Reading. C Programming Language. Basic syntax Whitespaces. Whitespaces (cont d) #include. Basic syntax Comments

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1 Reading C Programming Language Types, operators, expressions Control flow, functions Basic IO K&R Chapter 2 Types, Operators, and Expressions K&R Chapter 3 Control Flow K&R Chapter 7 Basic I/O NEWS Assignment 1 is posted ( Basic syntax Whitespaces Whitespaces (cont d) Space, tab, new line Separate words Ignored everywhere but in strings Comments /* */ are treated as whitespaces Equivalent int_a; int a; if(a==b) func(); if(_a_==_b_) func(_)_; if(a==b) /* case 1 */ printf( the same\n ); /*case 2 */ printf( different\n ); Not equivalent int_a; inta; if(a==b) if(a=_=b) Hello, world! Hello,world! Basic syntax Comments Begin with /*, end with */ If an occurrence of /* found, ignore everything until the first */ (therefore, cannot be nested /* /* */ */ is wrong!) Comments are treated as whitespaces Can span multiple lines Do not affect execution of the program at all Often used to temporarily remove parts of code without deleting it (commenting it out) #include #include includes contents of some file into your code as if contents were part of current file.h header file, contains declarations, C syntax #include <file.h> file is in the system include directory #include file.h file is in the current directory Some useful header files stdio.h declarations for printf(), getchar(), etc. math.h declarations for sin(), exp(), etc. string.h declarations for strlen(), etc. stdlib.h declarations for malloc(), etc. Split your code put declarations in.h, definitions in.c. 1

2 #define Syntax #define NAME VALUE NAME is replaced by VALUE everywhere (except in strings) Before #define A 0 #define B 1 #define C hello world /* my comment */ printf( %d %s,a+b, C); return 0; After printf( %d %s,0+1, hello world ); return 0; What is wrong with this program!? Try % cc E myfile.c (homework!) Variables and Constants Basic objects manipulated by programs. Declare before use as [qualifier] type var1, var2 ; Name is a combination of letters, _ (underscore), and digits must start with a letter or _ case sensitive, as pretty much everything in C and UNIX (X and x are different) Data Types Few basic data types char single byte (holds one character or a small integer) int an integer (normally 4 bytes) float single precision (normally 4 bytes) double double precision (8 bytes) Qualifiers short/long apply to int signed/unsigned apply to char, int long double: extended-precision floating point Constants 1234, 1234L, 1234UL int, long, unsigned long 12.34, 12.34f double, float 0177, 0x1F octal, hexadecimal 'x', '0', '\n', '\t', '\0 numeric value of the character in the machine s character set this is a string string constant (very different from Java!) See man -s4 limits Operators +, /, -, *, % (modulo, works only for integral types) ++, -- increment, decrement &,, ^, <<, >>, ~ bitwise and, or, exclusive or, left shift, right shift, one s complement (only with integral operands) >, >=, <, <=, ==,!=, &&, relational and logical (numeric value of true is 1, false 0; non-zero is true, 0 is false), && and evaluate left to right until the result is determined Type Conversion General rule Promote lower type to higher before operation long double double float long int int short char (for unsigned operands the rules are more complicated) char signed or unsigned? C language does not specify printable characters are always positive (ASCII )! logical NOT (converts 0 to 1 and non-zero to 0) 2

3 Type Conversion (cont d) In the assignment x = y; the value of y is converted to the type of x (dropping high order bits for ints) int i; char c; i = c; c = i; /* no information is lost */ c = i; i = c; /* may lose information */ Use cast when in doubt Examples of Type Conversion char c; int i; float f; double d; i=c+c; i=c+f; c=2.0f; d=f+f; F=d*2; Type Casting Explicit conversion, forcing a type (I know what I am doing, compiler!) Syntax (type) expression Example char x,y; (float)x (float)(x+y) vs. (float)x+y Assignment Op. & Expressions i = i+10 is equivalent to i += 10 += is called an assignment op. exp1 op= exp2 is equivalent to exp1 = (exp1) op (exp2) where exp1 and exp2 are expressions x *= y + 1 is equivalent to x = x * (y + 1) x = 2; y = 2; x *= y ; x has value 16 x = ; x >>= 2; x is x = ; x <<= 2; x is x = ; x = 0x02; x is Control Flow Specifies the order of computation C is similar to other languages, except perhaps for syntax An expression becomes a when followed by ; ; is a terminator (not a separator). The rules are tricky. A missing ; will produce lots of error messages. block: group of declarations and s within curly braces. No need for ; after if(expression) operator; if(expression) operator; operator; if(expression1) if(expression2) operator_a; operator_b; if- if(a==b) printf( true!\n ); a=a+b; (no semicolon!!!) printf( false\n ); if(a) if(1) if(a=b)!!! 3

4 Switch switch (expression) case constexp1: s case constexp2: s default: s Evaluate expression. If case matches expression value, then execution starts at that point and falls through default is executed if none of the cases matches while (expression) for (exp1; exp2; exp3) do while(expr) Loops (while and for) While expression evaluates to non-zero, execute. while(1) loops forever exp1; while(exp2) exp3; self-explanatory Precedence and Associativity Operators Associativity Comment () [] ->.! ~ * & (type) sizeof <= Unary + - * & * / % + - << >> < <= > = =!= & ^ &&?: <= exp1? exp2 : exp3 = += -= *= /= %= &= ^= = <<= >>= <=, Comma op (exp1,exp2) int x = ; int i = 1, j = 2, k = 3; i += j -= k +=2; int x = 1;! x x ~ x x x x ^ x P&A Example ( ( (2+ 3) - 1 ) + 4 ) i += ( j -= ( k+= 2)) ( 0 ) 1 1 (~ x ) x (11..10) x ( x ^ x) Another P&A Example #define PRINT(x) printf("%d\n", x) int x, y, z; x = 2; y = 1; z = 0; x = x && y z; PRINT(x!y && z); x = y = 1; z = x++ - 1; PRINT(x); PRINT(z); z += -x y; PRINT(x); PRINT(z); break and continue while ((c = getchar())!= EOF) switch( c ) case : case \n : case \t : nwhites++; break; default: others++; for (i=0; i < n; i++) if (a[i] < 0) continue; break causes innermost enclosing loop or switch to terminate continue causes next iteration of the enclosing for or while to begin. 4

5 Functions Functions are useful in several ways help break large computing tasks to smaller units allow code to be reused conveniently and efficiently hide details of operations: encapsulate operations make it easier to maintain programs Everything is a function in C, no procedures All functions are linked together via main which controls the sequence in which units are executed General guidelines Split your code into a set of simple functions! Avoid repetitions in your code. Functions (cont d) Communication between functions is by arguments, returned values, and external variables Arguments are passed by value. A C program is a set of external objects which are either variables or functions External variables are defined outside any function Functions are always external Function prototyping Declarations (what it does) and definitions (how it does it). Functions need to be declared before use Function definition can substitute declaration A declaration states type of return value and types of arguments (this is also called prototype) Examples of declarations int factorial(int); void foo(void); float bar(int a, char b); (notice the ; at the end) Function definition return-type function-name (argument declarations) Declarations and s int factorial (int n) int i, f; f = 1; for (i=2; i < n; i++) f *= i; return f; printf("%d\n", factorial(5)); Example of a function int mystery(unsigned short bits) unsigned i, mask = 1 << 15, total =0; Basic I/O getchar() read a character (from standard input) putchar( c ) print a character (to standard output) printf( ) print a formatted string for (i =1; i<=16; i++, bits <<= 1) if (bits & mask) ++total; return (total % 2) == 0? 1 : 0; (do not forget to #include <stdio.h>) Special characters EOF end of file \n new line \t tab 5

6 printf() Outputs a formatted string to standard output printf( template,arg1,arg2 ) Template is a string with conversion specifications (%). %d next argument is an int %s a string %f a float printf( this %d is %s an %f example\n,1, STUFF,3.14); this 1 is STUFF an 3.14 example #include <stdio.h> /*!!!!! */ #define IN 1 #define OUT 0 int c, nl, nw, nc, state; state = OUT; nl = nw = nc = 0; while (( c = getchar())!= EOF) ++ nc; printf("\ t% d %d %d\ n", nl, nw, nc); I/O in WC /* decision constructs */ if (c = = '\ n') ++ nl; if (c = = ' ' c = = '\ n' c = = '\ t') state = OUT; if (state = = OUT) state = IN; ++ nw; printf("\ t% d %d %d\ n", nl, nw, nc); return 0; See man fprintf for more information. Read chapters 2 and 3 of K&R. Conclusion Take a look at chapter 7 of K&R. Do your homework too! Next time more on C functions and program structure and basic testing. 6