Real World Applications: Computing Resistance Problem –Given single-character codes for the colored bands that mark a resistor, compute its resistance in ohms. The color codes are as follows: Black 0, Brown 1, Red 2, Orange 3, Yellow 4, Green 5, Blue 6, 9 Violet 7,Gray 8, White 9. –If the integer codes of the bands are (in order) c1, c2, and c3, the resistance in ohms is R = (10*c1+c2)*10 c3

Sample Input/Output What is the resistance for a resistor with color mark ? The colored bands are coded as follows: COLOR CODE Black > B Brown > N Red > R Orange > O Yellow > Y Green > G Blue > E Violet > V Gray > A White > W Enter three codes. ERO Resistance in ohms:

Main Program #include void print_codes(void); double decode_char(char code); main() { char code1, code2, code3; double R; double c1, c2, c3; int flag; print_codes(); printf("\n\nEnter three codes. "); code1 = getchar(); code2 = getchar(); code3 = getchar(); c1 = decode_char(code1); c2 = decode_char(code2); c3 = decode_char(code3); if(c1 == || c2 == || c3 == ) printf("\n\n\tBad code - cannot compute R.\n"); else { R = (10.0*c1 + c2)*pow(10.0, c3); printf("\n\n\tResistance in ohms:\t%f\n", R); } return EXIT_SUCCESS; }

Print_codes function /* This function prints a menu of color codes to guide the user in entering input. */ void print_codes(void) { printf("\n\n\tThe colored bands" " are coded as follows:\n\n\t"); printf("COLOR\t\t\tCODE\n\t"); printf("-----\t\t\t----\n\n"); printf("\tBlack > B\n"); printf("\tBrown > N\n"); printf("\tRed > R\n"); printf("\tOrange > O\n"); printf("\tYellow > Y\n"); printf("\tGreen > G\n"); printf("\tBlue > E\n"); printf("\tViolet > V\n"); printf("\tGray > A\n"); printf("\tWhite > W\n"); }

Decode_char function /* This function expects a character (color code) and returns a double precision floating-point number as its value. If the code is not legal, it returns a value that signals that fact. */ double decode_char(char code) { switch (code) { case 'B': return 0.0; case 'N': return 1.0; case 'R': return 2.0; case 'O': return 3.0; case 'Y': return 4.0; case 'G': return 5.0; case 'E': return 6.0; case 'V': return 7.0; case 'A': return 8.0; case 'W': return 9.0; default: return ; }

The Preprocessor The C Preprocessor processes a C source file before the compiler translates the program into object code. The preprocessor follows the programmer's (preprocessing) directives. All preprocessor directives start with the character #.

The CC command When you issue the cc or gcc command, –The C preprocessor (known as cpp on our system) processes the C source file. Processing means including the files in #include, replacing macro definition by its replacement text, etc. –C compiler (known as cc1 on our system) compiles the resulting file into object file. –The loader (known as ld) finally uses the object file to produce executable program.

#include... The preprocessor directive #include includes a copy of this file at the point of this command. The file is located at "standard" locations. #include "mydefs.h" includes the file mydefs.h from the working directory.

#define... #define EOF (-1) associates EOF with (-1) : every occurrence of EOF is replaced by the text (-1) by the preprocessor. The defined identity EOF is called a macro. Use this form of macro if a constant is used frequently in a program. Macro definition can only be logically one line. Use backslash to continue a line. E.g., #define EOF \ (-1)

Some macro definitions in Math.h #define M_E #define M_PI #define M_SQRT In your program, instead of using numerical numbers, you can use the symbolic names for the constants. E.g. X = sin(M_PI/4); y = pow(M_E, x);

Parameterized Macros #define SQR(x) ((x)*(x)) After this definition, the preprocessor replaces every occurrence of the form SQR(a) by the replacement text ((a)*(a)) For example: w = SQR(x) + 1; z = SQR(w+1); becomes w = ((x)*(x)) + 1; z = ((w+1)*(w+1));

Putting as many parentheses as possible in macros #define SQR(x) x*x The preprocessor replaces every occurrence of the form SQR(a) by the replacement text a*a For example: w = SQR(x) + 1; z = SQR(w+1); becomes w = x*x + 1; z = w+1*w+1; This is 2w+1, not (w+1) 2 as intended.

Parameterized Macros #define PRINT3(e1,e2,e3) \ printf("\n%c\t%c\t%d", \ (e1), (e2), (e3) ) After this definition, the preprocessor replaces every occurrence of the form PRINT3(a,b,c) by the replacement text printf("\n%c\t%c\t%d", (a), (b), (c) )

Macros Versus Functions #define min(x,y) \ ( ( (x)<(y) )?(x):(y) ) After this definition, the preprocessor replaces every occurrence of the form min(a,b) with the replacement text ( ( (a)<(b) )?(a):(b) ) where a and b can be ANY text. Macros are not functions. There are no arguments passing, or return value to talk about.

Conditional, Miscellaneous #undef MC cancels the definition made to the symbol MC. #define MC makes MC defined (not to any particular replacement text). #ifdef MC #define ROWS 1000 #define COLS 1000 #endif defines ROWS and COLS only if MC is defined.

Conditional, Miscellaneous #ifndef MC #define ROWS 1000 #define COLS 1000 #endif defines ROWS and COLS only if MC is not defined. #if defined(MC) #define ROWS 10 #elif defined(MM) #define ROWS 20 #else #define ROWS 0 #endif

Reading/Home Working Read Chapter 5, page 172 to 190. Work on Problems –Section 5.5, page 176, exercise 1, 3. –Section 5.6, page 190, exercise 1, 3, 7, 9, 15. Check your answers in the back of the textbook. Do not hand in.