = 1;
       y = -2;
     }
    else
      x = 3;
    

  • else if and else are optional.

    if (randNum < 0.2)
      x = 1;
    
    y = -2;
    

  • In C, an integer value represents whether the condition is TRUE or FALSE.
    • 0 means FALSE
    • non-zero value means TRUE

  • Comparisons (relational operators)
    

    a > b	greater than
    a >= b	greater than or equal to
    a < b	less than
    a <= b	less than or equal to
    a == b	equal
    a != b	not equal

  • The relational operators return 0 (False) or 1 (True).

    Let's say x = 3, and y = 1 now.

    Expression		what it evaluates to
    x == y		0 (false)
    x > y		1 (true)
    x != y		1 (true)
    x + 2 == y * 5		1 (true)

  • Note that condition in if statement doesn't have to be the comparion, and the following statement is valid:

    if (x)
      y = 1 / x ;
    

    Guess the behavior of the following code?

    if (a-b)
      a = -1;
    else
      a = 3;
    

  • Exercise:

    Copy /home/progClass/src/immig-emig/ifProb.c, compile it, and run it.

    Find out what is wrong with the code.

    #include <stdio.h>
    
    int
    main (void) {
      int x, y;
      
      x = 3;
    
      if (x = 1)
        printf("x is 1\n");
      else
        printf("x is NOT 1\n");
    
      y = 0;
    
      if (y = 0)
        printf("y is 0\n");
      else
        printf("y is NOT 0\n");
    
      return(0);
    }
    

• Arithmetic operation
  • Simple operations (+, -, *, /, %)

    y = a + (b - c) * d / e

    I think these arithmetic operators needs no explanation except %.

    % is called modulus, and it is very handy.

        x % y

    retuns the remainder when x is divided by y.

    0 % 3	= 0
    1 % 3	= 1
    2 % 3	= 2
    3 % 3	= 0
    4 % 3	= 1
    5 % 3	= 2
    6 % 3	= 0

  • Exercise:
    Make a small program to check, what values will be returned if you use negative values of x and/or y. (e.g., -100 % 3, -50 % -2 etc).
  • A little trickier ones

    Expression		equivalent to
    x += 3;		x = x + 3;
    x -= 5;		x = x - 5;
    x *= 2;		x = x * 2;
    x /= 4;		x = x / 4;
    x %= 6;		x = x % 6;
    x++ or ++x		x = x + 1;
    x-- or --x		x = x - 1;