Midterm Exam 2 Solutions, C programming

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1 Midterm Exam 2 Solutions, C programming April 26, 2010 Rules: Open book, open notes, open any printed or handwritten material. No electronic devices (except a music player). If you use a music player nobody else must be able to hear it and you must not use its controls during the exams, just put it on shuffle. For the exam assume that the basic data types of C have the sizes they actually have on all common compilers today, even though the ANSI standard does not specify those sizes. Five points per problem, 21 problems, total 105 points. 1. Suppose you have written a function that can sort an array of integers (of any size). How should this function be declared? (d) void sort(int x, int n); You must mention the number of elements to be sorted, because in C an array is just a pointer. 2. Fill in code to match the comments: int main(int argc, // finish this line correctly { int n = 7; // (you don t supply anything on this line) if( argc > 1) // if an argument is supplied on the command line { sscanf(argv[1],"%d",&n); // get n from the first command line argument else n = 7; 3. Show how to use the typedef command to create a type vector that is a synonym for an array of integers. typedef int vector[3]; 1

2 4. Define a function pair that takes two integers and returns an array of length 2 containing those two integers. Hint: Be sure that the space you use will continue to exist after your function returns! int *pair(int x, int y) { int *ans = (int *) calloc(2,sizeof(int)); ans[0] = x; ans[1] = y; return ans; 5. Correctly declare a two-dimensional array x of integers, where the first index can have values 0 to 5 and the second can have values 0 to 7. int x[6][8]; 6. Matrix multiplication is an extremely common programming task. Sammy Nguyen tries to program it in C, starting a follows: void MatrixMultiply( int x[][], int y[][], int *answer[][]) // multiply x by y and return the result indirectly by writing it into answer. This code, however, won t even compile. Exhibit a fixed version, that will at least compile, by just inserting something into Sammy s code without removing anything. void MatrixMultiply( int x[][3], int y[][3], int *answer[][3]) // multiply x by y and return the result indirectly by writing it into answer. 7. Here is Sammy s second attempt: void MatrixMultiply( int **x, int **y, int ***answer) // multiply x by y and return the result indirectly by writing it into answer. Pick the one choice that best analyzes why this attempt is also unsuccessful. (c) The function won t have any way to know how many rows and columns the matrices have (the best answer) (d) You can t pass a two-dimensional array into a function declared like that (also true and accepted as OK) 2

3 8. Which of the following is a recursive palindrome, as defined in your homework? Circle all the correct choices, if there are more than one. (c) TOTXTOTYTOTXTOT (e) XXXXXXXXXXXXXXX 9. The following code is intended to compute x to the power y, mod m efficiently. But the programmer has made a mistake. This code, while it will compute correct answers, will not run as fast as the fixed code you will supply. Fix this code by replacing ONE mistaken line with new code. Just write your answer beside the given code and cross out the line it should replace. int fastexp(int x, int y, int m) // assumes m*m fits into an int // returns x^y mod m { if(y==0) return 1; if(y==1) return y % m; if(y % 2 == 0) return (fastexp(x,y/2,m)*fastexp(x,y/2,m) %m); // delete this { int z = fastexp(x,y/2,m); return z*z; // replace with this return (x*fastexp(x,y-1,m)) % m; 10. Sammy Nguyen wants to include the system header file stdio.h and his own header file myfile.h in his C file. Give the two include commands he should use, in the proper order. #include <stdio.h> #include "myfile.h" 11. We want to work with some arrays. Since an array must be declared with a fixed number of elements in C, we have to pick a fixed size, but we will have many arrays of this size in our program. Show how to declare several arrays of size 100 in such a way that we can change the number 100 to some other number by changing it in just one place. #define N 100 int x[n]; double z[n]; 3

4 12. Sammy Nguyen has defined a function f that takes an int and returns an int. This function is called in several of his files. He wants to be sure that if he makes an error by calling the function wrong (for example by passing it a double instead of an int) that it will be a compile-time error and not a run-time error. Say the function is defined in file1.c and called in file1.c and file2.c. Show how to achieve Sammy s purpose by showing what lines of code need to be placed in each of his files, or, if no line of code is needed in one or more of his files, just write none instead of a line of code. (a) Line of code to place in file1.h: int f(int n); (b) Line of code to place in file2.h: none (c) Line of code to place in file1.c: #include file1.h (d) Line of code to place in file2.c: #include file1.h 13. Suppose that you have a free version and a paid version of your program. You wish to compile these versions from the same source, changing only a couple of lines to compile the other version. Show how this would be done, by supplying lines of code to go in the specified files and match the given comments. You want to arrange that the code from the paid version is not even present in the free version, since somebody might reverse-engineer it if it is present. (a) In your header file version.h: #define FREE // uncomment this line to make the free version #define PAID // uncomment this line to make the paid version (b) In various.c files: #ifdef PAID // if it s the paid version int f(void) { // this is just a place holder for some imaginary code you want to charge for #endif #ifdef FREE // if it s the free version 4

5 int f(void) { // this version of f is for the cheapskates #endif // code below here is for both versions 14. (Macros) What will be printed by the following code? (Write your answer beside the printf line.) #define CUBE(x) x*x*x void main(void) { printf("%d", CUBE(2+2)); Answer. 12 is printed. The macro expands to which evaluates to Correctly define a macro CUBE(x) to compute the cube of x. #define CUBE(x) ((x)*(x)*(x)) 16. What is the hexadecimal value of the expression 0xfffa & eee6? Answer eee2, since f, which is fifteen in decimal, is 1111 in binary and e = 14 = 1110 in binary, and a, which is ten in decimal, is 1010 in binary while 6 is 110, so the bitwise and of a and 6 is 0010, which is 2 in hexadecimal. 17. Below is most of the code for a function void f(int x) that will cause yes to be printed if the third bit of x is 1 and no to be printed if that bit is 0. Here the third bit of 4, which is 100 in binary, is 1, and its first and second bits are 0. Please supply the missing piece to match the comment. Do not write more than a few symbols, and do not write anything except in that part of that one line; in particular do not use any macros given in class. int f(int x) // test the third bit of x and print yes or no { if(x & 4) // see if the third bit is set printf("yes"); else printf("no"); 5

6 18. What is the decimal value of 1 << 10? Answer: What will be printed by the following code? Write your answer by the printf line. void main(void){ int x = 1; int i; for(i=0;i<5;i++) x << i; printf("%d",x); Answer: 1 will be printed. The value of x does not change just because you evaluate the expression x << i. 20. (a) Convert the number 2349 to a Roman numeral. Show how you did it = MM 300 = CCC 40 = LX 9 = IX final answer = MMCCCLXIX (b) Convert the Roman numeral MCMLXXIX to a decimal number. Show how you did it. thousands = M = 1000 hundreds = CM = 900 tens = LXXX = 80 units = IX = 9 final answer = = 1989 Also acceptable: M = 1000 C = -100 since followed by M M = 1000 L = 50 6

7 X = 10 X = 10 I = -1 since followed by X X = 10 total In your PigLatin homework, you had to get the initial consonants of a word. The concept of initial consonants was discussed in that assignment. Therefore you should have no trouble writing the following function: void f(char *x, char *consonants) // put the initial consonants of x into the array consonants. // x will be null terminated. // the array { char *m, *t; for(m=x,t=consonants;*m;m++,t++) { char u = toupper(*m); if(u == A u == E u == I u == O u = U ) { *t = \0 ; return; *t = *m; *t = \0 ; // no vowels in x You could also use integer indices instead of character pointers to move through x and consonants. 7