Variables in C

1. Variables in C Topics • Naming Variables • Declaring Variables • Using Variables • The Assignment Statement Reading • Sections 2.3 - 2.4

2. Preprocessor • All preprocessor directives (preprocessor commands) begin with “# ”. • The preprocessor reads and may change source in RAM before a program is compiled. But the source code as stored on disk is not modified. Possible actions: • Include other files to be included in your C code. • Performs various text replacements. • Strips comments and whitespace from the code. • For example: • #include <stdio.h> The preprocessor inserts the contents of the stdio.h at this place in your code. “<…>” means the file is located in the standard include file directory. • #define PI 3.14159The preprocessor replace every occurrence of PI with 3.14159 before it passes the file to the compiler.

3. Output Data on Screen • Use printf statement – a function call • To prompt user that the program is waiting for input data. Printf(“Enter number of exams: \n”); • Arguments to the function (format string and variables) are listed inside parentheses “()”. • Multiple arguments are separated by commas: “,”. • “\n” denotes a newline. • To output results to user on the screen. Printf(“Your grade is %d\n”, grade);

4. Input Data into Variables • The scanf function can be used to obtain input data from a user (usually from keyboard) or input file. • scanf ignores leading whitespace, i.e. • Tabs • Spaces • <Enter> or <Return> key • scanf (“%d”, &numExams); • Take user’s input and put it in the memory location of numExams. • First argument: Format control string: type of input data • %d decimal integer • %f floating number • %s string • The second argument: the memory location (address) of numExams. The “&” symbol is the address operator.

5. Naming Variables • Variable names in C • May only consist of letters, digits, and underscores. • May be as long as you like, but only the first 31 characters are significant • May not begin with a number. • May only begin with letters or underscores. • May not be a C reserved word (keyword).

6. Reserved Words (Keywords) in C • auto break • case char • const continue • default do • double else • enum extern • float for • goto if • int long • register return • short signed • sizeof static • struct switch • typedef union • unsigned void • volatile while

7. Naming Conventions • C programmers generally agree on the following conventions for naming variables. • Begin variable names with lowercase letters • Use meaningful identifiers • Separate “words” within identifiers with underscores or mixed upper and lower case. • Examples: surfaceArea surface_Area surface_area • Be consistent!

8. Naming Conventions (con’t) • Use all uppercase for symbolic constants (used in #define preprocessor directives). • Examples: • #define PI 3.14159 • #define AGE 18

9. Case Sensitivity • C is case sensitive • It matters whether an identifier, such as a variable name, is uppercase or lowercase. • Example: area Area AREA ArEa are all seen as different variables by the compiler.

10. Which Are Not Legal Identifiers? • AREA area_under_the_curve • 3D num45 • Last-Chance #values • x_yt3 pi • num$ %done • lucky***

11. Declaring Variables • Before using a variable, you must give the compiler some information about the variable; i.e., you must declare it. • The declaration statement includes the data type of the variable followed by variable name. • Examples of variable declarations: • int width ; • float area ; • Declarations must be placed at the beginning of a function before any executable statements.

12. numExams 2 FE07 Declaring Variables (con’t) • Variables in C correspond to locations (address) in the computer’s memory. • When we declare a variable • Space is set aside in memory to hold a value of the specified data type. • That space is associated with the variable name. • That space is associated with a unique address. • Every variable has a name, type, and value. • For example, • int numExams; • numExams = 2;

13. More About Variables C has three basic predefined data types: • Integers (whole numbers) • int, long int, short int, unsigned int • Floating point (real numbers) • float, double • Characters • char • At this point, you need only be concerned with the data types that are bolded.

14. Integer Variable int, long int, short int, unsigned int • Int is machine-dependent, can be positive, 0, or negative integer number. • Each compiler is free to choose appropriate size for its own hardware, provided size of short <= size of int <= size of long. Typically short is 16 bits, long is 32 bits, so int is either 16 or 32 bits. • Unsigned int is either 0 or positive integer.

15. Using Variables: Initialization • Variables may be be given initial values, or initialized, when declared. Examples: int length = 7 ; float diameter = 5.9 ; char initial = ‘A’ ; length 7 diameter 5.9 initial ‘A’

16. Using Variables: Initialization (con’t) • Do not “hide” the initialization • put initialized variables on a separate line • a comment is always a good idea • Example: int height ; /* rectangle height */ int width = 2 ; /* rectangle width */ int area ; /* rectangle area */ NOT int height, width = 2, area ;

17. Using Variables: Assignment • Variables may have values assigned to them through the use of an assignment statement. • Such a statement uses the assignment operator “=”. • This operator does not denote equality. It assigns the value of the righthand side of the statement (the expression) to the variable on the lefthand side. • Examples: width = 2; area = length * width ; Note that only single variables may appear on the lefthand side of the assignment operator.

18. Example: Declarations and Assignments length • #include <stdio.h> • int main( ) • { • int length, width, area ; • width = 2 ; • length = 4 * width ; • area = length * width ; garbage width garbage area garbage width 2 length 8 area 16

19. Example: Declarations and Assignments (cont’d) • printf (“The dimension of a rectangle: ”) ; • printf (“ width = %d ”, width) ; • printf (“ length = %d \n”, length) ; • printf (“The area of a rectangle = %d. \n”, area) ; • return 0 ; • }

20. Enhancing Our Example • What if the width were really 2.5? Our program, as it is, couldn’t handle it. • Unlike integers, floating point numbers can contain decimal portions. So, let’s use floating point, rather than integer. • Let’s also ask the user to enter the value of width, rather than “hard-coding” it in.

21. Enhanced Program #include <stdio.h> int main ( ) { float length, width, area ; printf (“Enter length of a rectangle : ”) ; scanf (“%f”, &length) ; printf (“Enter width of a rectangle : ”) ; scanf (“%f”, &width) ; area = length * width ; • printf (“The dimension of a rectangle: \n”) ; • printf (“ width = %d \n”, width) ; • printf (“ length = %d \n”, length) ; printf (“The area of a rectangle = %f. \n”, area) ; return 0 ; }

22. Final “Clean” Program #include <stdio.h> int main( ) { /* Variable Declarations */ float length ; /* length of a rectangle in decimal */ float width ; /* width of a rectangle in decimal */ float area ; /* area of a rectangle in decimal */ /* Get the dimension of a rectangle from the user */ printf (“Enter width of a rectangle : ”) ; scanf (“%f”, &width) ; printf (“Enter length of a rectangle : ”) ; scanf (“%f”, &length) ; /* Compute area of a rectangle */ area = length * width ;

23. Final “Clean” Program (con’t) /* Display the results */ printf (“The dimension of a rectangle is \n”) ; • printf (“ width = %f \n”, width) ; • printf (“ length = %f \n”, length) ; • printf (“The area of a rectangle = %f \n”, area) ; return 0 ; }

24. Sample Run (con’t) Sample output run: • Create a source code product.c:xemacs product.c • Compile source code product.c:gcc –ansi –Wall product.c • Run and test product.c:a.out • Sample screen dump: Enter width of a rectangle : 3.2 Enter length of a rectangle :5.4 The dimension of a rectangle is width = 3.200000 • length = 5.400000 • The area of a rectangle = 17.280001

25. Good Programming Practices • Place each variable declaration on its own line with a descriptive comment. • Place a comment before each logical “chunk” of code describing what it does. • Do not place a comment on the same line as code (with the exception of variable declarations). • Use spaces around all arithmetic and assignment operators. • Use blank lines to enhance readability.

26. Good Programming Practices (con’t) • Place a blank line between the last variable declaration and the first executable statement of the program. • Indent the body of the program 3 to 4 spaces -- be consistent!

27. Assignment and Next • Read 2.3 – 2.4, and 2.5 (optional) Next: • Arithmetic Operators