Outline • Midterm review • More c CS3157: Advanced – Preprocessor Programming – Bitwise operations – Character handling Lecture #8 – Math/random Oct 24 • Reading: Shlomo Hershkop – k&r ch chapter 4 shlomo@cs.columbia.edu – Next class chapter 6. 1 2 Announcements Pre-processor • the C pre-processor (cpp) is a macro-processor which • Midterms graded, will be handed back – manages a collection of macro definitions today. – reads a C program and transforms it • Excellent work on the most part. • pre-processor directives start with # at beginning of line used to: – $1 patterns – include files with C code (typically, “header” files containing – Pass by reference in perl definitions; file names end with .h) – define new macros (later – not today) – Excellent suggestions – conditionally compile parts of file (later – not today) • No lab on Wednesday (10/26). Do reading • gcc -E shows output of pre-processor • can be used independently of compiler 3 4 1
Pre-processor cont. Example #define MAXVALUE 100 #define name const-expression #define check(x) ((x) < MAXVALUE) #define name (param1,param2,...) expression if (check(i)) { ...} #undef symbol • becomes if ((i) < 100) {...} • replaces name with constant or expression • textual substitution • Caution: don’t treat macros like function calls • symbolic names for global constants #define valid(x) ((x) > 0 && (x) < 20) • is called like: • in-line functions (avoid function call overhead) if (valid(x++)) {...} • type-independent code • and will become: valid(x++) -> ((x++) > 0 && (x++) < 20) • and may not do what you intended... #define MAXLEN 255 5 6 • conditional compilation • ifdef • for boolean flags, easier: • pre-processor checks value of expression • if true, outputs code segment 1, otherwise code segment 2 #ifdef name • machine or OS-dependent code code segment 1 • can be used to comment out chunks of code— bad! • (but can be helpful for quick and dirty debugging :-) #else code segment 2 • example: #define OS linux #endif ... #if OS == linux • pre-processor checks if name has been puts( “Wow you are running Linux!" ); defined, e.g.: #else puts( "why are you running something else???" ); #define USEDB #endif • if so, use code segment 1, otherwise 2 7 8 2
Function Command Line Args int main( int argc, char *argv[] ) • Declaration: – Return-type function-name (parameters if any); • Definition: • argc is the argument count – Return-type function-name (parameters if any){ • argv is the argument vector – array of strings with command-line arguments declarations • the int value is the return value statements – convention: return value of 0 means success, } – > 0 means there was some kind of error – can also declare as void (no return value) 9 10 • Name of executable followed by space- • If no arguments, simplify: separated arguments int main() { $ a.out 1 23 "third arg" printf( "hello world" ); • this is stored like this: exit( 0 ); 1. a.out } 2. 1 3. 23 4. “third arg” • Uses exit() instead of return() — almost the same thing. • argc = 4 11 12 3
booleans Booleans II • C doesn’t have booleans • This works in general, but beware: • emulate as int or char, with values 0 (false) and 1 or non- if ( n == TRUE ) { zero (true) printf( "everything is a-okay" ); • allowed by flow control statements: } if ( n == 0 ) { printf( "something wrong" ); • if n is greater than zero, it will be non-zero, but } may not be 1; so the above is NOT the � • assignment returns zero - > false • same as: • you can define your own boolean: if ( n ) { #define FALSE 0 printf( "something is rotten in the state of denmark" ); #define TRUE 1 } 13 14 Logical operators Bitwise operators • in C logical operators are the same as in Java • there are also bitwise operators in C, in which • meaning C operator each bit is an operand: • AND && • Meaning c operator • OR || • bitwise AND & • NOT ! • bitwise or | • since there are no boolean types in C, these are mainly • Example: used to connect clauses in if and while statements int a = 8; /* this is 1000 in base 2 */ • remember that int b = 15; /* this is 1111 in base 2 */ – non-zero == true 1000 ( 8 ) • a & b = a | b= 1000 ( 8 ) | – zero == false & 1111 ( 15 ) 1111 ( 15 ) = = 1000 ( 8 ) 1111 ( 15 ) 15 16 4
Question Implicit convertions • implicit: • what is the output of the following code int a = 1; fragment? char b = 97; // converts int to char int s = a + b; // adds int and char, converts to int • int a = 12, b = 7; • promotion: char -> short -> int -> float -> double • if one operand is double, the other is made double • printf( "a && b = %d\n", a && b ); • else if either is float, the other is made float • printf( "a || b = %d\n", a || b ); int a = 3; float x = 97.6; • printf( "a & b = %d\n", a & b ); double y = 145.987; y = x * y; // x becomes double; result is double • printf( "a | b = %d\n", a | b ); x = x + a; // a becomes float; result is float • real (float or double) to int truncates 17 18 explicit Example • explicit: #include <stdio.h> • type casting #include <math.h> int a = 3; int main() { float x = 97.6; int j, i, x; double y = 145.987; double f = 12.00; y = (double)x * y; for ( j=0; j<10; j++ ) { x = x + (float)a; i = f; • – using functions (in math library...) x = (int)f; printf( "f=%.2f i=%d x=%d 1. floor() – rounds to largest integer not greater than x floor(f)=%.2f ceil(f)=%.2f round(f)=%.2f\n", f,i,x,floor(f),ceil(f),round(f) ); 2. ceil() - round to smallest integer not smaller than x f += 0.10; } // end for j 3. round() – rounds up from halfway integer values } // end main() 19 20 5
Output Be aware • f=12.00 i=12 x=12 floor(f)=12.00 ceil(f)=12.00 round(f)=12.00 • almost any conversion does something— but not • f=12.10 i=12 x=12 floor(f)=12.00 ceil(f)=13.00 round(f)=12.00 necessarily what you intended!! • f=12.20 i=12 x=12 floor(f)=12.00 ceil(f)=13.00 round(f)=12.00 • – example: • f=12.30 i=12 x=12 floor(f)=12.00 ceil(f)=13.00 round(f)=12.00 • f=12.40 i=12 x=12 floor(f)=12.00 ceil(f)=13.00 round(f)=12.00 int x = 100000; • f=12.50 i=12 x=12 floor(f)=12.00 ceil(f)=13.00 round(f)=12.00 • f=12.60 i=12 x=12 floor(f)=12.00 ceil(f)=13.00 round(f)=13.00 short s = x; • f=12.70 i=12 x=12 floor(f)=12.00 ceil(f)=13.00 round(f)=13.00 printf("%d %d\n", x, s); • f=12.80 i=12 x=12 floor(f)=12.00 ceil(f)=13.00 round(f)=13.00 • f=12.90 i=12 x=12 floor(f)=12.00 ceil(f)=13.00 round(f)=13.00 • – output is: 100000 -31072 • WHY? 21 22 math library math • Functions ceil() and floor() come from the math library • some other functions from the math library (these are function prototypes): • definitions: – double sqrt( double x ); – ceil( x ): returns the smallest integer not less than x, as a double – double pow( double x, double y ); – floor( x ): returns the largest integer not greater than x, as a – double exp( double x ); double – double log( double x ); • in order to use these functions, you need to do two – double sin( double x ); things: – double cos( double x ); 1. include the prototypes (i.e., function definitions) in the • exercise: write a program that calls each of these functions source code: #include <math.h> • questions: 2. include the library (i.e., functions’ object code) at link – can you make sense of /usr/include/math.h? time: – where are the definitions of the above functions? unix$ gcc abcd.c -lm – what are other math library functions? • exercise: can you write a program that rounds a floating point? 23 24 6
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