1. 1 Lecture 2  Input-Process-Output  The Hello-world program  A Feet-to-inches program  Variables, expressions, assignments & initialization  printf() and scanf()  #include  Readings: Chapter 1 Section 2 to 6

2. 2 The traditional first C program /* The traditional first program in honor of Dennis Ritchie who invented C at Bell Labs in 1972 */ #include int main() { printf("Hello, world!\n"); return 0; }

3. 3 Program Structure  Comments begin with /* and end with */  anything between the delimiters is ignored  “ #include …” - called include directives, tells the compiler to include the header file into the program  Every program must have a main function where program execution begins; inside the main function is a sequence of statements specifying the actions to be done.

4. 4 Some Syntax Rules  C is case-sensitive  Thus, “ main ” is not the same as “ Main ”  All keywords used in C are in lower-case  Syntax of preprocessing directives:  # must start at leftmost margin  no space after  no semi-colon at the end, each include directive must be on its own line

5. 5 Some Syntax Rules (cont’d)  Syntax of Statements:  most statements are ended with semicolons  spacing is not important  programmer can put in suitable spacing and indentation to increase readability

6. 6 The printf() function  printf() is a library function  It resides in the library with header file stdio.h  It outputs data to the screen  “Hello, world!\n” is the string to be printed; ‘\n’ represents a newline character  non-printable characters are preceded by \ in C

7. 7 The return statement  return signifies the end of a function  The number after the keyword return is sent back as the return value of the function main()  A return value of 0 usually means that the execution of the function is successful

8. 8 A Feet-to-inches program /* To convert length in feet to inches */ #include int main(void) { int inches, feet; scanf(“%d”, &feet); inches = 12 * feet; printf(“%d feet is equivalent to %d inches”, feet, inches); return 0; }

9. 9 Program dissection 1  int inches, feet;  Define inches and feet as integer variables  Variables must be defined before they are referred  scanf(“%d”, &feet);  The statement causes the computer to input an integer from keyboard and store it in variable feet  The first string is called a control string / formatter string  The formatter %d indicates that an integer is expected

10. 10 Program dissection 2  inches = 12 * feet;  It is NOT an equality statement  It is an assignment statement, which assigns the value of expression 12 * feet to the variable inches  The asterisk “ * ” stands for the multiplication operation  printf(“%d feet is equivalent to %d inches”, feet, inches);  The first string is a control or formatter string  The formatter %d in the control string causes the two parameters after the string to be printed as an integer

11. 11 Layout of a simple C program /* comments */ #include int main() { statement-1 statement-2 statement-3... return 0; }

12. 12 Variables  To store data (e.g. user input, intermediate result)  Implemented as memory bytes  Each variable must have a name and a type

13. 13 Variable Names  A variable name consists of a sequence of letters, digits and underscores, but must not begin with a digit  As a convention, user-defined variables always start with a lower-case letter  Certain reserved words (keywords) are used by C and cannot be used as variable names, e.g. int, return, etc.

14. 14 Data Types  There are 3 major data types in C:  int e.g. 12  float e.g. 2.54  char e.g. ‘A’, ‘e’, ‘#’, ‘ ’  Data of different types are stored using different encoding schemes and require different number of memory bytes  Exact schemes differ from system to system

15. 15 Typical Encoding Schemes  int –2’s complement –4 bytes (32 bits) –[-2 31, 2 31 -1] = [-2147483648, 2147483647]  float –Floating point representation –4 bytes –  3.4 x 10  38 (7 decimal places of accuracy)  char –ASCII –1 byte

16. 16 Variable Definitions  All variables should be defined (and initialized) before they are referenced  The purpose is to tell the computer the names and types of the variables so that  sufficient memory bytes are allocated for the variables; and  content in these memory bytes are interpreted appropriately.

17. 17 Expression  An expression is a meaningful combination of constants, variables and operators (e.g., +, -, *, / )  Constants are the simplest expressions, e.g., 7, ‘A’ and “Hello World\n”  Some operators only work on certain types of variables, e.g., ‘A’ * ‘B’ doesn ’ t make much sense

18. 18 Assignment  A variable is assigned (given) a value using the assignment operator “ = ”  An assignment expression consists of an =, a variable on its left and an expression on its right. E.g. inch = 12*feet  An assignment expression followed by a semi-colon is called an assignment statement. E.g. inch = 12*feet;  Some illegal assignment statements: a + b = c; /* illegal */ 12 = a; /* illegal */

19. 19 Variable Initialization  When variables are defined, they may also be initialized, e.g., char grade = ‘A’; int k = 10;  Only fathoms is initialized to 7 below int inches, feet, fathoms = 7;  Both inches and fathoms are initialized below int inches = 8, feet, fathoms = 7;

20. 20 The use of printf()  The first argument of printf() is a control string which may contain k formatters for the second to (k+1)-st arguments of printf(), e.g., printf(“%s attained %d courses\nAverage” “ mark is %f\nOverall grade is %c\n”, “Jimmy Liu”, 4, 66.5, ‘B’); The output is Jimmy Liu attained 4 courses Average mark is 66.500000 Overall grade is B

21. 21 The use of printf() ( cont’d )  printf() conversion characters (see p.16 of [Kelly & Pohl 2001]) cas a character das a decimal integer eas a floating point number in scientific notation ( float or double ) f as a floating point number ( float or double ) gin the e-format or f-format, whichever is shorter ( float or double ) sas a string

22. 22 The use of printf() ( cont’d )  The field width and the precision (for floating point number only) of an output data can be controlled, e.g., %5.2f means the field width is 5 (including the decimal point) with 2 decimal places  What will happen when the field width is too long or too short for the output data?

23. 23 The use of scanf()  scanf() is to read input from keyboard  Like printf(), the first argument is a control string, which is followed by an arbitrary number of arguments (and those arguments are addresses of data items that store the input)  Address operator is represented by an ampersand &, e.g., int feet; scanf(“%d”, &feet);

24. 24 The use of scanf() ( cont’d )  scanf() conversion characters (see p.18 of [Kelly & Pohl 2001]) cas a character das a decimal integer f as a floating point number (float) lf as a floating point number (double) Lf as a floating point number (long double) sas a string

25. 25 The use of scanf() ( cont’d )  When reading in numbers, scanf() will skip white space (blanks, newlines and tabs)  scanf() ends when  the input does not match the corresponding formatter in the control string  an end-of-file signal (EOF) is detected  Keyboard inputs are stored in keyboard buffer and they are “ consumed ” by scanf() ; excessive data inputs will be left in the buffer for subsequent scanf() statements