COMP2017 / COMP9017 Tutorial 1 Introduction to C
Welcome to C!
Welcome to a fresh new semester, tutorial and programming language. From previous semesters you should be familiar with command line programs such as ls and have written some bash scripts.
In addition to bash scripts, you may have noticed a number of compiled files as you’ve been rummag- ing around. These can be identified as unreadable messes of poorly rendered symbols. You can see one by running cat on just about any file in /bin, for example cat /bin/grep.
This mess of symbols is a series of assembly instructions that indicate a number of operations that a processor is to perform. Obviously writing this directly is a painful and time consuming exercise, so programming languages have been developed that ‘compile’ down to this assembly.
As is often the starting point for learning a new language, the syntax for hello world in C is as follows:
#include
int main() {
printf(“Hello World!\n”); return 0;
We can also see that the function is named ‘main’. This is a special function name in C such that when the code is executed, this will be the entry point to our program. The printf function is one of many standard output functions in C which will send a text stream to standard output buffer, allowing it to interface with the rest of linux.
While it is nice to have this sitting around in a file, in order to run this code we need to compile it. There are two major (one might even say competing) C compilers: gcc and clang. The name of the compiled file can be specified using the -o flag as follows.
gcc -o hello_world hello_world.c
#Or you can use clang
clang -o hello_world hello_world.c
Then you can simply execute the compiled code.
./hello_world
Try writing, compiling and running hello world in c. What does the compiled file look like? 1
Homework (Attempt before tutorial)
Question 1: Hello From C
YouaretowriteasimpleCprogramthatwilloutputHello World!totheterminal. $ ./hello
Hello World!
Question 2: Meet and Greet!
Like in previous semesters, you should be familiar with being able to query the user for data via scanner.nextLine() or input() and retrieve arguments from the command line.
Write a program that will ask for the user’s name and output their name mixed with a command line argument given.
$ ./greet Yo!
What is your name? David
For help on using the functions necessary to complete the question, you can retrieve information about thefollowingfunctionsscanfandfgetsbyusingman 3 scanfandman 3 fgets.
Question 3: Repeat
Write a program that reads characters from stdin and outputs them to stdout. For example:
$ ./repeat
Hello there
Hello there
Question 4: Echo 419
Write a program that outputs the arguments that are given to it. For example:
$ ./echo Echo 419 to Cortana, come in
Echo 419 to Cortana, come in
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The C Programming language is a statically typed language, therefore any variable must have a type associated with it. You can declare variables with following types and more:
• char and unsigned char
• int and unsigned int
• short and unsigned short • long and unsigned long
However, each type is defined by the cpu architecture, platform and compiler. When writing portable C code you may need to consider the differences of the primitive types between platform targets. C exposes unsigned types which can be prepended to integer types to eliminate the usage of the sign bit as part of the type encoding.
C provides access to memory allocations using array and pointer types. To declare an array in C you need to specify the size. C does support variable length arrays with the C99 standard however, but it typically considered bad practice.
int array_1[10];
int array_2[] = { 1, 2, 3, 4 };
int array_3[5] = {1, 2, 3}; // Will still be 5 elements
Your code should not depend on this feature your compiler will typically warn you if it has been mis- used. You will be able to dynamically allocate memory using functions such as malloc and navigating them through a pointer type.
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In-class Questions
Question 5: C Declarations and Initialisations
As a group, answer the following questions and discuss with your tutor about the following: 1. What are the differences between these declarations and initialisations?
const char * ptr = “hello”;
const char array[] = “hello”;
const char array2[] = { ‘h’, ‘e’, ‘l’, ‘l’, ‘o’ };
const char array3[] = { ‘h’, ‘e’, ‘l’, ‘l’, ‘o’, ‘\0’ }; const char array4[5] = { ‘h’, ‘e’, ‘l’, ‘l’, ‘o’ }; const char array5[6] = { ‘h’, ‘e’, ‘l’, ‘l’, ‘o’, 0 }; const char array6[20] = { ‘h’, ‘e’, ‘l’, ‘l’, ‘o’ }; const char array7[20] = { 0 };
const char array8[20] = “hello”;
2. Given the code above, what does the following output?
printf(“%zu %zu\n”, sizeof(ptr), sizeof(array)); printf(“%zu %zu\n”, sizeof(array2), sizeof(array3));
printf(“%zu %zu\n”, sizeof(*ptr), sizeof(&array)); printf(“%zu %zu\n”, sizeof(&array2), sizeof(&array3));
3. What does the following output, given that sizeof(int) is 4?
int x[] = { 1, 2, 3 };
int * p1 = x;
int * p2 = x + 1;
printf(“%zu %zu\n”, sizeof(x[0]), sizeof(x)); printf(“p1 value, p2 value: %d %d\n”, *p1, *p2); printf(“p1 value with offset: %d\n”, *(p1 + 1)); printf(“p2 value with offset: %d\n”, *(p2 – 1)); printf(“p1 value plus scalar: %d\n”, (*p1) + 2); printf(“p1 plus offset followed: %d\n”, *(p1 + 2) printf(“p1 plus offset followed: %d\n”, p1[2]);
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COMP2017 / COMP9017 Introduction to C
Question 6: Array and Pointer equivalence
The array and pointer type holds an address as its value, a common operation on a array and pointer types are dereferencing operations (*) which allows retrieval of the value stored at the address.
We are able to retrieve the address from a value type (as well as array and pointer types) by using the address operator (&). Supplying an integer value to the address, you can navigate the array or pointer using integer arithmetic, referencing and dereferencing operations.
Given these pointer statements, can you provide an equivalent statement?
*(p+10) = &r[20] = &(g[0]) = &*p =
p++ = &((r[5])[5]) =
What is the difference between pointers and arrays? Can we mix notation?
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Code Help, Add WeChat: cstutorcs
As a programmer you are able to modularise your code into functions. Similar to variables, functions have a type associated as part of the function signature.
The return type of a function requires a value of the same type to be passed back to the function caller. Only void functions do not return any value.
int add_two(int a, int b) { return a + b;
Question 7: Swap
Implement the swap function, your function should swap the values of a and b respectively.
void swap(int *a, int *b) { //
int main(void) { int a = 2; int b = 3;
swap(?, ?); //Specify the variables to swap printf(“%d %d\n”, a, b); // should print 3 2
return 0; }
Doesyourprogramexecutedifferentlyifyouchangeswaptypeparameterstoint a[]?
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Github
Question 8: atoi
Implement your own atoi function that converts a given string to an integer.
int atoi(const char s[]) { //
int main(void) {
printf(“%d\n”, atoi(“”)); printf(“%d\n”, atoi(“0”)); printf(“%d\n”, atoi(“0123”)); printf(“%d\n”, atoi(“1234”)); printf(“%d\n”, atoi(“-1234”));
return 0; }
Question 9: Reverse
Write a program that reverses every line of input, you may assume none of the input lines are longer than 100 characters. Use the fgets function instead of scanf for this exercise because scanf does not distinguish between spaces, newlines and tab characters. Refer to the isspace function for more information.
Sample input:
lorem ipsum
dolor sit amet
Sample output:
muspi merol
tema tis rolod
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Question 10: String Compare
Write a string comparison function that will compare the contents of two strings up to a certain amount. You can use the following function declaration as a starting point.
int string_compare(const char* w1, const char* w2, unsigned int len);
Question 11: Word search
Given a file, output the lines where the word exists. Use the fgets function to extract each line and use the previously constructed string_compare function. The file’s content will be passed via standard input.
$ ./wordsearch Sushi < restaurants.txt
Found: 157 Redfern St, Sushi Topia
Found: 425 Cleveland St, Sushi Suma
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