Description
Overview
In this lab, we will continue to investigate some of the interesting facts about how C interacts with memory. You will continue to practice the use of pointers to access memory.
You can refer to chapter 5 and 6 of K&R for references on pointers.
Getting Started
Before we begin any activities, create a directory (Lab_4) inside the CSE31 directory we created last week. You will save all your work from this lab here. Note that all the files shown in green below are the ones you will be submitting for this assignment.
You must have a clear idea of how to answer the lab activities before leaving lab to receive participation score.
How Type-Casting Works with Memory
From lecture, we have learned that an array is stored in a contiguous memory space. The address of each element of an array is assigned by the computer (operating system).
TPS (Think-Pair-Share) activity 1: Discuss questions 1 – 10 (25 minutes) while paired with your classmates assigned by your TA (you will be assigned to groups of 3-4 students) and record your answers in a text file named tpsAnswers.txt under a section labelled “TPS 1” (you will continue to use this file to record your answers to all the TPS questions that follow in the lab handout): 1. Open MemCast.c, compile and run the program. What do you expect the program to print? (%x in
printf allows an integer to be printed in Hex format).
2. Before changing the code, what do you expect the program to print if you print four_ints[0]
again at the end of the program?
3. Insert a print statement to output four_ints[0] at the end of the program and verify your answer
from (2).
4. Now add a print statement to the program so it will print out four_ints[1]. What does it print? Are
you surprised (or lost) by the results?
5. Let us study the code carefully and investigate what happened. No, the memory did not go crazy.
a. How many array(s) were allocated in this program? b. Are four_ints and four_c pointing to the same location? c. Verify your answer of (b) by printing out the values of four_ints and four_c.
6. Write a loop to print out the addresses and values (in Hex) of all the elements of four_ints. What is the difference in addresses between two consecutive elements? Discuss what this difference means. 7. Use a piece of paper to draw the layout of the array horizontally so that the smallest address begins
from the RIGHT-hand-side. Indicate the address and value of each element based on the results of (6). You can draw it digitally.
8. Now, write a loop to print out the same information for four_c as you did in (6). What is the difference in addresses between two consecutive elements? Discuss what this difference means.
9. Use the same piece of paper (or file) from (7) to draw a similar structure for four_c. 10. By comparing the layout of the array pointed by the two pointers, what do you observe in terms of how
C accesses memory when the index of an array is incremented?
Your TA will “invite” one of you randomly after the activity to share what you have discussed.
2D Arrays with Malloc
In C, a 1-dimensional array can be declared as a pointer pointed to a dynamically allocated memory location:
int* array = (int*)malloc(n * sizeof(int)); // n is the size of array
We can also use a double pointer (**) to construct a 2D array. In fact, a 2D array is just multiple rows of 1D arrays. You can also view it as an array of arrays as shown below:
In the example above, each row is an array of int. Since each row is an array of int, we need an int* pointing to each row. As a result, we need to have an array of int* to point at different rows of int.
TPS activity 2: Discuss questions 1 – 7 (25 minutes) with your TPS partners in your assigned group and record your answers in tpsAnswers.txt under a section labelled “TPS 2”: 1. Open Array2D.c. This program will create a n x n array of int. Explain what line #8 does. 2. Since every array must be allocated in the memory before it can be used, we need to allocate the rows
one by one. To do this, we need to be able to access the pointers from the first array (pointed by arr). Assuming i is the index of that array, how do we access the ith value of the array?
3. Without using array notations ([]), insert code to complete allocating all the rows and initialize all contents to be 0. Your code should work with different values for n. Hint: if j is the index of each row, how do you access arr[i][j] in pointer notation?
4. To verify whether you have created your array correctly, we need a function to print out the array. The
function printArray has been declared. It takes in both the array to be printed and size of array. Why do we need to pass the size as an argument?
5. Complete printArray so it prints out the content and layout of an array correctly. 6. Now, let us modify the content of the array. Insert code to make the array into a diagonal matrix that
looks like the following (again, do not limit the size to 5):
1 0 0 0 0
0 2 0 0 0
0 0 3 0 0
0 0 0 4 0
0 0 0 0 5
7. Call printArray to print out your new array and verify result.
Individual Assignment 1: Matrix Multiplication
Now we know how 2D arrays work. Let us put them in practical use (you must not use any array notions ([]) in this assignment): 1. Open MatrixMult.c. and define in main() two n x n matrices (arrays) using malloc. 2. Implement printArray function so you can call it to print a 2D array. 3. In main(), call printArray to print out the 2 arrays you have defined in (1). 4. Implement matMult so it multiplies the 2 input arrays and return the resulting array. Pay attention to
the input arguments and return type.
5. In main(), call matMult to multiply the 2 arrays you have defined in (1). 6. In main(), call printArray to print out the resulting array you receive in (5). 7. You will need to declare any variables that are necessary.
Collaboration
You must credit anyone you worked with in any of the following three different ways: 1. Given help to 2. Gotten help from 3. Collaborated with and worked together
What to hand in
When you are done with this lab assignment, submit all your work through CatCourses.
Before you submit, make sure you have done the following: • Your code compiles and runs on a Linux machine (without the need for special libraries). • Attached MemCast.c, Array2D.c, MatrixMult.c, and tpsAnswers.txt. • Filled in your collaborator’s name (if any) in the “Comments…” textbox at the submission page.
Also, remember to demonstrate your code to the TA or instructor before the end of the grace period.
