Programming

Conditional Control -- if, switch

This group of control statements enables you to select at run-time which block of code is executed. To make this selection based on whether a condition is true or false, use the if statement (which may include else or elseif). To select from a number of possible options depending on the value of an expression, use the switch and case statements (which may include otherwise).

if, else, and elseif

if evaluates a logical expression and executes a group of statements based on the value of the expression. In its simplest form, its syntax is

• if logical_expression
      statements
  end
  

If the logical expression is true (that is, if it evaluates to logical 1), MATLAB executes all the statements between the if and end lines. It resumes execution at the line following the end statement. If the condition is false (evaluates to logical 0), MATLAB skips all the statements between the if and end lines, and resumes execution at the line following the end statement.

For example,

• if rem(a, 2) == 0
      disp('a is even')
      b = a/2;
  end
  

You can nest any number of if statements.

If the logical expression evaluates to a nonscalar value, all the elements of the argument must be nonzero. For example, assume X is a matrix. Then the statement

• if X
      statements
  end
  

is equivalent to

• if all(X(:))
      statements
  end
  

The else and elseif statements further conditionalize the if statement:

• The else statement has no logical condition. The statements associated with it execute if the preceding if (and possibly elseif condition) evaluates to logical 0 (false).
• The elseif statement has a logical condition that it evaluates if the preceding if (and possibly elseif condition) is false. The statements associated with it execute if its logical condition evaluates to logical 1 (true). You can have multiple elseif statements within an if block.

• if n < 0             % If n negative, display error message.
      disp('Input must be positive');
  elseif rem(n,2) == 0 % If n positive and even, divide by 2.
      A = n/2;
  else
      A = (n+1)/2;     % If n positive and odd, increment and divide.
  end
  

if Statements and Empty Arrays.   An if condition that reduces to an empty array represents a false condition. That is,

• if A
     S1
  else
     S0
  end
  

executes statement S0 when A is an empty array.

switch, case, and otherwise

switch executes certain statements based on the value of a variable or expression. Its basic form is

• switch expression (scalar or string)
     case value1
        statements       % Executes if expression is value1
     case value2
        statements       % Executes if expression is value2
      .
      .
      .
     otherwise
        statements       % Executes if expression does not
                         % match any case
  end
  

This block consists of

• The word switch followed by an expression to evaluate.
• Any number of case groups. These groups consist of the word case followed by a possible value for the expression, all on a single line. Subsequent lines contain the statements to execute for the given value of the expression. These can be any valid MATLAB statement including another switch block. Execution of a case group ends when MATLAB encounters the next case statement or the otherwise statement. Only the first matching case is executed.
• An optional otherwise group. This consists of the word otherwise, followed by the statements to execute if the expression's value is not handled by any of the preceding case groups. Execution of the otherwise group ends at the end statement.
• An end statement.

switch works by comparing the input expression to each case value. For numeric expressions, a case statement is true if (value==expression). For string expressions, a case statement is true if strcmp(value,expression).

The code below shows a simple example of the switch statement. It checks the variable input_num for certain values. If input_num is -1, 0, or 1, the case statements display the value as text. If input_num is none of these values, execution drops to the otherwise statement and the code displays the text 'other value'.

• switch input_num
      case -1
          disp('negative one');
      case 0
          disp('zero');
      case 1
          disp('positive one');
      otherwise
           disp('other value');
  end
  

Note    For C programmers, unlike the C language switch construct, the MATLAB switch does not "fall through." That is, if the first case statement is true, other case statements do not execute. Therefore, break statements are not used.

switch can handle multiple conditions in a single case statement by enclosing the case expression in a cell array.

• switch var
      case 1
          disp('1')
      case {2,3,4}
          disp('2 or 3 or 4')
      case 5
          disp('5')
      otherwise
          disp('something else')
  end
  

Program Control Statements

Loop Control -- for, while, continue, break

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