University of California San Diego Department of Electrical and Computer Engineering. ECE 15 Midterm Exam

Transcription

1 University of California San Diego Department of Electrical and Computer Engineering ECE 15 Midterm Exam Tuesday, February 17, :30 p.m. 1:50 p.m. Room 109, Pepper Canyon Hall Name Class Account: ece15 Student ID Number Signature INSTRUCTIONS The exam consists of six problems worth a total of 100 points. Write your answers in the spaces provided. If you need extra space, please use the back of the previous page. Partial credit will be given only for substantial progress. Good luck! Grading points 2. 8 points 3. 7 points points points points TOTAL

2 Problem 1 (14 points) Consider the following C program. The program contains numerous errors, which you will be asked to correct. Nevertheless, it is easy to figure out what the program is supposed to do. #define N MAX = 64 int main(void) int n, i; printf(enter a positive integer less than %d:, N MAX); scanf("%c", n); if!(1 <= n < N MAX) printf("your integer %d is out of range\n", N MAX); for (i == 1; i =< n; ++i); for (j = 1, j =< n; j++) if (j = i) printf("1") else printf("0"); printf("/n"); a. Briefly explain in your own words what this program is supposed to do. Use at most two or three sentences to do so.

3 b. Correct all the errors in the program, directly in the box below. You do not need to provide an explanation for the corrections. However, you do need to make sure that the corrected program will compile successfully (under any ANSI C compliant compiler), run without runtime errors, and do what you said it is supposed to do in part (a). Also note that you should introduce only those corrections that are, indeed, necessary. #define N MAX = 64 int main(void) int n, i; printf(enter a positive integer less than %d:, N MAX); scanf("%c", n); if!(1 <= n < N MAX) printf("your integer %d is out of range\n", N MAX); for (i == 1; i =< n; ++i); for (j = 1, j =< n; j++) if (j = i) printf("1") else printf("0"); printf("/n");

4 Problem 2 (8 points) Each of the following exercises contains several lines of code excerpted from a C program. Your task is to determine, in each case, which is more likely: these lines of code will cause a compilation error, they will cause a run-time error, or they will not cause any errors. Recall that: Compilation errors are errors that prevent the creation of an executable file, since the corresponding C code is not valid. In what follows, you should assume that any piece of code that is not valid according to the ANSI C standard will cause a compilation error. Run-time errors are errors that pass the compilation stage successfully, but cause an error during the execution of a program. A classic example of a run-time error is division by zero. For the purposes of this problem, if a program enters into an infinite loop (from which is does not exit by itself), this should be also considered a run-time error. Grading: For each of the three items below, 4 points if the correct box is the only one checked and 0 points in all other cases. Explanation is optional; in certain scenarios it might lead to partial credit. a. char c; for (c = A ; c < Z ; c++) printf("%d", c+10); compilation error run-time error no errors Optional explanation: b. int i = 1, j = 1; for (i; i <= 10; i++) j /= 5-i; compilation error run-time error no errors Optional explanation:

5 Problem 3 (7 points) Consider the following simple C program, which reads a single character from the user and then prints a message to the screen. char c; scanf("%c", &c); if (c == A ) printf("yay!\n"); else if (c == B ) printf("more study, less play.\n"); else if (c == C ) printf("do the homework!\n"); else printf("drop now!\n"); Rewrite this program while converting the if-else chain above to a switch statement. Write at most one line of code per ruled line in the box. You can also skip as many ruled lines as you like. char c; scanf("%c", &c);

6 Problem 4 (17 points) Consider the following C program. int a[8] = 8,5,1,7,2,6,3,4; int i, j, temp; for (i = 0; i < 8; i++) for (j = i+1; j < 8; j++) if (a[j] < a[i]) temp = a[i]; a[i] = a[j]; a[j] = temp; printf("--> "); for (i = 0; i < 8; i++) printf("%d ", a[i]); a. Briefly explain in your own words what the program does. Use at most one or two sentences to do so. What is the output of this program? The program output is: -->

7 b. Rewrite this program without using for loops. You can use any other loops you like, in particular the while loop, but your program should not contain the for keyword. The output of your program has to be identical to the output of the original program. Write at most one line of code per ruled line in the box. You can also skip as many ruled lines as you like. int a[8] = 8,5,1,7,2,6,3,4; int i, j, temp;

8 Problem 5 (18 points) Each of the following exercises contains several lines of code excerpted from a C program. In each case, you should assume that the program file looks like this:... /* lines of code below */ You should also assume that the program compiles and runs successfully. Your task is to determine the output of the program. Write your answers in the space provided. Explanations are not required. a. int x = 5, y; printf("--> %.2lf", x/10); printf(" %.2lf", x/10.0); printf(" %.2lf", (double) x/10); y = (double) x/10; printf(" %.2lf", y); The program output is: --> b. int a = 0, b = -2, c = 0; if (!a-- (b+=2) && c) printf("true "); else printf("false "); if (a++ && (b-- --c) ) printf("true"); else printf("false"); The program output is:

9 c. int x,y,z; x = 0; for (y = 0; y < 3; y++) for (z = 0; z < 9; z++) if (z == 2) continue; if (z == 5) break; x++; printf("--> x = %d y = %d z = %d", x,y,z); The program output is: --> x = y = z =

10 Problem 6 (36 points) Your goal in this problem is to write a C program, called vowels.c, that counts the number of vowels in a string of characters. The vowels in English text are the characters a, e, i, o, u as well as their uppercase counterparts A, E, I, O, and U (although the letters y, Y often produce a vowel sound, they are generally not considered vowels). In this problem, we will treat uppercase and lowercase vowel characters as being the same. For example, the familiar word Abracadabra contains 5 vowels, namely five a s. The phrase "The Future Ain t What It Used To Be" contains 12 vowels: two a s, four e s, two i s, one o, and three u s. On the other hand, the phrase "29 nymphs shyly fly by mythy crypts" does not contain any vowels. Write a C program that prompts the user to enter a string of characters (terminated by the newline character \n), counts the number of vowels in this string, and reports the results to the user. Here are two sample runs of this program: /home/userxyz/ece15/midterm> vowels Please enter a string: abracadabra Your string contains 5 vowels: 5 a s /home/userxyz/ece15/midterm> vowels Please enter a string: The Future Ain t What It Used To Be Your string contains 12 vowels: 2 a s, 4 e s, 2 i s, 1 o s, 3 u s In this problem, the maximum length of a string of characters is STRING LENGTH, which is a symbolic constant. You can assume that a string entered by the user will not exceed STRING LENGTH characters. You should not assume, however, that this string will necessarily contain vowels. If the user string does not contain vowels, your program should produce an appropriate message and terminate. Here is the corresponding sample run: /home/userxyz/ece15/midterm> vowels Please enter a string: 29 nymphs shyly fly by mythy crypts Your string does not contain any vowels. Hint: You can read characters from stdin using the standard library function getchar. For example, if c is a variable of type char, then c = getchar() reads a single character from the user and assigns it to c. Run this in a loop, while checking for the newline character \n. Hint: In order to count how many times each of the five vowels a or A, e or E, i or I, o or O, u or U appears in the user string, compute a histogram (see Lecture Unit #5). Notes: You must use the program outline provided on the next page. Write at most one line of code per ruled line in the box. You can skip as many ruled lines as you like, but your program should not exceed the length allotted to it on the next page. Comments are not required, but could help you receive partial credit: programs that are not correct and are difficult to understand will receive zero credit, even if they are partially correct.

11 #define STRING LENGTH 80