Lab 2: pointers and arrays

CS 305 Lab 2: pointers and arrays

This lab introduces you to pointers, arrays, malloc and free.
This lab has a total of 100 possible points (30 points from pre-lab and 70 points from lab).
Objectives
Upon completion of the laboratory, you will be able to do the following:
• Explain the results of pointer arithmetic and comparisons (prelab).
• Access array elements using pointers (lab).
• Use malloc or calloc to create dynamically sized arrays (lab).
• Free memory (lab).
Part 1: Log in (choose one person of the pair to log in)
1. Follow the procedure you used in lab 1 to log into the computer.
2. Go into your cs305 directory and make a new directory called lab2 in your cs305 directory.
cd cs305
mkdir lab2
3. Get the lab2 files. Download starter.zip from moodle to your lab2 folder. On Mobaxterm
navigate to your lab2 folder. In the terminal window, type:
ls
Hopefully, you can see your zip file there. To unzip it, type:
unzip starter.zip
List your directory contents again. You should see a new directory called starter in addition to the zip
file.
ls
Part 2: Pointers and arrays
In C, there is a close relationship between pointers and arrays.
First, with just a couple of exceptions, any time an array name is given in a program, it is converted (by
the compiler) to be a pointer to the first element of the array. The exceptions are when the array is the
immediate operand of the sizeof function or the "create-pointer-to" (unary &) operator. The
semantics of the [] operator in a[i] mean *((a)+(i))):
• a is converted to be a pointer to its first element,
• i is added to this pointer, giving a pointer to the element in the ith position of a, and
• the * operator makes the reference.
One of the anomalies of C is that the semantics of [] allows i[a] to be used as an alternative to
a[i]. (You could experiment with this: instead of a[0], write 0[a] when you want to access the
first element in array a. Just because you can do this does not mean you should in your code…Tammy
does not recommend using the 0[a] form.)
Second, any time a function's formal parameter is given as an array type, it is converted (by the
compiler) to be a pointer type: "array of int" is converted to "pointer to int"; "array of pointer to char" is
converted to "pointer to pointer to char".
1. Given this knowledge, examine the program array.c. Compile and run the program. Explain why:
• no type-check errors are reported for any of the three declarations of the function abc, even
though the parameter declarations appear to be inconsistent. The “extern” keyword declares
the type for the function abc, while the actual function definition is defined once below the
three declarations.
Compile and run the program:
gcc –o array array.c
./array
2. Why does the array seem to change size when the function is called?
Part 3: Pointers accessing elements out-of-bounds (switch driver)
C does not do bounds-checking on arrays for you, so it is possible to use pointers to access memory that
does not store contents of the array. This is a bit scary, so you should be vigilant in writing C code by
doing bounds-checking.
Examine the program bad_pointers.c. This program creates two arrays (one of size 4 and one of
size 8) and creates pointers to each array. Compile and run the code and explain how you think the 
memory for array a and array b are allocated given the printing you see. Draw a picture of the memory
layout.
Add to the bottom of the main function:
• Print the values of a[0] through a[25]. (Note that the array a really contains 8 integers. The
variable i is already declared in the file.)
Does printing the values confirm your memory layout above? YES NO
What are the second half of the 25 values coming from?
_____________________________________________________________________________
Checkpoint 1 [25 points]: Show your answers to the above questions and execute your code for your
lab instructor/assistant.
Part 4: Allocate an array on the heap (dynamically-sized array) (switch driver)
Open and examine the file array_alloc.c.
The main function in array_alloc.c:
• prompts the user for a non-negative integer; re-prompts if necessary
• calls the get_array function with the given argument, which returns (a pointer to) a dynamicallyallocated array. If the result of get_array is non-null, the main function will assume that it is to
an array of double whose length is the integer argument.
• prints the values in the array by calling the printArray function
• frees the memory allocated for the_array; note that you should also free memory that is
allocated on the heap when your program is done using that memory
As implemented, the get_array function simply returns null. Modify it so that it:
• checks the argument for being negative. If it is negative, it is treated as if the argument was 0
• allocates an array of doubles of the given length (hint: use the calloc function or malloc
function to do this)
• initializes the array so that for all values n in its range, the value at index n of the array is n *
10.0.
Compile and run your program.
Checkpoint 2 [20 points]: Show your modified array_alloc.c program to your lab instructor/assistant.
Demonstrate its execution.
Part 5: Allocate a 2-dimensional array on the heap (switch driver)
1. Open and inspect the file 2darray.c.
The main function in 2darray.c:
• prompts the user for the height and width of the 2D array
• calls the make2Darray function to create a 2D array of ints with that size and returns a pointer
to the dynamically-allocated array
• prints the values in the array to the screen using print2Darray
As implemented, the make2Darray function allocates memory to store the 2D array. Inspect this
function to see how the allocation and freeing of memory works.
Why is the type of a (int **)?_______________________________________________________
Why is the type of a[i] (int *)?____________________________________________________
As implemented, the free2Darray function frees the memory for the 2D array. Inspect this function to
see how the array is “freed”: first each row is “freed” and then the array is “freed”.
2. Compile and run the program.
3. Add code to the function make2Darray to do the following:
• Initialize each element in the 2D array with its row number. For example, a height 4 and width 3
array would have the values:
0 0 0
1 1 1
2 2 2
3 3 3
Checkpoint 3 [25 points]: Show your answers above and your modified 2darray.c program to your
instructor or lab assistant. Demonstrate its execution.
Part 6: Finish up
Close any emacs windows. Close Mobaxterm and log off the PC.
If any checkpoints are not finished, they are due in one week. You may submit screenshots and printed
files of code to complete the checkpoints. Be sure your name is on the submitted work.