LAB 6 (2017 June 22/27) Array of pointers. Dynamic memory allocation.

Transcription

1 LAB 6 (2017 June 22/27) Array of pointers. Dynamic memory allocation. Due: June 30 (Fri) 11:59 pm Part I Array of pointers vs. 2D arrays. Command line arguments. 1. Problem A Motivation In class lots people had difficulty understanding array of char pointers -- how to access pointee strings, how to access characters in the pointee strings, and what a pointer array decays to (Week 8 slide 27-30). This practice aims at helping you get started. 1. Download file lab6a.c, play with it. Look at the existing implementations. Observe that, a. to print an integer via its pointer, the argument to printf is at the pointee level b. to print a char array (string), the argument to printf is at the pointer level. That is, pass array name or char * c. to print a char in an array, the argument to printf is at the array element level, and there are 4 ways of doing that. The basic rule is that arr[i] == *(arr+i) == *(p+i)== p[i] where p = arr = &arr[0] 2. Next, complete the program by following the comments 3. Finally, uncomment the last two lines of code, and run the program again a. observe how the character i in Jupiter is accessed. b. familiarize yourself with the different ways of accessing element level of the pointer array, and convince yourself that although they look daunting, they make sense. c. notice how the parentheses are necessary to enforce the order of evaluation The final outputs of the program should be red 329 % a.out 10 hello hello hello h h h h e e e e o o o o Mercury Mercury Venus Venus Jupiter Jupiter Saturn Saturn Neptune Neptune 1

2 Mercury Venus Jupiter Saturn Neptune i i i i M M M M red 330 % Submit your program using submit 2031 lab6 lab6a.c 2. Problem B Subject Similarities and differences between 2D char array and array of char pointers, both of which can be used to store rows of string inputs. Write an ANSI-C program that reads user input strings line by line, until a line of xxx is entered (similar to lab4). The program then outputs the inputs after reordering the first 6 rows of inputs. Assume that there are no more than 30 lines of inputs, and also assume that there are at least 6 lines of inputs. Assume that each line contains no more than 50 characters. Note: each line of input may contain spaces. use fgets(.., 50, stdin) to read in a line. Note that a trailing \n is also read in. use a 2D array to store the user inputs. That is, similar to lab4, define something like records[30][50] when all the inputs have been read in (indicated by xxx ), exchange row 0 and row 1 in main(), and then send the array to a function exchange() to exchange some other rows. define a function void exchange(char[][50])which takes as argument an 2D array, and swaps the record in row 2 with that in row 3, and swaps row 4 with the row 5. Assuming the array has at least 6 rows. define a function void printarray(char[][50], int n)which takes as argument an 2D array, and then prints the first n rows of the records on stdout. Use this function in main to display all the rows of the array. Sample Inputs/Outputs: indigo 329 % a.out Enter string: this is input 0, orange is orange Enter string: this is input 1, apple is green Enter string: this is input 2, cherry is red Enter string: this is input 3, banana is yellow Enter string: this is input 4, do you like them? Enter string: this is input 5, yes bye Enter string: this is input 6, bye Enter string: xxx ===================== 2

3 this is input 1, apple is green this is input 0, orange is orange this is input 3, banana is yellow this is input 2, cherry is red this is input 5, yes bye this is input 4, do you like them? this is input 6, bye Name your program lab6b.c and submit your program using submit 2031 lab6 lab6b.c After you submit, as an additional practice, change the formal argument in one of the function definitions (and the corresponding declaration) from char[][50] to char [][], for example, void exchange(char[][]), and compile. What do you get? 3 Problem C Subject Similarities and differences between 2D char array and array of char pointers. Store strings using Array of (char) Pointers. Pass array of pointers to functions. Swap records of an array of char pointers. download the program lab6c.c and start from there. Observe how an array of char pointers is declared and initialized. define a function void printarray(char ** records, int n) which takes as argument an array of char pointers, and prints first n pointees of records on stdout, one line for each pointee of the array. Use pointer notation only, don t use array index notation. Note that the argument is declared as a pointer to pointer char **, which is what an array of char pointer char * record [] is decayed to when it is passed to a function (why?) define a function void exchange(char * records[])which takes as argument an array of char pointers, and swaps the pointee of the third element pointer (element 2) with the fourth element pointer, and swap the pointee of the fifth element with the 6 th pointer o You should accomplish the swapping without copying/moving the original string data. This is one of the advantages of using array of char pointers against 2-D array. call printarray in main to display the original array. then in main, exchange pointees of the first (element 0) and the 2 nd (element 1) pointers of the array, and then send the array to function exchange() to exchange some other pointees. finally, call function printarray() again to display the (reordered) pointees of the pointer arrays. Sample Inputs/Outputs: red 329 % a.out 3

4 this is input 0, orange is orange this is input 1, apple is green this is input 2, cherry is red this is input 3, banana is yellow this is input 4, do you like them? this is input 5, yes bye this is input 6, bye ===================== this is input 1, apple is green this is input 0, orange is orange this is input 3, banana is yellow this is input 2, cherry is red this is input 5, yes bye this is input 4, do you like them? this is input 6, bye red 330 % Submit your program using submit 2031 lab6 lab6c.c 4. Problem D Subject: Command line arguments (program parameters) and pass array of pointer to functions Write a (short) ANSI-C program that reads command line inputs, which are integers, and then outputs the sum of the input integers as well as the total number of integers. define a function int getsum(char *[], ), which takes (probably among others) as argument an array of char pointers and returns the sum of the value of the pointees of the array elements. define a function int getsump(char **, ), which takes (probably among others) as argument an array of char pointers and returns the sum of the value of the pointees of the array elements. Use pointer notation only, don t use array index notation. display all the command line arguments, and then display the sum twice -- first the result from getsum() and then the result from getsump(), as shown in the sample outputs below. assume that each pointee of the pointer elements, which is a char array, is a valid integer literal, such as 42. assume also that there are at least two input integers Sample Inputs/Outputs: red 377 % gcc commandline.c red 378 % a.out arguments excluding "a.out" = 3 3 indigo 380 % a.out

5 11 arguments excluding "a.out" = indigo 228 % gcc commandline.c -o executablefile indigo 229 % executablefile arguments excluding "executablefile" = red 379 % Name the program commandline.c and submit using submit 2031 lab6 commandline.c 5 Problem E Part II Dynamic memory allocation Subject: Dynamically allocate array space, using malloc or calloc. Write a (short) ANSI program that prompts the user for the size of an int array, and then create the array dynamically. prompts the user for an integer, and then allocate the array space dynamically, using malloc or calloc. assuming the size is always greater than 2, then set the first and second element of the array to be 100 and 200 respectively. then asks the user for the values for the rest of the array elements. finally, display the array, by defining and calling a function void printarray(int *,..) should check if memory allocation is successful. should use free to deallocate the space (at the end of main). Sample Inputs/Outputs: red 376 % a.out Enter array size: 4 Element [2]: 55 Element [3]: red 377 % a.out Enter array size: 6 Element [2]: 55 Element [3]: 33 Element [4]: 40 Element [5]: 450 5

6 red 378 % red 379 % a.out Enter array size: -3 red 378 % Program exits peacefully. No segmentation fault. Name the program lab6e.c and submit using submit 2031 lab6 lab6e.c 6 Problem F Subject Swapping records stored in Array of char pointers. Dynamically allocate array space using malloc or calloc. Similar to problem B, write an ANSI-C program that reads input strings line by line, until a line of xxx is entered. The program then displays the inputs after reordering them. Assume that there are no more than 30 lines of records. Also assume that there are at least 2 lines of inputs. Assume that each line contains no more than 50 characters. Note: each line of input may contain spaces. use fgets(char *, 50, stdin) to read in a line. Note that a trailing \n is also read in. use an array of char pointers to store the user inputs. That is, define something like records * [30] use malloc or calloc to dynamically allocate memory for each row and then copy the input to the allocated memory. o For each element pointer, you could allocate a fix space, e.g, 50 bytes, which is big enough to hold any input line. As in the above 2D array case, each input line occupies the same amount of memory space o A better approach is to allocate memory space according to the length of each line of input, creating a ragged array. This is another advantage of using array of char pointers against 2-D array. when all the inputs have been read in (signified by xxx ), exchange the pointee of the first element (element 0) with that of the 2 nd element, and then send the pointer array to function exchange() to exchange some other rows. define a function void exchange(char **, int)which takes as argument an array of char pointers, and then starting from the 3 rd row, swaps the record with the next line. That is, swap the third row (element 2) with that in the fourth row (element 3), and swap the 5 th row with the 6 th row, and so on. Assume the array has at least 2 rows, and has even number of element pointers Use pointer notation only, don t use array index notation. define a function void printarray(char**, int n)which takes as argument an array of char pointers, and then prints first n pointees on stdout, one line for each pointee of the array element. Use pointer notation only, don t use array index notation. 6

7 Use printarray in main to print the reordered array. Sample Inputs/Outputs red 329 % a.out Enter string: this is input 0, orange is orange Enter string: this is input 1, apple is green Enter string: this is input 2, cherry is red Enter string: this is input 3, banana is yellow Enter string: this is input 4, do you like them? Enter string: this is input 5, yes bye Enter string: this is input 6, thanks, bye Enter string: this is input 7, bye Enter string: xxx ===================== this is input 1, apple is green this is input 0, orange is orange this is input 3, banana is yellow this is input 2, cherry is red this is input 5, yes bye this is input 4, do you like them? this is input 7, bye this is input 6, thanks, bye red 330 % Name your program lab6f.c and submit your program using submit 2031 lab6 lab6f.c 7. Problem G Subject Array of pointers. Dynamic memory allocation. Heap. In addition to allocating memory dynamically, another important feature of malloc/calloc/realloc is that it is a way in C to request a memory space in HEAP, rather than STACK. Local variables declared in a function are stored in stack, and they are deallocated when the function returns. Heap memory space, however, is not deallocated when function returns. The provided programs setarr1.c and setarr2.c try to set an array of integer pointers, through a void function setarr(). The function tries to set the pointers at index i to point to an integer of value 2*i. The programs then try to print out the pointees of the first 5 pointer elements, which should be 0,2,4,6,8. 1. download, compile (using flag -Wall) and run the provided program setarr1.c, and observe what happens. Write at the end of the program your explanation of the outputs. 2. download, compile (using flag -Wall) and run the provided program setarr2.c, and observe what happens. 7

8 Is this version better than the previous version? A little bit, as it does not crash. Write at the end of the program your explanation of the outputs. 3. both the two programs compile successfully but either does not work or does not work correctly. Fix the program by modifying the function setarr(). The program should output 0,2,4,6,8. Name the new program setarr3.c. Submit all the 3 programs using submit 2031 lab6 setarr1.c setarr2.c setarr3.c or submit 2031 lab6 setarr?.c In summary, for this lab, you should submit the following files: lab6a.c, lab6b.c lab6c.c commandline.c lab6e.c lab6f.c setarr1.c setarr2.c setarr3.c Common Notes All submitted files should contain the following header: /*************************************** * CSE2031 Lab 6 * * Filename: Name of file * * Author: Last name, first name * * Your address * * eecs_num: Your eecs login username * ****************************************/ In addition, all programs should follow the following guidelines: Include the stdio.h library in the header of your.c files. Use /* */ to comment your program. You are not encouraged to use // Give a return type int for main(), and return 0 at the end of main() 8