ENCM 335 Fall 2018 Lab 2 for the Week of September 24

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1 page 1 of 8 ENCM 335 Fall 2018 Lab 2 for the Week of September 24 Steve Norman Department of Electrical & Computer Engineering University of Calgary September 2018 Lab instructions and other documents for ENCM 335 can be found at Administrative details Each student must hand in their own assignment Later in the course, you may be allowed to work in pairs on some assignments. Due Dates The Due Date for this assignment is 3:30pm Friday, September 28. The Late Due Date is 3:30pm Monday, October 1. The penalty for handing in an assignment after the Due Date but before the Late Due Date is 3 marks. In other words, X/Y becomes (X 3)/Y if the assignment is late. There will be no credit for assignments turned in after the Late Due Date; they will be returned unmarked. Marking scheme A B C D E F G total 1 mark 10 marks 3 marks 2 marks unmarked unmarked 10 marks 26 marks How to package and hand in your assignments Please see the information in the Lab 1 instructions. A few words about unmarked exercises Please don t skip exercises that won t be marked. There are two main benefits to doing the unmarked exercises: Doing them will sometimes teach you things that you need to know to do marked exercises later in the assignment.

2 ENCM 335 Fall 2018 Lab 2 page 2 of 8 Sometimes they help solidify understanding of an important concept that will be tested on a quiz, the midterm or the final exam. Function interface comments Please find the document called Function Interface Comments on the Lab Assignment Information page. This document describes the format we will use to document C functions in lectures, labs, and tutorials, so it s important that you read this document carefully. For Lab 2, you can skip Section 4.3 of Function Interface Comments. This section is about pointers and arrays, which is a Lab 3 topic. Exercise A: Pointers are numbers Lecture material on pointers should have made it clear that pointer variables and pointer function parameters are containers for addresses, and that addresses are really just numbers. When programming with pointers, you normally don t actually care what these numbers are. But being able to print addresses takes some of the mystery away from the topic of pointers. The best way to do it with the C printf function is to use the %p format specifier. There s no exact standard for how printf will display an address, but it s common to use hexadecimal (base sixteen) format, and often 0x appears in front of the number to identify it as hexadecimal the 0x is not part of the number, but rather a way to say that the following digits are hex digits., Part I Download the file lab2exa.c. Read the source file; ask questions in the lab if there is anything you don t understand. In Cygwin Terminal, build an executable and run it. Use the program output to complete a table that looks like this: variable ga gb la lb address, Part II There is nothing to hand in for this part. Have a look at Figure 1, which shows the output of the program using various combinations of operating systems and compilers. The exact same thing happens three times to the values of the int variables, but it s interesting to note that the sets of addresses involved are very different from one platform to the next. Hand in your table from Part I. If you did the work on on your own computer, not a computer in ICT 320, add a note to say what operating system you were using.

3 ENCM 335 Fall 2018 Lab 2 page 3 of 8 Figure 1: Output of the program of Exercise A, with executables built on three different platforms. output on my Linux machine using gcc... The address in left is 0x The value of the int in *left is 0. The address in right is 0x The value of the int in *right is 44. After swapping, ga is 44 and gb is 0. The address in left is 0x7fff96ad5e78. The value of the int in *left is 22. The address in right is 0x7fff96ad5e7c. The value of the int in *right is 33. After swapping, la is 33 and lb is 22. output on my Mac using the clang compiler... The address in left is 0x c. The value of the int in *left is 0. The address in right is 0x The value of the int in *right is 44. After swapping, ga is 44 and gb is 0. The address in left is 0x7fff5c768b98. The value of the int in *left is 22. The address in right is 0x7fff5c768b94. The value of the int in *right is 33. After swapping, la is 33 and lb is 22. output from an x64 console application built using Visual C++ on Windows The address in left is 00007FF6B509C170. The value of the int in *left is 0. The address in right is 00007FF6B509C000. The value of the int in *right is 44. After swapping, ga is 44 and gb is 0. The address in left is AA5477F734. The value of the int in *left is 22. The address in right is AA5477F754. The value of the int in *right is 33. After swapping, la is 33 and lb is 22.

4 ENCM 335 Fall 2018 Lab 2 page 4 of 8 Exercise B: Introduction to pointers This is one of the most important exercises in ENCM 335. If you don t become comfortable with pointers, you will not be able to work with C. Spend as much time on this exercise as is necessary to understand exactly what is happening at every step in the given programs., Part I Download the file lab2exb-parti.c. Using pencil and paper, trace through the execution of the program to determine what the output will be. For each of points two through five, make drawings in the style... AR main delta 555 gamma 333 beta?? alpha no params... which is an accurate representation of the program state at point one. Draw the diagrams neatly by hand. Compile the program and run it to see whether your prediction of the output was correct. If the output is not what you expect, you will probably have to correct one or more of your diagrams., Part II Assume the following addresses for the variables in lab2exb-parti.c: alpha 7120 beta 7112 gamma 7108 delta 7104 (Those addresses are made-up and don t resemble addresses that the variables might get with any operating system and C compiler I know of.) Redraw the diagrams for points one to five in Part I, using numbers instead of blobs and arrows to show the contents of pointer variables., Part III Download the file lab2exb-partiii.c. Repeat Part I, but this time make diagrams for points one, two and three in lab3exb-partiii.c., Part IV Download the file lab2exb-partiv.c. Repeat Part I, but this time make diagrams for points one and two in lab3exb-partiv.c. Hand in all your diagrams from Parts I through IV.

5 ENCM 335 Fall 2018 Lab 2 page 5 of 8 Exercise C: Days, hours, minutes, and seconds Download the file lab2exc.c. Read it carefully. Add code to main and write a function definition for to_dhms. Do not modify the function prototype for to_dhms the given parameter types are correct! Hand in printouts of your completed source code and terminal input/output showing several tests of the program, including inputs of seconds and seconds. Exercise D: scanf in an infinite loop (Some of the text here reviews ideas seen in lectures and/or in code you looked at in Lab 1. But there may be a few new ideas mixed in, so please read this carefully.) scanf is a Standard C Library function that can be used to read terminal input into variables. The first argument to scanf is a string constant that looks somewhat like a printf control string. The remaining arguments are addresses of variables into which scanf will put input data. To detect input error after calling scanf, a program must check the return value from scanf. Before continuing to discuss scanf, it is important to make sure you know exactly what is meant by the term return value. This is most easily explained by example. Suppose scan count and k are both variables of type int. Then in the function call scan count = scanf("%d", &k); the return value is what is copied into scan count. The function call will probably also change the value of k, but it is wrong and confusing to say that scanf returns a value into k. It is correct to say: An important side effect of calling scanf is to change the value of k. When scanf is used to read a single number, it returns one of three values: 1, 0, or EOF. EOF is a negative constant defined in <stdio.h>. The value 1 means that scanf succeeded, that it actually read a number. The value 0 means that scanf encountered characters it couldn t convert into a number. The value EOF means that scanf reached the end of an input file or that some other error occurred. Now that you know that scanf returns a value, you may wonder why a statement like scanf("%d", &k); is allowed in C. It s allowed because in C the value of an expression may be ignored in forming a statement, as in the allowed but useless statement x + y; A frequent special case of this rule is that it s allowable to ignore the return value from a function call. For instance, the return value from printf is almost always ignored. It s a bad idea to ignore the return value from scanf, because in doing so you pass up a chance to detect an input error. Here s one more thing you should know about scanf: When it fails to read a number because the first character it reads can t be used as part of a number, it leaves that character on the input stream, and the next call to scanf or to some other input function will see that same character again. As you will see in this exercise, that can cause an infinite loop.

6 ENCM 335 Fall 2018 Lab 2 page 6 of 8 Before you start, note these facts: When a program is running in a Cygwin Terminal window, typing Ctrl+C (holding down the Ctrl key while pressing the C key) will usually kill the program immediately. The same thing works in a Linux or macos terminal window. That s useful if you suspect that a program has gone into an infinite loop. Typing Ctrl+D at the beginning of an input line in a Cygwin, Linux or macos terminal window indicates end of file that there are no more characters to be supplied as input to whatever program is running. Download the files avg1.c and avg2.c. Build executable files and run them both a few times with valid input. Then run them and make deliberate typing errors type letters or punctuation where numbers are expected. Answer these questions: 1. Do the two programs behave the same way with valid input? If not, what is the difference in behaviour, and why is there a difference? 2. Do the two programs behave the same way with bad input? If not, what is the difference in behaviour, and why is there a difference? Hand in answers to questions 1 and 2 in. Exercise E: More weird scanf behaviour This exercise will not be marked. An important concept to understand about scanf (and many other C input functions) is that a C program sees input as a stream of characters, not as a sequence of lines of text. scanf will consume just enough characters to do its job, or to find out that it can t do its job. As mentioned in Exercise D, remaining characters on the input stream are not consumed instead they remain in the stream, waiting for the next input operation. Download the file use-scanf.c. Build an executable and run it several times, first with sensible input, then with various forms of bad input. What happens when you enter letters in response to the prompt for an integer? What if you type two integers on the same line? What if you type 67.89? What if you type one integer followed by letters? (You don t have to write down the answers.) Explanation Here is what happened when you typed in in response to the prompt for an integer. This is a picture of the program s input stream buffer a region of memory that holds on to characters from lines of user input:

7 ENCM 335 Fall 2018 Lab 2 page 7 of \n consumed characters first character to be read by second scanf call The picture shows the stream of input characters after the first call to scanf was complete. Here is what the first call did: Read the 6, saw that it was a digit and could be used as part of int input. Read the 7, saw that it was a digit and could be used as part of int input. Read the., saw that it was not a digit and could be not be used as part of int input. The. was put back on the input stream so that it could be read by the next input operation. Exercise F: Utility functions for numerical input This exercise will not be marked, but you are expected to copy and paste some code from this exercise into your Exercise G program. Using library functions such as fgetc, which reads a single character from an input stream, and fgets, which reads an entire line, it is possible to write code to work around some of the difficulties of using scanf to read numbers. One reasonable approach is this: Whenever an error is detected, consume an entire line of input and ask the user to try again at the beginning of a fresh input line. However, that s a lot of work, and probably not worthwhile for programs that get just a few numbers from a user. It makes more sense for the program to fail immediately if input error is detected. It s also useful to echo the input repeat it back to the user so that she or he can tell whether the program got the input that she or he intended to provide. Download the file get-numbers.c. Read the code, then build an executable and run it a few times to see if it behaves the way you expect. Then add a definition for get_double_or_die, and add code to main that will allow you to test your definition. Exercise G: Writing a complete program The program you will write here will be a simple utility for a long-distance runner. The input will be a distance and a time, and the output will be the average times taken per kilometre and per mile. Write a program that asks a user for a distance in kilometres, as a double, and a time interval, as an int and a double: a number of minutes, and a number of seconds. If the program succeeds in reading all three numbers, it should report the output specified above. Here is a sample of a successful dialogue with the program, with slanted typewriter font used to show what was typed by a user:

8 ENCM 335 Fall 2018 Lab 2 page 8 of 8 Please enter a distance in km, using type double I read a double value of Please enter a number of minute, using type int. 39 I read an int value of 39. Please enter a number of seconds, using type double I read a double value of Distance run: km. Time of run: 39 minute(s), second(s). Average time for 1 km: 3 minute(s), seconds(s). Average time for 1 mile: 6 minute(s), seconds(s). And here is a sample of an unsuccessful dialogue: Please enter a distance in km, using type double. No! You can t make me! I could not read a double. I am quitting. For the most part, you may organize your program however you see fit, except for the following, which I insist on, so that you get practice with some of the key C features used in Exercises A F: The three input numbers should be collected by a function with exactly this prototype: void get_input(double *km_in, int *minutes_in, double *seconds_in); The get_input function should make calls to get_double_or_die and get_int_or_die you can copy and paste the prototypes and definitions from Exercise F. Note: 1 mile is exactly metres. Hand in a complete listing of your program source code, and printed copy of terminal input/output showing a few tests, including: km in 179 minutes, 33.6 seconds; km in 3 minutes, 59.2 seconds; various kinds of inputs that the program can t read.