CpSc 1011 Lab 11 Pointers

Transcription

1 CpSc 1011 Lab 11 Pointers Overview Pointers are an integral part of C and C++. While they may be the cause of many headaches, they allow for a great deal of control and flexibility. Because they are low-level constructs, they require a solid understanding of the memory model behind it all. Not every language has pointers, but what you learn about memory management will apply to most every language. Unlike most of your previous labs this semester, this one will not require you to write and submit a program. This lab is adapted from a lab used elsewhere, and it involves a good amount of guided self-exploration. We ll give you the right questions to ask along the way. Many of the questions will be open-ended, so when answering them, be sure to ask if you are not 100% sure what s going on! Please use the answer sheet provided. You will submit the answer sheet to Vocareum. You may work individually or in groups of no more than 3. The code snippets here are screenshots, so you won t be able to copy and paste them (sorry!) but they re short enough to type. This lab will give you the opportunity to: Understand and use pointers and pointer operations Discover the relationship between arrays, strings, and pointers Explore passing pointers as function parameters Explore the const modifier with pointers Warm Up First, let s practice with pointers to basic C data types and some related pointer operations. We ll see referencing, dereferencing, and arithmetic operations. 1. Compile and run this program: 2. Suppose after the declaration of the variables on line 4, we have the following: Symbol Table: Name Type Address integer1 int 0x7fff8c1ff994 p1 int * 0x7fff8c1ff988 p2 int ** 0x7fff8c1ff980 Memory Chunk (address on the left, data on the right): 0x7fff8c1ff994 0x7fff8c1ff988 0x7fff8c1ff980 a. Fill in the above memory chunk table to reflect the changes that lines 6, 7, and 8 cause. 1

2 b. In we substitute lines with the following two statements, then what will be the output of the program? (*p1)++; printf( integer1= %d\n, *p1); c. Will the output be the same if we substitute the above two lines with the following two statements? integer1++; printf( integer1= %d\n, *p1); d. Will the output be the same if we substitute the above two lines with the following two statements? *p1++; printf( integer1= %d\n, *p1); e. Explain why the above outputs are the same or different from each other. 3. Consider the following program and answer the questions below. What does variable p2 represent? Will this program successfully compile without warnings? Why or why not? (Try it!) 2

3 Arrays and Pointers Now let s look at the relationship between pointer notation and array notation. (Hint: they both accomplish the exact same thing!) 4. Compile and run the following program and answer the corresponding questions. a. What is the output of the program? b. What does array_of_integers evaluate to? If you dereference it, what value do you get? Try it. c. Suppose we change lines to the following: for (i = 0;i < 10; i++) *(ptr_of_array + i) = i; Will the output change? Why or why not? d. Suppose we change lines of the original program to the following statements: for (i = 0;i < 10; i++) printf( %d\n, *(ptr_of_array + i)); Will the output change? Why or why not? e. Suppose we change lines of the original program to the following statements: for (i = 0;i < 10; i++) *(ptr_of_array++) = i; Will the output change? Why or why not? Now, does the variable ptr_of_array have the same value as the variable ptr_of_first_element? Why or why not? 3

4 5. Consider the following code: Fill out this memory model table (you may indicate repetition with arrows or with marks if necessary): Label of Address of Content of Meaning of x addr_x 10 an integer px addr_px addr_x a pointer to an integer a[0] a[1] a[2] a[3] a[4] a[5] a[6] a[7] a[8] a[9] b b[0] b[1] b[2] b[3] b[4] p1 p2 parray addr_a[0] addr_a[1] addr_a[2] addr_a[3] addr_a[4] addr_a[5] addr_a[6] addr_a[7] addr_a[8] addr_a[9] addr_b addr_b[0] addr_b[1] addr_b[2] addr_b[3] addr_b[4] addr_p1 addr_p2 addr_parray 4

5 Label of Address of Content of Meaning of ap1 ap1[0] ap1[1] ap1[2] addr_ap1 addr_ap1[0] addr_ap1[1] addr_ap1[2] ap2 ap2[0] ap2[1] addr_ap2 addr_ap2[0] addr_ap2[1] a. Will the program successfully compile without warnings if we substitute line 14 of the program with the following statement? Explain why or why not. int (*parray)[10] = &b; b. What if we substitute line 14 of the program with the following statement? Explain. int (*parray)[] = &b; c. Will the program successfully compile without warnings if we substitute line 22 of the program with the following statement? Explain why or why not. ap2[0] = &(*parray); 5

6 Function and Pointers In this part, you will get to play with pointers as function parameters and practice C s version of pass-by-reference. 6. What is the output of the following code? Why? The const Modifier and Pointers In this part, you will play with code involving the const modifier with pointers. 7. Write and try to compile the following code: a. What lines of code cannot be successfully compiled? What are the error messages given by the computer? Explain why. Try commenting out the lines that don t compile and then compile again that might help you understand what s going on. b. What prints out on line 9? What is the value of x at that point? 6