Computer Science 15-100, Fall 2008
Class Notes:  Code Tracing

1. Trace variable declaration and assignment
2. Trace multiple variable declarations
3. Trace multiple steps on one line
4. Trace a conditional
5. Trace a loop
6. Trace a method call
   1. Without source code
   2. With source code

Code Tracing

1. Trace variable declaration and assignment

   1  class Foo {
   2    public static void main(String[] args) {
   3      int x = 3;
   4      System.out.println("Now x = " + x);
   5      x += 5;
   6      System.out.println("And now x = " + x);
   7    }
   8  }

   Trace:

   	Method: 'main'
   Line	Action	x
   2	start	---
   3	set x to 3	3
   4	print "Now x = 3"
   5	set x to 8	8
   6	print "And now x = 8"
   7	return

    

2. Trace multiple variable declarations

   1  class Foo {
   2    public static void main(String[] args) {
   3      int x = 3;
   4      System.out.println("Here x = " + x);
   5      x += 5;
   6      int y = 4;
   7      System.out.println("And here x,y = " + x + "," + y);
   8    }
   9  }

   Trace:

   	Method: 'main'
   Line	Action	x	y
   2	start	---	---
   3	set x to 3	3
   4	print "Here x = 3"
   5	set x to 8	8
   6	set y to 4		4
   7	print "And here x,y = 8,4"
   8	return

    

3. Trace multiple steps on one line

   1  class Foo {
   2    public static void main(String[] args) {
   3      int x = 3, y = 4;
   4      System.out.println("Here x,y = " + x + "," + y);
   5      y = 2 * (x = 5);
   6      System.out.println("And here x,y = " + x + "," + y);
   7      }
   8  }

   Trace:

   	Method: 'main'
   Line	Action	x	y
   2	start	---	---
   3	set x to 3	3
   3	set y to 4		4
   4	print "Here x,y = 3,4"
   5	set x = 5	5
   5	set y = 10		10
   6	print "And here x,y = 5,10"
   7	return

    

4. Trace a conditional

    1  class Foo {
    2    public static void main(String[] args) {
    3      int i = -3;
    4      if (i > 0)
    5        System.out.println(i + " is positive");
    6      else if (i == 0)
    7        System.out.println(i + " is zero");
    8      else
    9        System.out.println(i + " is negative");
   10    }
   11  }

   Trace:

   	Method: 'main'
   Line	Action	i
   2	start	---
   3	set i to -3	-3
   4	test if (-3 > 0)
   6	test if (-3 == 0)
   9	print "-3 is negative"
   10	return

    

5. Trace a loop

   1  class Foo {
   2    public static void main(String[] args) {
   3      String s = "ab";
   4      for (int i=0; i<s.length(); i++)
   5        System.out.println(s.charAt(i));
   6    }
   7  }

   Trace:

   	Method: 'main'
   Line	Action	s	i
   2	start	---	---
   3	set s to "abc"	"abc"
   4	set i to 0		0
   4	test if (0 < 2)
   5	print "a"
   4	set i to 1		1
   4	test if (1 < 2)
   5	print "b"
   4	set i to 2		2
   4	test if (2 < 2)
   6	return

    

6. Trace a method call
    
   1. Without source code

      1  class Foo {
      2    public static void main(String[] args) {
      3      int x = 3, y = 4;
      4      int lesser = Math.min(x,y);
      5      System.out.println("lesser = " + lesser);
      6    }
      7  }

      Trace:

      	Method: 'main'
      Line	Action	x	y	lesser
      2	start	---	---	---
      3	set x to 3	3
      3	set y to 4		4
      4	call Math.min(3,4) returns 3
      4	set lesser to 3			3
      5	print "lesser = 3"
      6	return

      Another Example:

      1  class Foo {
      2    public static void main(String[] args) {
      3      int x = 3, y = 4;
      4      int z = Math.min(x, (int)Math.sqrt(y));
      5      System.out.println("z = " + z);
      6    }
      7  }

      Trace:

      	Method: 'main'
      Line	Action	x	y	z
      2	start	---	---	---
      3	set x to 3	3
      3	set y to 4		4
      4	call Math.sqrt(4) returns 2.0
      4	call Math.min(3,2) returns 2
      4	set z to 2			2
      5	print "z = 2"
      6	return

       

   2. With source code

       1  class Foo {
       2    public static int times(int x, int y) {
       3      return x*y;
       4    }
       5
       6    public static void main(String[] args) {
       7      int x = 3, y = 4;
       8      int z = times(x,y);
       9      System.out.println("z = " + z);
      10    }
      11  }

      Trace:

      	Method: 'main'
      Line	Action	x	y	z
      6	start	---	---	---
      7	set x to 3	3
      7	set y to 4		4
      8	call times(3,4)

      Method: 'times'     Line Action x y 2 start 3 4 3 return 12

      8	returns 12
      8	set z to 12			12
      9	print "z = 12"
      10	return

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