Conditionas and Logical Expressions

• We are done with our basic study of data structures (almost)
• We are about to begin our study of control structures (almost)
• To this point, the flow of control in our program has been simple
  • We executed the first statement
  • We executed the second statement
  • ...
  • We executed the last statement
• It has been essentually sequential or linear.
• We will soon study a selection, or branching structure
• The computer will decide which statement or set of statements to execute baised upon the value of an expression that it evaluates.
• But first, a new data type
• Type bool
  • reserved word: bool
  • bool flag;
  • values : true, false;
  • bool flag1 = false, done = true;
  • bools are used directly, but more frequently indirectly
  • Operations on bools
    • && logical and, both must be true for the result to be true
      • true && true -> true
      • true && false -> false
      • false && true -> false
      • false && false -> false
    • || logical or, one or the other or both must be true
      • true || true -> true
      • true || false -> true
      • false || true -> true
      • false || false -> false
    • ! uniary not, flip the value.
      • ! true -> false
      • ! false -> true
  • Operations that produce bools
    • ==, are the two operands the same
    • !=, are the two operands different
    • >, >=, is the first greater than, greater than or equal to, the second
    • <, <= , less than, less than or equal to
    • For most types, (int, char, string, float, bool, ...)
    • 3 > 4 -> false
    • 'c' < 'd' -> true
    • "Hello World" == "Hello World" -> true
  • Combining these
    • (x<-4) || (x>4)
    • (day == "Monday") && (hour > 7)
  • When you compare characters, you compare the ascii value of that character.
    • So 'A' < 'a' -> true
    • 'a' > 'b' -> false
  • When you compare strings,
    • If the compare the values at position 0, if they are different report this, and exit, if they are the same compare the values at position 1, ...
    • If one string ends before the second and they are the same up to that point, the smaller is less
    • If they are the same the entire way through, they are equal
    • "Hello" < "hello" -> true
    • "Hello" < "HEllo" -> false
    • "Hello" < "Hello World" -> true
  • When you compare ints it is as you expect.
  • It is dangerous to compare floats, roundoff error will cause strange behavior.
    • it is better to do something like
    • fabs(x-y) < 0.001
    • then x < y
    • try 1.0/3.0 + 1.0/3.0+1.0/3.0 == 1
  • Short circuit evaluation
    • (4 == 1) && (7 < ??? ) will always be false, so it does not even look at the second part
    • (4 == 4) || (????) will always be true, ...