C Tutorial

1
C Tutorial

• CSU480
• Donghui Zhang

Adapted from Wei Qians C tutorial slides.
2
What we do

• Hello World Program
• Data Types Variables
• printf()
• Arithmetic Logical Operations
• Conditionals
• Loops
• Arrays Strings

3

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

4
Hello World Program

• The source code
• include
• int main()
• printf("Hello World\n")
• return(0)

5

• 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.
  

6

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

7
Data Types Variables

• Data Types
• char - character variable 1 bytes
• int - integer variable 4
• short - short integer 2
• long - long integer 4
• float - single precision real number 4
• double - double precision real variable 8
• unsigned - cant be used with float and double
  type.
  
• unsigned variables can not represent
• negative values.

8

• 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)

9

• 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.

10

• 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.
  

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• An example
• int global 10 //global variable
• int func (int x)
• static int stat_var //static local variable
• int temp //(normal) local variable
• int name50 //(normal) local variable

12
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)

13

• Format Identifiers
• d decimal integers
• x hex integer
• c character
• f float and double number
• s string
• p pointer
• 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.

14

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

15
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

16

• Comparing them
• int i 10
• int j 15
• float k 15.0
• j / i ?
• j i ?
• k / i ?
• k i ?

17

• The Answer
• j / i 1
• j i 5
• k / i 1.5
• k i It is illegal.

18

• Whats wrong with the following code

int grade2 // A4, B3, C2, D1, F0
int hour2 // credit hour
int total_grade grade0hour0
grade1hour1 int total_hour hour0 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 (4332) / 5 3.6,
• but the above program generates gpa3.0
• How to fix?

float gpa total_grade 1.0 / total_hour
19
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?

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• 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?
  
• Dont be confused
• and have different meanings from and .
• and are bitwise operators.
• Some practices
• Please compute the value of the following
  logical expressions?

21

• 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 )

22

• 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?

23
Conditionals

• if statement
• Three basic formats,
• if (expression)
• statement
• if (expression)
• statement
• else
• statement

24

• if (expression)
• statement
• else if (expression)
• statement
• else
• statement

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• 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

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• The switch statement
• switch (expression) 
• case item1
• statement
• break
• case item2
• statement
• break
• default
• statement
• break

27

• 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!

28

• 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

• Whats wrong with the code?

• If c is not A or B or C, return an arbitrary
  integer.
  

• Solution default!

29

• 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

30
Loops

• for statement
• for (expression1 expression2 expression3)
• statement
• expression1 initializes
• expression2 is the terminate test
• expression3 is the modifier

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• An example
• int x
• for (x0 x
• printf("xd \n",x)

32

• The while statement
• while (expression)
• statement
• while loop exits only when the expression is
  false.
  
• An example
• int x 0
• while (x
• printf("xd \n",x)
• x

33
Arrays Strings

• Arrays
• int ids50
• char name100
• int table_of_num3040
• Accessing an array
• ids0 40
• i ids1 j
• table_of_num34 100
• Note In C (and Java) Array subscripts start at
  0 and end one less than the array size.

34
Arrays Strings

• A common mistake
• int ids50
• ids10 98 // ERROR array out of bound!!!

35

• 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 name8, and we store into it a string
  Dave.
  
• Note the length of this string 4, but it
  occupies 5 bytes.

36
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  

37

• 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

38
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...

39
Memory
word 0
word 1
word 2

• 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.

Stack
Heap
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Memory
0
1
2
3

• Local variables are stored in stack, growing
  upwards.
  
• Multiple-byte variables address is the smallest
  byte number.
  
• Complete/half word req.

900
904
908
V4
912
V3
V2
916
V1
920
924
928
932
936
940
944
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Whats the print out?

• int main()
• int d
• char c
• short s
• int p
• int arr2
• 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)

0
1
2
3
900
904
arr
908
912
p
c
s
916
d
920
924
928
932
936
940
944
Solution 920, 919, 916, 912, 904
Note according to a bug in gcc, the address of
arr will be 900 instead of 904.
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Usage of pointer

• p d
• p 10
• c (char)1
• p arr
• (p1) 5
• p0 d
• ( (char)p 1 ) c

0
1
2
3
900
904
arr0
908
arr1
920
912
p
1
c
s
916
10
d
920
924
928
932
936
940
944
Note according to a bug in gcc, the address of
arr will be 900 instead of 904.
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Usage of pointer

• p d
• p 10
• c (char)1
• p arr
• (p1) 5 // int p
• p0 d
• ( (char)p 1 ) c
• Q arr0 ?

0
1
2
3
900
904
arr0
10
908
5
arr1
904
912
p
1
c
s
916
10
d
920
924
928
932
936
940
A 266!
944
Note according to a bug in gcc, the address of
arr will be 900 instead of 904.
44

• 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)

45

• 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.

46

• 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.
  

47

• 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)

48
Usage of pointer

• p (int) malloc(sizeof(int)3)
• p2 arr1 3
• s (short)( (p2) )
• free( p )

0
1
2
3
900
904
arr0
266
908
5
arr1
904
912
p
1
c
s
916
10
d
920
924
928
932
936
940
944
Note according to a bug in gcc, the address of
arr will be 900 instead of 904.
49
Usage of pointer

• p (int) malloc(sizeof(int)3)
• p2 arr1 3
• s (short)( (p2) )
• free( p )

0
1
2
3
900
904
arr0
266
908
5
arr1
932
912
p
1
15
c
s
916
Q what if you say p20 afterwards?
10
d
920
924
A run time error.
928
Q what if you do not call free(p)?
932
936
A memory leak.
p2 15
940
944
Note according to a bug in gcc, the address of
arr will be 900 instead of 904.
50
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 name50
• int id
• int age
• int major

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• 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

52

• 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

53
Case Study Linked List
void DeleteAll( Node head )
while ( head ! NULL ) Node current
head head head-next free( curr
ent ) int main() Node head NU
LL Insert( head, 1234 ) Insert( head, 45
67 ) DeleteAll( head ) return 0

• 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

Flawed!
54
Case Study Linked List
void DeleteAll( Node head )
while ( head ! NULL ) Node current
head head head-next free( curr
ent ) int main() Node head NU
LL Insert( head, 1234 ) Insert( head,
4567 ) DeleteAll( head ) return 0

• 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

Correct version!
55
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

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• What value is argc and argv?
• Lets add two printf statement to get the value
  of argc and argv.
  
• include
• int main(int argc, char argv))
• int i0
• printf("Hello World\n")
• printf(The argc is d \n, argc)
• for(i0 i
• printf(The dth element in argv is s\n,
  i, argvi)
  
• return(0)

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• 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
  argv1 is string 10, not a number. You must
  convert it into a number before you can use it.

58
Programming Tips

• Replacing numbers in your code with macros
• define MAX_NAME_LEN 50
• char nameMAX_NAME_LEN
• Avoiding global variables
• Giving variables and functions a nice name
• Dont repeat your code
• Dont let the function body to exceed one screen

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• Indenting your code
• If(expression)
• if(expression)
• Commenting your code
• Dont rush into coding. Plan first.
• Printing out more debugging information
• Using debugger

60
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