Skygorgia Portal - Tracing Functions

Tracing Functions View Code

Tracing code is an excellent way to help you think in programming. By doing so you can see the processes of the program better, and you can check for bugs more easily. To trace code it is recommend that you have a physical copy of it in front of you, a pencil, and a sheet of scratch paper. (Faulty print outs make great scrap.)

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Blocks

Go line by line reading the code. Mark on the paper what happens in the code.
Have a section of the paper to re sprent the screen. If something is printed to the screen write it there. Don't forget about non printable characters. I use a dot · to represent spaces, an arrow » for tab, and the paragraph symbol ¶ for a new line.
When a variable is declared, write the name of the variable with a line under it. When something is assigned to it write the value under the name. If it receives a new value cross the old one out.
Don't forget arithmetic expressions.

cout << "Print variables";
int A, B;
A = 5;  B = 2; 
cout << "The sum is " << A+B;
B *= 3

Print variableThe sum is 6

A: 5 13
B: 2

If you couldn't tell, the left block is the example code, this is the scratch work, and the right is the thought process.

Print to the screen "Print variables".
Make variables A and B.
A has value 5, and B has value 2.
Print to screen, same place, "The sum is·".
B has value of 2*3.

Logic

Here you have to be careful and follow the code flow. Remember go line by line.
Remember the only the next line is evaluated unless their is a block.

int A = 4, B = 3;
char ch = '4';
if ( A <= B )
   ch = 't';
else
{
   int c = 4 / b * 2 - 1;
   cout << c << B+A << endl;
}
cout << "A+B" << ch;

17¶
A+B4

A: 4
B: 3
ch: 4
c: 1

Declare variables A with value 4, and B with 3.
Declare a character ch with value '4'.
Check if A is less than B. It is not skip to the else.
Declare c with value 4/3*2-1 = 1*2-1 = 2-1 = 1
Print the value of c, 1, then value of B+A, 7, then a new line.
Print the string, "A+B", then the value of ch, '4'.

Functions

Treat it like a block of codes but now keep track of the parameters.
When the function is call write all its parameters in a different section and make whether its "by value" or "by reference.
Personally I mark a a by reference by a double ended arrow to the original arrow.
Work out the code, when you are finished make sure that the by reference variables copy their values to the actual variable.
Don't forget to cross out the function parameters when the function is complete.

void swap(int& A, int& B)
{
   int temp = A;
	 A = B;
	 B = temp  
}
void swap2(int A, int B)
{
   int temp = A;
   A = B;
   B = temp  
}
int f(int c, int v)
{
   return c + c * v;
}
int main()
{
   double g = 1.98, t = .09;
   int num = 3, x = 3, y = 1;
   cout << "Total" << f(g, t) << endl;
   swap(x, y); swap2(num, y)
   cout << num << ' ' << x << ' ' << y;  
}

Total2.1582¶
5 1 3

g: 1.98
t: .09
num: 5
x: 31 (after swap())
y: 13 (after swap())

f()
c:1.98
v:.09

swap()
A(ref): 31
B(ref): 13
temp: 3

swap2()
A: 32
B: 23
temp: 3

Since this simple block involves a lot of calls, the explanation is below.

Declare double's g as 1.98 and t as .09.
Declare ints num as 5, x as 3, and y as 1.
Print "Total" then call f(g, t)
1. Declare variables "by value" c and v with the values of f an g.
2. Return the value of the expression, 1.98 + 1.98 * .09 = 2.1582
With the return value of 2.1582 print it screen, then a new line.
Call swap with x and y.
1. Declare variables "by reference" A and B with the values of x an y.
2. Declare temp with value of A.
3. Assign B to A, then temp to B.
Copy the value of A and B to x and y, since they were by reference.
Call swap2 with num and y.
1. Declare variables "by value" A and B with the values of num an y.
2. Declare temp with value of A.
3. Assign B to A, then temp to B.
Print out num, space x, space, then y.

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