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Selection Statements
+ also known as condition statements
+ so far, programs seen as top-down sequence of instructions
+ often problems require choices
+ need to express situations as TRUE and FALSE
+ use tests to cause branches in the program
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Logic
+ Boolean: test logical TRUTH or FALSEHOOD
+ Boolean Expressions: expressions with boolean values and operators
+ logical values:
  0 (false) and 1 (true)
+ logical operators:
  & (and), | (or), ~ (not), xor (exclusive or)
+ relational operators:
  >, >=, <, <=, ==, ~=
+ examples) 
  1 < 2            % less than
  1 == 1           % equal (true)
  1 == 2           % equal (false)
  1==1   &   2==2  % true
  1 > 2  |  1 < 2  % true
  xor(1 < 2,1 < 3) % false
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Branches:
+ making choices
+ use $if$ (skip $switch$)
+ see $help if$
  General form of the $if$ statement is
 
      IF expression
        statements
      ELSEIF expression
        statements
      ELSE
        statements
      END

+ Must have $if$ and $end$
+ $else$ and $elseif$s are optional
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% Example 1
% Test if a number is positive
  x = input('Enter a number: ');
  if x > 0
     disp(['Your number is positive.'])
  end
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% Example 2
% Test if a number is positive
  x = input('Enter a number: ');
  if x > 0
     disp(['Your number is positive.'])
  else
     disp(['Your number isn't positive.'])
  end
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% Example 3
% Test sign of a number (see $help sign$ for built-in function)
  x = input('Enter a number: ');
  if x > 0
     disp(['Your number is positive.'])
  elseif x < 0
     disp(['Your number is negative.'])
  else
     disp(['Your number is zero.']) % but can you guarantee that???
  end
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% Example 4
% Test sign of a number (see $help sign$ for built-in function)
  x = input('Enter a number: ');
  if x > 0
     disp(['Your number is positive.'])
  elseif x < 0
     disp(['Your number is negative.'])
  elseif abs(x) < eps
     disp(['Your number is zero.']) % but can you guarantee that???
  else
     disp(['I don't know what the hell your number is.'])
  end
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% Example 5
% Test sign of a number
  test = input('Enter the number to test: ');
  if isnumeric(test)
     if x > 0
        disp(['Your number is positive.'])
     elseif x < 0
        disp(['Your number is negative.'])
     elseif abs(x) < eps
        disp(['Your number is zero.'])
     else
        disp(['I don't know what the hell your number is.'])
     end
  else
     disp(['I said "Enter a number." Now look what you have done!'])
  end
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More about selection statements

% example) swap values
 
  disp(['swapping values'])
  v1 = input('give me value1: ');
  v2 = input('give me value2: ');

  % swapping using a "temporary" variable
  % $tmp$ acts as a placeholder
  tmp = v1;
  v1  = v2;
  v2  = tmp;

  disp(['v1 now has v2 value: ' num2str(v1)])
  disp(['v2 now has v1 value: ' num2str(v2)])
  disp(['press any key to continue'])

% now, apply to problem: 
%   Find the maximum of stored value and input value
%   Keep the smaller value in variable called $min$

  % input
    disp(['finding a max value'])
    max = input('give me a value: ');
    val = input('give me another value: ');

  % testing and swapping
    if (val > max)
      min = max;
      max = val;
    else
      min = val;
    end

  % output
    min
    max

  % curiosity
    val       % what happens to $v1$? nothing
    clear val % if you want to erase $v1$
  % val       % now unassigned!
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% Example 7
% Temperature range
    t = input('Enter the temperature: ');
    if isnumeric(t)
       if t > 70
          if t < 80
	     disp('comfy');
	  else
	     disp('hot!');
	  end
       end
    end
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