Lab-2 – Built in Arrays in C

Lab-2 –
Department of Electrical & Computer Engineering

Notes:
• In this lab, you are allowed to work with a partner (groups of three or more are NOT allowed). If you
decided to work with a partner, please submit only one lab report with both names.
• Material related to the exercises D, E, and F will be discussed on Monday and Tuesday next week
(September 28-29).
Objectives:
This lab consists of several exercises, mostly designed helping you to understand: Using arrays and built-in strings,
pointer arithmetic, and C structs.
Due Date: 5:00 PM on Friday Oct 2nd. All of your work should be in a single PDF file that is easy for your TA to
read and mark. For instructions about how to provide your lab reports, study the posted document on the D2L called:
How to hand in your lab assignment.
Marking Scheme
You shouldn’t submit anything for the exercises that no marks are assigned to them.
Exercise Marks
A 8 marks
B 8 marks
C 6 marks
D No marks
E 6 marks
F 2 marks
Exercise A – Built in Arrays in C
Read This First - A Few Facts About Built in Arrays in C:
A built-in array in C is a data structure that can store a fixed size of sequential collection of elements of the same
type. It is part of the language and doesn’t need to include or import any library or header file. Here is a quick
overview of a few facts about arrays in C:
Fact 1: When you declare an array with n elements of type T, as a local variable, a chunk of memory equal to the
size of T multiplied by n will be allocated. In the following examples the size of x is 80 bytes, which is 8 (size of
double) multiplied by10 (number of elements):
double x [10]; /* size of x is: 10 * 8 = 80 bytes */
double y[] = {2.3, 3.0, 4.0}; /* size of y is 24 bytes */
C also provides an operator called sizeof that can be used to find the size of a data object in bytes. It can be
applied either to a type or an expression. Here are examples of using sizeof operator:
int n = (int) sizeof(x); /* n == 80 */
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int m = (int) sizeof(double) * 10; /* m == 80 */
The value produced by sizeof operator is of type size_t, which is some sort of integer type; exactly which type it
is depends on the particular implementation of C you are using. In this code segment we have used the type cast
operator (int) to convert size_t type to the exact type of int on the left-hand side of the assignment operator.
The syntax sizeof(something) looks like a function call, but it isn't. When the compiler sees the expression
sizeof(x), it simply replaces the expression with the size of x in bytes.
Fact 2: Pointer and arrays are closely intertwined in C. Most of the time, when we use the name of an array in an
expression, that name is automatically treated like a pointer to the first element of the array:
int ia[] = { 4, 6, 9};
int *ip = ia;
Here is an exception to this fact, when passing the name of array ia to the sizeof operator it is not treated as a
pointer. It is treated just like an array – the value of y in the following example will be 12.
int y = (int) sizeof(ia);
Similarly, for proper type-match when the name of an array is passed to a function, the corresponding argument of
the function has to be a pointer. For example a call to a function such as:
func(ia, 3);
Means the prototype of the function func should have two argument as follows::
void func(int*, int);
Fact 3: Arrays cannot be simply copied by using a single assignment statement that copies source array into an
entire destination array. The following example produces a compilation error:
double x[3] = {7.5, 43.2, 0.3};
double y[3] ;
y = x; /*ERROR */
Fact 4: Arrays in C cannot be resized. Therefore, they are often declared with a ``worst-case'' size. In the following
example we assumed the maximum number of data at some point may or may not reach to 100, but at this point we
are using only the first four elements of the array data:
double data [100] = {120.40, 200.00, 34.56, 99.88} ;
Fact 5: When we pass a numeric array to a function we should also pass an integer argument to the function
indicating the actual number of elements to be used.
double x[10] = {2.50, 3.20, 33.0}; /* Note only first 3 elements of x are used */
double y[] = {5.00, 2.00};
my_function(x, 3); /* my_function should use the first 3 elements */
my_fucntion(y, 2); /* my_function should use entire array, 2 elements */
C-strings are exceptions: You don't have to worry about this exception in this lab -- we will discuss it during the
lectures.
Fact 6: An array notation (square brackets) as a formal argument of a function is in fact a pointer. For example:
int foo(int a[], int n);
, is exactly the same as:
int foo(int *a, int n);
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And, both are exactly same as:
int foo(int a[100], int n); // compiler ignore the number 100 between []
What to Do:
Download the file lab1exe_A.c from D2L. Read the comment at the top of the file, and then try to predict the
output of the program. Compile and run the program to check that your prediction of the output was correct.
Note: Some compilers may give warnings about size of pointers, but you still should be able to run the program.
Then,
1. Draw memory diagrams for point one, two, three, and four.
2. Add labels to diagram at point two to indicate the size in bytes for each variable, array, and function argument.
What to Submit:
Submit a properly scanned copy of your AR diagrams for point one, point two (with labels), point three, and point
four as part of your lab report.
Exercise B (8 marks): Duplicating string library functions
Read This First:
A few facts about array of character and strings in C
Fact 1: C-string in an array of character, and the end of a string is marked by ‘\0’, which is equivalent to integer 0.
An array of character without null-terminator (‘\0’) is not considered a valid C-string and any operations or function
calls that needs an argument of type ‘valid C-string’ may fail because of lack of ‘\0’. Here are a few examples:
char s1[6];
s1 is an array of character, and if it is passed as a C-string to a library function such as printf:
printf(“%s”, s1);
the result is undefined: For example, it may print garbage, or depending on different compilers, it may print
nothing, or other undefined outcomes.
In the following example:
char s2[3] = “AB”;
s2 is an array of character, and a valid C-string, because compiler copies string constant “AB” from string
constant area on the static segment of the memory into the elements of s2, and puts a ‘\0’ after the third
element. As a result a printf statement works fine and prints the string: AB
In the following example:
char s3[3] = “XYZ”;
s3 is an array of character that contains ABC but is NOT a valid C-string because compiler copies string constant
“XYZ” from string constant area on the static segment into the elements of s3 but cannot put a ‘\0’ after the
string. Therefore, the result of passing s3 to the printf function is again undefined.
In the following example:
char s4[10] = “KLM”;
s4 is an array of character that its first 3 characters are KLM, and the following elements are ALL ‘\0’. The reason
is that, if even one element of the arrays in C is initialized the rest of them will be padded with zeros and they will
not be garbage anymore.
Fact 2: Same as any other type of arrays, array of characters cannot be copied to each other. For example, the
following code segment produces a compilation error.
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char s4[10] = “KLM”;
char s5[10];
s5 = s4;
The last line causes a compilation error. Therefore if you really need to make s5 a copy of s4, you should do it
element by element, using a loop:
for (int i = 0; i < 3; i++)
 s5[i] = s4[i];
s5[i] = ‘\0’;
printf(“%s”, s5); // prints KLM
please notice that you need to add the ‘\0’ manually to make s5 a valid C-string. Otherwise it will be only usable
as an array of characters with KLM in the first three characters and the rest remains garbage.
It is also good to know that in the above code segment, I could also use a C Library function called strlen,
instead using number 3 in the for loop:
for (int i = 0; i < strlen(s5); i++)
 s5[i] = s4[i];
s5[i] = ‘\0’;
printf(“%s”, s5); // prints KLM
strlen returns the length of string, which in this example is 3. To be able to use C-String Library function such
strlen, you must include the header file called <string.h.
There is another way to make a C-string a copy of another C-string and that is by using a C-String Library
function called strcpy. Here is an example:
char s4[10] = “KLM”;
char s5[10];
strcpy(s5, s4);
printf(“%s”, s5); // prints KLM
Fact 3: We cannot use operators such as , <, =, <=, ==, or &&. || to have a logical operation on C-strings. For
example, to compare two string we cannot use the following statement:
char s4[10] = “KLM”;
char s5[10] = “ABC”;
if(s4 = s5) {
 // do something
}
This code segment gives logical error. When you are comparing the name of the two arrays s4 and s5, you are in
fact comparing the addresses of the first elements of the arrays (remember: the name of the array is the address of
the first element). To be able to compare C-Strings there is a C Library function called strcmp, the function returns
a positive number, if its first argument is lexicologically greater than its second argument. It returns a negative
number, if the second argument is greater than its first argument. Otherwise, if they are identical, it returns zero.
Fact 4: Unlike some other languages, the C-string doesn't do any boundary checking and if you try to write or read
spaces before the first element, or after the last element, the compiler will not give you any compilation error. It is
your responsibility to be aware of this fact and avoid any illegal operation.
Fact 5: Unlike some other languages, the C-string doesn't allow you to append strings using operators + or +=. You
have to use the C library function called strcat. Here is a very brief program that shows how strcat: works
int main(void){
 char s[8];
 strcpy(s, "foo");
 strcat(s, "bar");
 return 0;
}
This is what the array s would look like before and after the call to strcat: 
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Doing the concatenation involves three steps:
• Finding the '\0' character at the end of the destination string (the end of "foo" in the example).
• Copying all the characters from the source string ("bar" in the example).
• Adding a '\0' character at the end of the modified destination string.
A similar function called strncat with the following function prototype:
char* strncat(char* dest, const char* source, int n);
Appends the first n characters of string source to string dest, and returns a char* to dest. If the length of the C
string in source is less than n, only the content up to the terminating null character, ‘\0’, is copied.
Here are couple of examples:
char str1[20] = “abcd”;
char str2[] = “xyz”;
char str3[] = “MT”;
strncat(str1, str2, 2); /* appends xy the end of str1 */
printf (“\n%s”, str1); /* Prints: abcdxy */
strncat(str1, str3, 10); /* appends MT to the end of str1 */
printf (“\n%s”, str1); /* Prints: abcdxyMT*/
There are also other functions that can use to manipulate C-stroings. Also, there a few functions that you can be
used to a single character. For example, a function such is islower returns true if its argument is a lower-case
character:
char s4[10] = “KLM”;
if(islower(s4[0])) { // this statement returns true if first element is a lower case
 // do something
}
Please read the set slides about C character and string functions posted on the D2L to learn more about
other functions.
In this exercise you are going to write your own version of some of the above mentioned C-String library functions. In
a practical programming project, writing a function that does the same job as a function in the library would be a
serious waste of time. On the other hand, it can be a very helpful exercise for a student learning the fundamentals of
C.
What to do:
From D2L download the file lab2exe_B.c. Study the file and write down your prediction for the program output.
Compile the program and run it to check your prediction. If your prediction was wrong, try to understand why.
Make another copy of lab2exe_B.c; call the copy my_lab2exe_B.c. In my_lab2exe_A.c, add a function
definition for my_strlen that calculates the length of a c-string. Then, replace all of the calls to strlen in the
function main with calls to my_strlen, and then make sure your modified program produces the same output as
the original.
Once my_strlen is working, add a function definition for my_strncat to my_lab2exe_A.c. Don't make any
function calls within your definition of my_strncat. Fill in the missing parts of the function interface comment for
my_strncat. Replace all of the calls to strncat in main with calls to my_strncat. 
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Your other task in this exercise is to add function prototype, function interface comment, and the function definition
for a function called my_strcmp. This function should work like strcmp C library function.
If you don't already know the details of how strcmp works, here is a closer look at this library function:
The function prototype of this function is:
int strcmp(const char* str1, const char* str2);
strcmp compares str1 and str2, and returns 0 if the two strings are identical. Otherwise, returns a positive
number if str1 is greater than str2, and a negative number if str2 is greater than str1.
Method one: In some platforms the return value of strcmp is:
• 1, if str1 is “ADC” and str2 is “ABC”.
• -1, if the str1 is “ABC” and str2 is “ADC”.
• 0, if the str1 is “ABC” and str2 is also “ABC”.
Method two: Some other compiler may operate differently. They may return 0 when str1 and str2 are identical.
Otherwise, the may returns the ASCII value differences of the first two characters that are different. For examples:
• If str1 is “ADC” and str2 is “ABC”, it returns 2.
• If the str1 is “ABC” and str2 is “ADC”, it returns -2.
• If the str1 is “ABC” and str2 is also “ABC”, it returns 0.
If you try the following code on one of the recent Mac computers:
char str1[10] = “ABCDE”;
char str2[10] = “ABCD”;
int y = strcmp (str1, str2);
The value of y will be 69, because str1[4] == ‘E’ which its ASCII value is 69 and str2[4] == 0.
For the purpose of this exercise your function my_strcmp should use method two.
Submit the completed source file and your program output(s) that shows your program work, as part of your lab
report on the.
Exercise C (6 marks): Pointer arithmetic
What to do:
Download the file lab2exe_C.c from D2L.Read the program and draw a memory diagram for the second time the
program gets to point one, which occurs during the second call main makes to function what . If you have difficulty
tracing the program you may want to add some calls to printf in what , then run the program to collect extra
information. For example, this code could be used to print addresses from a couple of the pointer variables:
printf("p: %p; max: %p\n", p, max);
%p tells printf to print an address. With most (maybe all) C libraries, the address will be printed using hexadecimal
(base sixteen) representation.
Submit your diagram as part of your lab report, on the D2L.
Exercise D: Drawing pictures of struct variables
Read This First:
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A structure type in C is a type that specifies the format of a record with one or more members, where each member
has a specified name and type. These members are stored on memory in the order that they are declared in the
definition of the structure, and the address of the first member is identical to the address of the structure object itself.
For example, if we consider the following definition for structure course:
struct course{
char code[5];
int number;
char year[4];
};
And, the following declaration of an instance of struct course:
struct course my_course = {“ENCM”, 339, “2nd”};
The address of member my_course.code is identical to the address of my_course, and the address of member
my_course.number is greater than the address of the previous member, my_course.code. However, the
address of the member my_course.number will not be necessarily the address of the following byte right after the
end of memory space allocated for the member my_course.code. It means there might be gaps, or unused bytes
between the members. The compiler may align the members of a structure for certain kind of the addresses, such
as 32-bit boundaries, to ensure fast access to the members. As a result, the size of an instance of structure such as
course is not necessarily equal to the sum of the size of the members; it might be greater. In summary: the size in
bytes of a struct is always greater than or equal to the sum of the sizes of its member variables.
Read This Second – Structures and Pointers
In principle a pointer to a C structure type is not much different from other types of pointers. They are basically
supposed to hold the address of a struct object and they are of the same size as other pointers. It is important to
realize that no matter how big and complicated a struct type is, pointers to that struct type are small and simple.
Remember that the address of a variable is the lowest address of all the bytes used to store it. So the address of a
struct variable is the address of the first byte of the region of memory used to store all the members.
Please notice, when drawing AR diagrams make sure to be clear whether the arrowhead points to the entire
struct instance or to a member of the structure. Please see the following example:
struct course* struct_ptr = & my_course;
char* member_ptr = mty_course.year;
What To Do
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The main point of this exercise is to help you visualize the way struct variables and pointers to struct variables
are organized on the computer memory.
Download the file lab2exe_D.c from D2L. Read the program carefully, and try to predict the output. Run the
program. Ask a lab instructor or teaching assistant for help if you are surprised by any of the output.
Draw stack diagrams for point one, and point two.
This exercise will not be marked, and you shouldn't submit anything.
Exercise E (6 marks): Writing Functions with C struct
Read This First
In general, most of the real-world C programs are divides into two different parts: Implementation part that normally
consists of one or more source code with the extension of .c, and the interface part that may include one or more
header files with the extension.h.
The .h files may contain declaration/prototype of one or more functions, and some other general and global
definitions or declarations such as:
• Declaration of global constants
• Definition of strut data types
• C pre-processor that will be discussed in detail during future lecture
In general the header files that will be created by programmer must be in the following format, confined between Cpre-processors typed in red.
#ifdef XYZ // you can replace term XYZ with any other word that you like
#define XYZ // what ever term is use in previous line must be exactly used here
 ... // your items such as global constants, function prototypes, goes here
 ... // etc.
#endif
The .c files normally should contain the definition of the functions and must include the .h file that contains the
function prototypes and other items as indicated above, using C pre-processor directive. When we include the
system header files like <stdio.h, we use angle-bracket < . However, when we include our own header
files or other programmers’ header files we must use double quotation marks:
#include “your_file_name.h”
To better understand the format of .h files, here is partial example, that shows two files point.c, and point.h
Header file Implementation file
// point.h
#ifndef MY_POINT
#define MY_POINT
// definiton of structure Point
struct Point {
 double x;
 double y; // Point’s y-coordinate
 char label;
};
// function prototypes
void foo(struct Point *p, int x, int y);
void bar(int *a, int n);
// declaration of a global constant
const double PI = 3.145;
#endif
// point.c
#include "point.h"
int main(void){
 struct Point centre;
 int a, b;
 foo(centre, a, b);
 ...
 bar(a, b);

 return 0;
}
void foo(struct Point *p, int x, int y) {
 // implementation of function foo
}
void bar(int *a, int n){
 // implementation of function bar
}
For more details please read the set of slides called Program_Modules posted on the D2L.
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Read This Second
Complex numbers have the form a + j b, where a and b are real numbers and j is a square root of -1. The
quantity i is called the real part and the quantity b is called the imaginary part. Here is a summary of the rules for
complex arithmetic:
The complex number module you will work in this exercise has a terrible, awkward design. The point of this bad
design is to force you to think about some of the different ways functions can work with struct variables--using
value arguments, pointer arguments, and return statements. Don't use this as a design model!
What To Do
Download files lab2exe_E.c, lab2exe_E.h, and lab2exe_E_main.c from D2L.
Step 1. Read through the three source files. Build and executable and run it. To compile this program you should
enter the following command.
gcc –Wall lab2exe_E.c lab2exe_E_main.c
Note: You only compile .c files. The header file lab2exe_E.h should not appear in the command.
Step 2. In the file lab2exe_E.c the definition of cplx_add is given. You should add the definitions for
cplx_subtract , and cplx_multiply.
Do NOT change the prototypes for any of these functions; one of the points of this exercise is to get used to the
different methods available for working with structs and functions.
Step 3. Add code to main to call the functions you wrote in Step 2 and print out the values of w-z , and w*z.
Run your program, using input values of w = 1.5 + j 0.75, and z = -2.5 - j 0.5 .
Check the results by hand or with a calculator.
What To Submit:
Submit only the definition of three functions: cplx_add, cplx_subract, and cplx_multiply as part of your lab report on
the D2L.
Exercise F (2 marks): More Practice to Use Struct Types
What to Do:
Download file lab2exe_F.c from D2L. Compile and run the program and observe the program output. Notice how
sizeof works with C struct types. Now if you pay attention to the program output, you will notice that that output
for the distances between points are incorrect, because the definition of function distance that calculates the
distance between two 3-D points, is incomplete. Your task in this exercise is to complete the implementation of this
function. 
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What To Submit: Submit the copy of your source code for function distance and your program output as part of
your lab report in PDF format.