Tracing Programs

Tracing Programs

Question #1 – Tracing Programs

As with previous exams, trace the following program by tracking the values of all variables in the table provided below.

Program to Trace

#include <stdio.h 

int main(){

1                      int array[5] = { 0,1,2,3,4} ;

2                      int * p1, **p2 ;

3                      int * parray[2] = { NULL , NULL };

4                      p1= & array[2] ;

5                      *p1 = 20 ;

6                      p2 = & p1 ;

7                      ** p2 = 30 ;

8                      parray[0] = & array[0];  

9                      parray[1] = & **p2;  

10        *parray[0] = 30 ;  

11        *parray[1] = 300;


Trace Table to Fill

In order to make it easier for you to trace pointer values, here is a table telling you the memory addresses of all the variables in the above program.

array[0] to array[4]
parray[0] to parray[1]
Memory Address

The following table is where you will be tracing your program;

Use ??? to denote an undefined value; e.g. non-initialized variable.

Use error to denote the dereference of a NULL or undefined pointer.

As for previous exam, a given line should only modify the columns of the variables which it modified. However, because we use pointers, you might have to modify other columns if your line just dereferenced a pointer to modify another variable.

Question #2 – Testing Programs

You are going to write tests for a function. Unlike previous exams, we do not provide you with the source of this function but only its requirements. Therefore, your tests will have to allow us to verify that an arbitrary implementation fulfills requirements.

Program Requirements

The signature / prototype of the function; 

char* str_reverse(char * str); 

Its requirements are as follows;

Work with any value for the char*

Return a newly allocated string which is reversed compared to the original

Return a copy of an empty string

Return NULL if the string is NULL

Returns NULL if the string is more than 10 characters long, including end of string marker

Tests Table to Fill

Feel free to add / remove rows if necessary

Question #3 – Refactoring Programs

Refactor the following code; i.e. improve its quality without modifying its behavior;

Use meaningful names for variables, parameters & functions

Provide proper documentation as required in the PAs

Provide proper indentation & programming style similar  to the examples from the textbook, videos & PAs

Remove useless code

Simplify program

Improve performance

Improve readability

Handle bad parameter values by returning NULL

Handle memory allocation problems by returning NULL

You will provide your version in the “Your Modified Version” subsection which is already pre-filled with the original. Then, for each improvement you applied, use the table in the “Summary” subsection to explain what you did & why you did it.

Program to Refactor

Summary of Improvements

Feel free to add rows to, or remove rows from, this table as needed;

Question #4 – Debugging Programs

The following function has all sorts of problems in implementing the requirements. We need you to list the errors you found in the table below along with how you would fix them. You will then provide the fixed version of the function. Here is the documentation for the function. You will use this information as requirements;


Takes two non-null, non-empty, strings of same length, containing only lower-case alphabetical characters

Return a newly allocated string of same length than s1

For each character at index n in this new string, its ASCII index should be 141 plus half of the positive difference between the ASCII values of the characters at index n in s1 & s2


s1 & s2 non-null, non-empty strings of same size

Return Values

NULL if the parameters are not what we expected

NULL if memory allocation was a problem

Address of a newly allocated string as described above

Bugs Table

Identify each bug you find & provide the lines # where it happens. In addition, explain what the problem was, then how you fixed it. If the same bug appears at different line, just enter it one time in the table but specify all the lines where it happens.

Apply all the fixes you listed in the table to the code below to make it your fixed version. Please note that if a bug is fixed below but not in the table, or the other way around, you won’t get credit for it. You need to have both the bug and its explanations in the table and have it fixed below.
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