C Tutorial CSU480 Donghui Zhang Adapted from Wei Qian’s C tutorial slides.

What we do Hello World Program Data Types & Variables printf() Arithmetic & Logical Operations Conditionals Loops Arrays & Strings

Pointers Functions Command-Line Argument Data Structure Memory Allocation Programming Tips

Hello World Program The source code #include int main() { printf("Hello World\n"); return(0); }

How to compile gcc hello.c –o hello gcc is the compiling command. hello.c is the source file, and hello is compiler-generated executable file.

How to execute./hello “./ ” indicates the following file “hello” resides under the current execution directory.

Data Types & Variables Data Types char ->character variable1 bytes int->integer variable4 short ->short integer 2 long ->long integer4 float -> single precision real number 4 double ->double precision real variable 8 unsigned -> can’t be used with float and double type. unsigned variables can not represent negative values.

Variable Declaration int length = 100; char num = ‘9’; //The actual value is 57 float deposit = 240.5; unsigned short ID = 0x5544; Try the following statements, and see what happens unsigned char value = -1; printf(“The value is %d \n”, value); unsigned char value = 300; printf(“The value is %d \n”, value);

unsigned char value = -1; printf(“The value is %d \n”, value); The value is 255. unsigned char value = 300; printf(“The value is %d \n”, value); The value is 44.

Local variable Local variables are declared within the body of a function, and can only be used within that function. Static variable Another class of local variable is the static type. It is specified by the keyword static in the variable declaration. The most striking difference from a non-static local variable is, a static variable is not destroyed on exit from the function. Global variable A global variable declaration looks normal, but is located outside any of the program's functions. So it is accessible to all functions.

An example int global = 10;//global variable int func (int x) { static int stat_var; //static local variable int temp; //(normal) local variable int name[50]; //(normal) local variable …… }

printf() The printf() function can be instructed to print integers, floats and string properly. The general syntax is printf( “format”, variables); An example int stud_id = 5200; char * name = “Mike”; printf(“%s ‘s ID is %d \n”, name, stud_id);

Format Identifiers %d decimal integers %xhex integer %c character %ffloat and double number %sstring %ppointer How to specify display space for a variable printf(“The student id is %5d \n”, stud_id); The value of stud_id will occupy 5 characters space in the print-out.

Why “\n” It introduces a new line on the terminal screen. More about printf() man printf

Arithmetic Operations Arithmetic operators int i = 10; int j = 15; int add = i + j; int diff = j – i; int product = i * j; int quotient = j / i; Int residual = j % i; i++;//Increase by 1 i--;//Decrease by 1

Comparing them int i = 10; int j = 15; float k = 15.0; j / i = ? j % i = ? k / i = ? k % i = ?

The Answer j / i = 1; j % i = 5; k / i = 1.5; k % i It is illegal.

What’s wrong with the following code: int grade[2]; // A=4, B=3, C=2, D=1, F=0 int hour[2]; // credit hour int total_grade = grade[0]*hour[0] + grade[1]*hour[1]; int total_hour = hour[0] + hour[1]; float gpa = total_grade / total_hour; Always gets an integer GPA. E.g. if grades are A and B, and hours are 3 and 2. correct gpa = (4*3+3*2) / 5 = 3.6, but the above program generates gpa=3.0 How to fix? float gpa = total_grade * 1.0 / total_hour;

Logical Operations What is “true” and “false” in C In C, there is no specific data type to represent “true” and “false”. C uses value “0” to represent “false”, and uses non-zero value to stand for “true”. Logical Operators A && B=>A and B A || B=> A or B A == B=>Is A equal to B? A != B=> Is A not equal to B?

A > B=>Is A greater than B? A >= B => Is A greater than or equal to B? A Is A less than B? A Is A less than or equal to B? Don’t be confused && and || have different meanings from & and |. & and | are bitwise operators. Some practices Please compute the value of the following logical expressions?

int i = 10; int j = 15; int k = 15; int m = 0; if( i if( i != j || k if( j k) => if( j == k && m) => if(i) => if(m || j && i ) =>

int i = 10; int j = 15; int k = 15; int m = 0; if( i false if( i != j || k true if( j k) => true if( j == k && m) => false if(i) => true if(m || j && i ) => true Did you get the correct answers?

Conditionals if statement Three basic formats, if (expression){ statement … } if (expression) { statement … }else{ statement … }

if (expression) { statement … } else if (expression) { statement … } else{ statement … }

An example if(score >= 90){ a_cnt ++; }else if(score >= 80){ b_cnt++; }else if(score >= 70){ c_cnt++; }else if (score>= 60){ d_cnt++ }else{ f_cnt++ }

The switch statement switch (expression) { case item1: statement; break; case item2: statement; break; default: statement; break; }

What does Func( ‘B’ ) return? int Func( char c ) int grade; switch ( c ) { case ‘A’: grade = 4; case ‘B’: grade = 3; case ‘C’: grade = 2; } return grade; } Answer: 2! Reason: ‘case’ is a label. Jumps to a location inside switch and executes sequentially. Solution: break!

Correct version: int Func( char c ) int grade; switch ( c ) { case ‘A’: grade = 4; break; case ‘B’: grade = 3; break; case ‘C’: grade = 2; break; } return grade; } What’s wrong with the code? If c is not A or B or C, return an arbitrary integer. Solution: default!

int Func( char c ) int grade; switch ( c ) { case ‘A’: grade = 4; break; case ‘B’: grade = 3; break; case ‘C’: grade = 2; break; default: grade = 0; // or assert(false) or anything you like } return grade; }

Loops for statement for (expression1; expression2; expression3){ statement … } expression1 initializes; expression2 is the terminate test; expression3 is the modifier

An example int x; for (x=0; x<3; x++) { printf("x=%d \n",x); }

The while statement while (expression) { statement … } while loop exits only when the expression is false. An example int x = 0; while (x<3) { printf("x=%d \n",x); x++; }

Arrays & Strings Arrays int ids[50]; char name[100]; int table_of_num[30][40]; Accessing an array ids[0] = 40; i = ids[1] + j; table_of_num[3][4] = 100; Note: In C (and Java) Array subscripts start at 0 and end one less than the array size.

Arrays & Strings A common mistake: int ids[50]; ids[10] = 98; // ERROR: array out of bound!!!

Strings Strings are defined as arrays of characters. The only difference from a character array is, a symbol “\0” is used to indicate the end of a string. For example, suppose we have a character array, char name[8], and we store into it a string “Dave”. Note: the length of this string 4, but it occupies 5 bytes. Dave\0

Functions Functions are easy to use; they allow complicated programs to be broken into small blocks, each of which is easier to write, read, and maintain. How a function looks like returntype function_name(parameters … ) { localvariables declaration; functioncode; return result; }

Sample function int addition(int x, int y) { int add; add = x + y; return add; } How to make a call? int main() { int result; int i = 5, j = 6; result = addition(i, j); return 0; }

Pointers Pointer is the most beautiful part of C, but also brings most trouble to C programmers. Over 90% bugs in the C programs came from pointers. What is a pointer A pointer is a variable which contains the address in memory of another variable. In C we have a specific type for pointers. Either it is a “int*”, “char*”, “void*”, it all occupy 4 bytes. But first study memory layout...

Memory A sequential list of words, starting from 0. 32bit architecture (e.g. Win32 programming): each word is 4 bytes. A 256MB RAM has about 64 million words. Number each word using its smallest byte number. word 0 word 1 word Stack Heap

Memory Local variables are stored in stack, growing upwards. Multiple-byte variable’s address is the smallest byte number. Complete/half word req V1 V2 V3 V

What ’ s the print out? int main() { int d; char c; short s; int* p; int arr[2]; printf( “%p, %p, %p, %p, %p\n”, &d, &c, &s, &p, arr ); return 0; } // suppose &d = 920 (in practice a // 4-byte hex number such as // 0x22FC3A08) d c s p Solution: 920, 919, 916, 912, 904 arr Note: according to a bug in gcc, the address of arr will be 900 instead of 904.

Usage of pointer p = &d; *p = 10; c = (char)1; p = arr; *(p+1) = 5; p[0] = d; *( (char*)p + 1 ) = c; d c s p Note: according to a bug in gcc, the address of arr will be 900 instead of 904. arr[1] arr[0] = 920 = 10 =

Usage of pointer p = &d; *p = 10; c = (char)1; p = arr; *(p+1) = 5; // int* p; p[0] = d; *( (char*)p + 1 ) = c; Q: arr[0] = ? d c s p Note: according to a bug in gcc, the address of arr will be 900 instead of 904. arr[1] arr[0] = 904 = 10 = = 5 = 10 A: 266!

Pass pointer parameters into function void swap(int *px, int *py) { int temp; temp = *px; *px = *py; *py = temp; } int a = 5; int b = 6; swap(&a, &b); What will happen int * a; int * b; swap(a, b);

You will have a running error. If the code happens inside the kernel, you might crash the system. Note: never read/write the content of a uninitialized pointer.

Dynamic memory allocation Allows the program determine how much memory it needs at run time, and allocate exactly the right amount of storage. The region of memory where dynamic allocation and deallocation of memory can take place is called the heap. Note: the program has the responsibility to free the dynamic memory it allocated.

Functions for the dynamic memory allocation void *malloc(size_t number_of_bytes); allocates dynamic memory size_t sizeof(type); returns the number of bytes of type void free(void * p) releases dynamic memory allocation An example of dynamic memory allocation int * ids;//id arrays int num_of_ids = 40; ids = malloc( sizeof(int) * num_of_ids); …….. Processing …... free(ids);

Usage of pointer p = (int*) malloc(sizeof(int)*3); p[2] = arr[1] * 3; s = (short)( *(p+2) ); free( p ); d c s p Note: according to a bug in gcc, the address of arr will be 900 instead of 904. arr[1] arr[0] = 904 = 10 = = 5 = 266

Usage of pointer p = (int*) malloc(sizeof(int)*3); p[2] = arr[1] * 3; s = (short)( *(p+2) ); free( p ); d c s p Note: according to a bug in gcc, the address of arr will be 900 instead of 904. arr[1] arr[0] = 932 = 10 = = 5 = 266 p[2] = 15 = 15 Q: what if you say p[2]=0 afterwards? A: run time error. Q: what if you do not call free(p)? A: memory leak.

Data Structure A data structure is a collection of one or more variables, possibly of different types. An example of student record struct stud_record{ char name[50]; int id; int age; int major; …… };

A data structure is also a data type struct stud_record my_record; struct stud_record * pointer; pointer = & my_record; Accessing a field inside a data structure my_record.id = 10; or pointer->id = 10;

Allocating a data structure instance struct stud_record * pointer; pointer = malloc(sizeof(struct stud_record)); pointer->id = 10; Never calculate the size of data structure yourself. Give it to sizeof() function. Note: can use typedef, e.g. typedef struct stud_record {..... } Record;

Case Study: Linked List #include typedef struct tmpNode{ int ssn; struct tmpNode * next; } Node; void Insert( Node* head, int ssn ) { Node* node = (Node*)malloc(sizeof(Node)); node->ssn = ssn; node->next = head; head = node; } void DeleteAll( Node* head ) { while ( head != NULL ) { Node* current = head; head = head->next; free( current ); } int main() { Node* head = NULL; Insert( head, 1234 ); Insert( head, 4567 ); DeleteAll( head ); return 0; } Flawed!

Case Study: Linked List #include typedef struct tmpNode{ int ssn; struct tmpNode * next; } Node; void Insert( Node** phead, int ssn ) { Node* node = (Node*)malloc(sizeof(Node)); node->ssn = ssn; node->next = *phead; *phead = node; } void DeleteAll( Node* head ) { while ( head != NULL ) { Node* current = head; head = head->next; free( current ); } int main() { Node* head = NULL; Insert( &head, 1234 ); Insert( &head, 4567 ); DeleteAll( head ); return 0; } Correct version!

Command-Line Argument In C you can pass arguments to main() function. Main() prototype int main(int argc, char * argv[]); argc indicates the number of arguments argv is an array of input string pointers. How to pass your own arguments./hello 10

What value is argc and argv? Let’s add two printf statement to get the value of argc and argv. #include int main( int argc, char * argv[]); ) { int i=0; printf("Hello World\n"); printf( “ The argc is %d \n ”, argc); for(i=0; i < argc; i++){ printf( “ The %dth element in argv is %s\n ”, i, argv[i]); } return(0); }

The output The argc is 2 The 0th element in argv is./hello The 1th element in argv is 10 The trick is the system always passes the name of the executable file as the first argument to the main() function. How to use your argument Be careful. Your arguments to main() are always in string format. Taking the above program for example, the argv[1] is string “ 10 ”, not a number. You must convert it into a number before you can use it.

Programming Tips Replacing numbers in your code with macros #define MAX_NAME_LEN 50; char name[MAX_NAME_LEN]; Avoiding global variables Giving variables and functions a nice name Don’t repeat your code Don’t let the function body to exceed one screen

Indenting your code If(expression) { if(expression) { …… } Commenting your code Don’t rush into coding. Plan first. Printing out more debugging information Using debugger

C Vs. C++ C++ is a superset of C C++ has all the characteristics of C Object-oriented! Using g++ to compile your source code