CMSC104

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Lecture 9: Conditionals

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Reading Assigned:   4.1 - 4.3 (Note - We will cover logical operators later)

Homework Due:  None

Homework Assigned:  Homework 04 -- assigned last class

Classwork Assigned:  None

Topics Covered:

• Homework 4 questions
• Example questions for quiz
• Finish (well - start really) last lecture
• Since we didn't go over last lecture last class, we may delay this lecture one day - or may start it but only get half way - we'll see
• Conditionals
  • Used to change the flow of program code
  • Used to allow the program to respond to different input
  • Two main constructs in C
    • IF - ELSE
    • SWITCH
  • Conditional tests - operators
    • Multiple tests can be used to determine if something is "true" or not
    • Your code can use these tests to then change what it will do
    • Eg. for homework 4, after we calculate the grade, we could print "PASS" or "FAIL" depending on the class grade:

      /* Print PASS if average is good enough to pass */
      void print_pass(double avg)
      {
        if (avg >= 70) {
          printf("PASS\n");
        }
      }
      

    • Relational operators
      • Can compare variables, constants, and expressions.
      • Used to test if things are equal, less than, greater than, etc.
        • <   true if left is less than right
        • >   true if left is greater than right
        • <=   true if left is less than or equal to right
        • >=   true if left is greater than or equal to right
        • ==   true if left is equal to right (note TWO ='s)
        • !=   true if left is not equal to right
      • The result of a relational operator is an integer with the value 0 (false) or 1 (true)
      • if will evaluate its condition and run the conditional code if the value is non-zero
      • Variable being compared to another variable

        if (a == b) {
          printf("a is equal to b\n");
        }
        

      • Constant being compared to a variable

        if (100 != var) {
          printf("var is not equal to 100\n");
        }
        

      • Constant being compared to an expression (assume my_function() returns some int or double)

        if (70.0 <= my_function() * 20.0) {
          printf("Calculated value is greater than 70\n");
        }
        

  • IF - ELSE details
    • if and else are reserved words in C
    • Takes the form if (condition) { condition_true_code; } else { condition_false_code; }
    • The condition_true_code can be one statement, or a series of statements
    • The condition_true_code will execute ONLY if condition was evaluated to true
    • The condition_false_code can be one statement, or a series of statements
    • The condition_false_code will execute ONLY if condition was evaluated to false
    • The else { condition_false_code; } is optional and can be left off if the program doesn't need it
    • Note that when there is only one statement to execute after the if, the {}'s are optional

      if (x == 1)
      printf("This code will only print when x is 1\n");
      printf("This code will always print\n");
      

    • This can cause hard to diagnose bugs, so it is recommended to always use {}'s with if
    • The {}'s also make it easier to see what block of code is associated with the condition

      if (x == 1) {
        printf("This code will only print when x is 1\n");
      }
      printf("This code will always print\n");
      

    • An example of using if with else

      if (avg >= 70) {
        printf("PASS\n");
      } else {
        printf("FAIL\n");
      }
      

    • Note that spaces and newlines don't matter, the following is the same to the compiler as the last example

      if (avg >= 70) { printf("PASS\n"); } else { printf("FAIL\n"); }
      

    • And with optional {}'s, the compiler will generate the same code for the following as well

      if (avg >= 70) printf("PASS\n"); else printf("FAIL\n");
      

    • And this is the same too

      if (avg >= 70)
      printf("PASS\n");
      else
      printf("FAIL\n");
      

    • One final pathological example that is the same to the compiler, but not to us having to read it!

      if(
      avg >=
      70) printf(
      "PASS\n"
      );else
      printf(        "FAIL\n")
      ;
      

    • A correct (but normally not what you want) example:

      if (var) {
        printf("This code will print whenever var is non-zero\n");
      }
      

    • This example checks to condition inside the ()'s following if, which simply evaluates var
    • The printf will run when var is true (non-zero), and won't run when var is false (zero)
    • Compiler will not warn you about this, so be careful
  • A sample program showing combining user input, with if, else, and conditionals to compare two values
  • An improved sample program showing combining user input, with if, else, and conditionals to compare two values
  • Note that the second program shows a more complex chaining of if-else-if that will be covered later