CMPT 125 Assignment 2 Solutions

Transcription

1 CMPT 25 Assigmet 2 Solutios Questio (20 marks total) a) Let s cosider a iteger array of size 0. (0 marks, each part is 2 marks) it a[0]; I. How would you assig a poiter, called pa, to store the address of elemet 0 of this array? Write the C code for your aswer. it *pa = &a[0]; or it *pa = a; II. Usig pa, how would you obtai the value of the ext elemet (elemet ) of the array? pa poits to the first elemet of the array. So the ext elemet would be *(pa+) III. Explai i 2-3 seteces what the statemet pa = a[]; would do. The statemet would chage the address to which pa poits to the value stored i a[], which is meat to store a iteger, ot a address. While this code will compile, tryig to access the value of pa by usig *pa may cause a segmetatio fault (memory error). IV. Is the statemet a = pa; valid (would it cause compilatio errors)? How about a++;? Explai briefly i 3-4 seteces. No. There is oe key differece betwee a array ame ad poiter. A poiter is a variable, so pa=a ad pa++ are legal. But a array ame is ot a variable; costructios like a=pa ad a++ are thus illegal. V. Write C code to prit all the elemets from this array. Oe possible solutio: #iclude <stdio.h> it mai() { it a[0] = {0,,2,3,4,5,6,7,8,9; for (it *i = a; i <= a+9; i++) { pritf( %d\, *i); b) Cosider the two variables below. (5 marks) char amessage[] = "ow is the time"; char *pmessage = "ow is the time"; Page of 7

2 I. What is the differece betwee the two variables amessage ad pmessage? ( mark) char amessage[] is a array, whereas char *pmessage is a poiter iitialized to poit to a strig costat. II. Write a C code to chage the character t from time to uppercase T i the variable amessage. (5 marks) Oe possible solutio: #iclude <stdio.h> #iclude <strig.h> it mai() { char amessage[] = "ow is the time"; it c = 0; while (amessage[c]!= '\0') { if(amessage[c] == 't' && amessage[c+] == 'i') { amessage[c] = 'T'; pritf("letter: %c\", amessage[c]); c++; c) Cosider the followig code. (5 marks) void strcopy(char s[], char s2[]) { it le = strle(s2); for (it i = 0; i<=le; i++) { s[i] = s2[i]; it mai () { char s2[0] = "copy this"; char s[0]; strcopy(s, s2); The above code copies the character from s2 to s usig the cocept of array, i the strcopy fuctio. Implemet aother fuctio strcopy2 that achieves the same result, but that takes as iput poiters to character arrays that is, complete the strcopy2 fuctio i the followig code so that the code copies the cotet of s2 ito s. void strcopy2(char *s, char *s2) { Page 2 of 7

3 while (*s2!= '\0') { *s = *s2; s++; s2++; it mai () { char s2[0] = "copy this"; char s[0]; strcopy2(s, s2); Page 3 of 7

4 Questio 2 (0 marks, each part is 2 marks) Assume that each of the expressios below gives the processig time T() spet by a algorithm for solvig a problem of size. Fid the domiat term(s) havig the steepest icrease i ad specify the lowest Big-O complexity of each algorithm. Expressio Domiat term(s) O( ) 0.00 log 4 + log 2 (log 2 ) 0.00 log 4 O(log ) 3 log 2 + (log 2 ) 2 3 log 2 O( 3 log ) O( 3 ) , O(.5 ) 3 log 8 + log 2 (log 2 (log 2 )) 3 log 8 O(log ) Note that the base of log does t matter (by default, it s base 2 i this class). I additio, costats are dropped whe usig the Big-O otatio. To compare two terms to see which is domiat, oe ca take their ratio ad let. Example : Iitial evaluatio of gives, so we use L Hopital s rule ad take the derivative of the top ad bottom. log 2 (log 2 ) lim 0.00 log 4 = 000 lim l 2 log 2 l 2 l 4 log = costat lim 2 = costat lim log 2 = 0 This meas that the 0.00 log 4 term becomes much larger as. Note that the costat i frot, o matter how small, does ot make a differece as. Example 2: First we ca cacel a buch of thigs. (log 2 ) 2 lim 3 log 2 = lim log 2 2 Page 4 of 7

5 At this poit oe ca see that the 2 will is much bigger tha the log 2, so the domiat term is 3 log 2. However, to be completely pedatic, let s take. Iitial evaluatio of gives, so we use L Hopital s rule ad take the derivative of the top ad bottom. log 2 lim 2 = lim l 2 2 = l 2 lim 2 2 = 0 Questio 3 (8 marks, each part is 2 marks) For the followig parts, try to get the best Big-O estimate that you ca ad briefly justify your aswers. Part a) it i, j; it = 00; for (i = ; i <= ; i++) { for (j = 3*i; j <= ; j++) { pritf("programmig is fu\"); The ier loop goes from j = 3i to j =. This is 3i + times. Now, i goes from to, so the total umber of pritf calls is i= ( 3i + ). This is at most i= (), which is 2, so the total ruig time is O( 2 ). Part b) it i, j; it = ; for (i = ; i <= ; i++) { for (j = ; j <= 0000; j++) { pritf("%d %d\", i, j); Outer loop is executed O() times. For every executio of the outer loop, ier loop is executed 0000 times, which is a costat umber of times. So, total ruig time is 0000 = O(). Part c) Page 5 of 7

6 it i = 0; it = 0; it j; while (i < ) { j = i; while (i < ) { pritf("hello %d\", i); i = j; Outer loop executes times. For every executio of the outer loop, the ier loop is executed for j = i (the value of i i the outer loop) to, which is i + times. So the total executio cout of pritf is i= ( i + ), which is at most i= () = 2 = O( 2 ). Part d) it i = 0; it = 0; it j; while (i < ) { j = i; while (i < ) { pritf("hello %d\", i); break; i = j; Outer loop executes O() times. Ier loop has a break statemet so it becomes O(), costat. Total ruig time is O(). Questio 4 (5 marks) Write a program i C that prompts a user to select ay five beverages of your choice (e.g Coke, Lemo Tea, etc.). User should be able to select oe of these optios usig oly itegers from 0 to 4 as iputs. Oce a user has made the choice, prit the chose drik alog with its calorie iformatio. You may look up the calorie iformatio olie. Example iput: 3 Example output: Selected drik: Coca Cola Calorie cout: 40 Calories Page 6 of 7

7 Notes: You ca assume that the user will iput itegers. You do ot eed to hadle ivalid iputs of other types (such as strigs, floats, etc.). Your program should gracefully termiate if a user eters wrog iput three times. I additio, after every ivalid attempt, your program should tell the user how may tries he/she has remaiig. For example, the prompt should work somethig like this: o Prompt : User eters 6. Your program should tell that iput is ivalid ad eter agai. Also prit the remaiig tries (2). o Prompt 2: User eters -. Your program should tell that iput is ivalid ad eter agai. Also prit the remaiig tries (). o Prompt 3: User eters 200. Your program should tell that iput is ivalid ad termiate. Also prit the remaiig tries (0). o If the user eters a correct iput o or before the last try, the program should behave ormally, ad output should be like the provided example. #iclude <stdio.h> #iclude <strig.h> it beverages() { pritf("\please make a selectio of your favourite beverage by typig a umber\" "0 - Coca Cola\" " - Pepsi\" "2 - Lemo Tea\" "3 - Redbull\" "4 - Coffee\"); it tries = 3; it selectio; cost char *display[] = { "Selected drik: Coca Cola. Calorie cout: 40 Calories\", "Selected drik: Pepsi. Calorie cout: 50 Calories\", "Selected drik: Lemo Tea. Calorie cout: 90 Calories\", "Selected drik: Redbull. Calorie cout: 200 Calories\", "Selected drik: Coffee. Calorie cout: 60 Calories\", ; while(tries > 0) { tries--; pritf("eter a umber\"); scaf("%d", &selectio); if(selectio >= 0 && selectio <= 4) { pritf(display[selectio]); break; else { pritf("iput is ivalid. Remaiig tries (%d)\", tries); retur 0; Page 7 of 7