CS3241 Tutorial

1
CS3241 Tutorial

• Michael S. Brown
• Pavel Korshunov
• Tutorial 1
• 21 Aug 2009

Tutorials Page http//www.comp.nus.edu.sg/pavelk
or/cs3241/
2
About Me

• Full Name
• Pavel Valerievich Korshunov ?
• Call me PASHA (from Russia)
• Chinese name ??
• Email pavelkor_at_gmail.com
• Website www.comp.nus.edu.sg/pavelkor
• Tutorial website www.comp.nus.edu.sg/pavelkor/cs
  3241
• Office COM1, 02-46, (TA Office)

3
Typical C-code

• include ltstdio.hgt //include standard libraries,
  stdio has printf inside
• include ltmath.hgt //math has sin(), cos(), abs()
  and other math functions
• double useless_global_variable34.5 //global
  variables
• void whatsup (int problem) //prototype of the
  function that is defined below
• int main () //this is the standard main
  function, where program starts
• int p1 //variables always defined first
• whatsup(p) //then, do other stuff logic of
  your program
• return 0
• void whatsup (int problem) //define here the
  function that we declared above
• if (problem)
• printf (Whats up? You got a problem
  dude?) //print stuff

4
Have to learn C No Choice

• Main data types you can use
• int integers
• double float numbers
• char characters
• int iam1 double iamfloating4.98 char cc1
• Arrays are like everywhere else
• int arr45, 6, 7, 2 //indexes start from 0

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Input and output

• Read from input with scanf() or getchar()
• Print with printf()
• int age100
• printf(People of Earth! I am d years old\n,
  age)
• d says where the variable age will be
  printed. d means it will be printed as integer.
  \n prints new line.
• Different ways to format printing string (google
  it)
• Printing to files is fprintf(), open files with
  fopen()

6
typedef

• Use typedef to create a new data type
• typedef int3 apples
• apples apple1 //now apple1 is array int3
• typedef is commonly used together with structures

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Structures

• typedef struct student
• char name100
• char gender
• int age
• student
• //first student is the name of structure,
  second student is the new type
• student pasha //we create pasha of type
  student
• pasha.namePasha //assign him a name
• pasha.genderM //gender, and so on
• pasha.age29

8
Type casting

• Unlike Java, not all data types are automatically
  converted to each other, hence use explicit
  typecasting
• Common examples
• int inum (int) 4.5 //after type casting,
  4.5 becomes 4 (all decimal points are dropped).
• double dd (double) 1/5 //if you do not
  typecast, 1/5 will be computed as integer
  operation with result 0. Writing (double) makes
  division a float operation with result 0.2.

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Pointers

• Analogy
• The variable is like a person
• Pointer is like a GPS location of the person

int a3int ppa
Variable a in memory
value
3
p is pointer to a
!
0010
Address of the variable in memory
0010
value
0300
Address of the pointer in memory
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Arrays and memory
memory
4 bytes each value
int arr10 arr03 arr12 arr232
Values in array
3
2
32

4
0010
0014
0018
0022
arr
0010
Addresses
0458
arr24 means 0010240018 gt at address 0018
write 4
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Arrays and memory continued
memory
4 bytes each value
int arr10 int pa paarr0
Values in array
3
2
4

7
arr
0010
0014
0018
0022
0010
pa
0458
0010
Addresses
0670
(pa2)7 means (001024)0018 gt at address
0018 write 7
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Pointers and new memory

• int parr //nothing is created in memory
• parr (int)malloc(sizeof(int)10)
• malloc allocates block of bytes in memory
• sizeof(int) number of bytes in int (its 4)
• sizeof(int)10 410 50 bytes
• malloc returns (void), which is type-casted to
  (int)
• (parr3)5 //writes 5 into 4th element of array
• free(parr) //to free the memory after use

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2D and 3D arrays

• Any array is essentially a pointer to a place in
  memory. The following define 2D arrays
• int arr33 //3x3 matrix is created in memory
• int pparr //this does not specify the size of
  the array and no actual array is created in
  memory
• pparrarr00 //pparr now points to 2D array

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2D and 3D arrays
int arr33 3,2,1, 2,6,9, 56,3,4
To make all elements 0, define arrayint
arr460
3
2
1
In memory
3
2
1
2
6
9
56

4
2
6
9
0026
0030
0034
56
3
4
0010
0014
0018
0022
arr
0010
0458
Row
Size of int
arr204 means 0010234040034 to this
address 4 is written
Total number in a row
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Random function

• rand() - integer from 0 to RAND_MAX
• But first initiate with srand(). It seeds
  random generator.
• srand( time(NULL)) //just use it this way
• Random integer between a and b
• int i rand() (b-a1) a
• Random float/double
• double fn ((double)rand()/RAND_MAX)(b-a) a

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Math functions

• You would need trigonometry functions
• include ltmath.hgt
• define PI 3.1415 //this is how you define a
  constant in C
• double half_pie sin(PI/2)
• double qt_pie cos(PI/4)
• printf(We got sinf and cosf \n, half_pie,
  qt_pie )