Imperative Programming

Transcription

1 Imperative Programming Week 2 Exercises model answers 1. Write a program that copies its standard input to its standard output. Test the program by using it to display its own source code../copyio < copyio.c censored! int c= 0; while (EOF!= (c= getchar())) putchar(c); 2. Write a program to count the number of characters in a file. Test it on its own source code, or on a huge file..../count < /usr/share/dict/words chars int nchars= 0; while (EOF!= getchar()) ++nchars; printf("%d chars\n", nchars); 3. Extend your program to count the number of lines too../count < /usr/share/dict/words chars, lines

2 int nchars= 0, nlines= 0; int c= 0; while (EOF!= (c= getchar())) ++nchars; if ( \n == c) ++nlines; printf("%d chars, %d lines\n", nchars, nlines); 4. Just for fun, modify your program so that it also prints each letter or newline character that it reads from the input. All other characters should be printed as spaces../count < count.c include stdio h include ctype h int main...etc chars, 22 lines int nchars= 0, nlines= 0; int c= 0; while (EOF!= (c= getchar())) ++nchars; if (isalpha(c)) putchar(c); else if ( \n == c) ++nlines; putchar( \n ); else putchar( ); printf("%d chars, %d lines\n", nchars, nlines); 2

3 5. Modify your program so that it also counts the number of words that appear in the input. Consider a single word to be a sequence of one or more letters../count < count.c 512 chars, 59 words, 21 lines int nchars= 0, nlines= 0, nwords= 0; int inword= 0, c= 0; while (EOF!= (c= getchar())) ++nchars; if ( \n == c) ++nlines; if (isalpha(c)) if (!inword) // transition from outside to inside word inword= 1; ++nwords; else inword= 0; // definitely outside a word printf("%d chars, %d words, %d lines\n", nchars, nwords, nlines); 6. alternating CaPs go way back to the early days of cyberculture. They were sometimes seen on Usenet newsgroups (an early Internet discussion forum) and FidoNet (a dial-up telephone bulletin board system) in the 1980s and 90s. To avoid wearing out your shift key, write a program that automates the process of alternating the case of text../alternate < alternate.c #include <StDiO.h> #InClUdE <ctype.h> InT main()...etc... 3

4 int uppit= 0, c= 0; while (EOF!= (c= getchar())) if (isalpha(c)) if (uppit) c= toupper(c); else // downit c= tolower(c); uppit=!uppit; putchar(c); // only toggles when we have a letter 7. While the above is true, everyone from back then knows that the cool way to mix letter case is to KeeP only THe VoWeLS in LoWeRCaSe. THiS improves LeGiBiLiTy WHiLe STiLL GeTTiNG THe PoiNT across. Modify your program accordingly../downvowels < downvowels.c #include <STDio.H> #include <CTYPe.H> int MaiN()...etc... int isvowel(int c) c= tolower(c); return a == c e == c i == c o == c u == c; int c= 0; while (EOF!= (c= getchar())) if (isvowel(c)) putchar(tolower(c)); else putchar(toupper(c)); 4

5 8. Write a function that calculates the factorial of its argument. (For extra kudos, make it recursive.) Test it by writing a loop in main that prints the integers from 0 to 19 along with their factorials../factorials factorial 0 = 1 factorial 1 = 1 factorial 2 = 2 factorial 3 = 6 factorial 4 = 24 factorial 5 = etc... int factorial(int n) if (n < 2) return 1; return n * factorial(n - 1); int i= 0; while (i < 20) printf("factorial %d = %d\n", i, factorial(i)); ++i; 9. Figure out why the program in the last question went off the rails towards the end, and then fix it. long factorial(long n) if (n < 2) return 1; return n * factorial(n - 1); long i= 0; while (i < 20) printf("factorial %ld = %ld\n", i, factorial(i)); ++i; Programming project: checksums Checksums are vital for ensuring the integrity of data. One of the simplest ways to create a checksum is to add up the integer values of all the characters in a message and use the final sum as a checksum. 10. Write a program that computes the additive checksum of its input characters and then prints it 5

6 when the message ends. To allow the final checksum to be included with the message (and to avoid arithmetic overflow) the checksum should be restricted to 8 bits. Therefore, perform your addition operation modulo 255 so that the checksum cannot exceed 254. Test your program by typing hello checksum(enter)ˆd into it../checksum hello checksum 150 don t forget to press Control-D after ENTER int sum= 0, c= 0; while (EOF!= (c= getchar())) sum= (sum + c) % 255; printf("%d\n", sum); Fletcher-16 checksum Test your checksum program again using the input hell ochecksum (the space and the o have been transposed). Your checksum is not sensitive to the order of the characters. 11. Make your checksum program sensitive to the order of the characters in the message. You can do this by calculating two checksums at the same time: The first checksum (let s call it sum1) is exactly as above; it sums the sequence of characters (modulo 255) in the message. The second checksum (let s call it sum2) uses the exact same algorithm to calculate a checksum for the sequence of values of sum1 after each character in the message has been processed. In other words, each time around the loop you update sum1 to be the sum of the characters seen so far, and then update sum2 to be the sum of the sum1 checksums seen so far. This is called the Fletcher-16 checksum, after its inventor and the number of bits of checksum (two 8-bit sums) that you have computed. At the end of the message print sum2, a space, and then sum1. Test your new checksum program using the same two input messages as above../fletcher16 hello checksum /fletcher16 hell ochecksum

7 int s2= 0, s1= 0, c= 0; while (EOF!= (c= getchar())) s1= (s1 + c) % 255; s2= (s2 + s1) % 255; printf("%d %d\n", s2, s1); If you are very bored, try to find two (different, non-trivial) messages that have the same checksum. Adler checksum The probability of a communication error modifying a (long) message without modifying its Fletcher- 16 checksum is about 1/255 2, or 1 in 65025, or %. Not spectacular. If you were to send one (long) message per second over an unreliable connection, how much time (on average) would elapse before a corrupted message is not detected by the Fletcher-16 algorithm? 9 hours 1 minute and 52.5 seconds 12. Modify your program to calculate the Adler checksum of the message. The algorithm is the same as for Fletcher, except that it uses a modulus of 65,521 and the two checksum values are initialised with 1 instead of 0. Test the new version on the same input../adler hello checksum /adler hell ochecksum int s2= 1, s1= 1, c= 0; while (EOF!= (c= getchar())) s1= (s1 + c) % 65521; s2= (s2 + s1) % 65521; printf("%d %d\n", s2, s1); How many bits of checksum are being calculated by your program now? 32 7

8 What is the probability of a communication error modifying a (long) message without modifying its Adler checksum? 1/ , or 1 in , or % If you were to send one (long) message per second over an unreliable connection, how much time (on average) would elapse before a corrupted message is not detected by the Adler algorithm? 68 years If you are very bored, should you entertain yourself trying to find two (different, non-trivial) messages that have the same Adler checksum? probably not 8