C is readable... :-) What does this function do? void what(char *s, char *t) { while (*s++ = *t++) ; } How about this function? int why(char *s, char *t) { while(*s && *s == *t) s++, t++; return *s == *t;

printf( ) ... super polymorphism? printf( ) is a function, written in C. The definition must include a list of its parameters. So, how can we write... printf("hi there"); // 1 parameter printf("n=%d", 4); // 2 parameters, one int printf("x=%f", 0.5); // 2 parameters, one float printf("%s %d %f","hi",n,x); // 4 parameters! printf("%s%s%d%d%f","2","3",4,5,6.0); // 6 !! printf( ) accepts an infinite variety of argument lists. How???

Variable Parameters C allows variable number of parameters using "var args", also called "stdarg". #include <stdarg.h> int sum(int num, ...) /* "..." indicates var args */ { va_list ap; int d, sum = 0; /* second value (num) is the name of the param before start of variable parameters */ va_start(ap, num); /* initialize */ for(int k=0; k < num; k++) { /* read next arg. Expect an "int" */ d = va_arg(ap, int); sum += d; } va_end(ap); /* end va_arg processing */ return sum;

The Problem with C's var args: C variable parameters are not type-safe. The compiler does not check that actual arguments are of the correct type. If you use the wrong data, you get a run-time error: dynamic semantic or logic error, your choice :-) /* using sum from the previous slide */ int sum1, sum2, sum3; /* this is correct! */ sum1 = sum(4, 3, 12, 88, -45); // add 4 integers /* wrong number of arguments! */ sum2 = sum(4, 3, 12, 88); // crash! /* wrong data type of arguments! */ sum3 = sum(4, 3, 1.2, 88, "help"); // crash!

Java printf( )... super polymorphism? The Java Format class has a format( ) method; PrintStream (System.out) has a printf( ) method. Both of these methods accept a String argument followed by any number of other arguments! Is this more super polymorphism? System.out.printf("hi there %s", you.toString() ); System.out.printf("%s is %s", "now", new Date()); // works with primitive types, too! int a = 5, b = 12; System.out.printf("%d + %d = %d", a, b, a+b); printf("x=%f", 0.5); // 2 parameters, one float

Java also has variable arguments Java 5.0 adopts syntax similar to C, but more strict: printf(String format, Object ... arg ); defines arg as an array of Object. /** Define a method that can sum any number * of integer arguments. */ public static int sum(int num, int ... x) { int total = 0; if ( x.length < num ) num = x.length; for( int k=0; k<num; k++ ) total += x[k]; return total; } the variable length argument must be the last param. all arguments must be the same or compatible type.

Java variable argument is safer The compiler will check that all arguments match the parameter data type. Since the formal parameter is an array, the method can verify the number of actual arguments. /* using sum from the previous slide */ int sum1, sum2, sum3; /* this is correct! */ sum1 = sum(4, 3, 12, 88, -45); /* wrong number of arguments! */ sum2 = sum(4, 3, 12, 88); // no problem /* wrong data type of arguments! */ sum3 = sum(4, 3, 1.2, 88, "help"); // Compile-time error

Safe string input (1) gets( ) reads a string from the input into a character array: char s[255]; gets(s); // not safe if ( *s != NULL ) mysub(s); // not safe gets( ) is considered UNSAFE. Why?

Safe string input (2) fgets( ) is a safer alternative to gets( ): usage: fgets(char *s, int sizeof_s, FILE *stream ) char s[255]; fgets(s, sizeof(s)-1, stdin); if ( *s != NULL ) mysub(s); // OK

Syntax/Semantics Example

Classify the errors in this code 1: #include <stdio.h> 2: int gcd(int u#, double v); 3: { if ( v = 0 ) return 0; 4: else return gcd(v, u# % v); 5: } 6: main() 7: { int x, y; 8: printf("Input two integers: "); 9: scanf("%d%d", x, y); 10: printf("The gcd is %d\n", GCD(x,y) ); 11: return; 12: }

Errors Line 2, 4: "u#" is a lexical error (not legal identifier) Line 2: "double v" is a static semantic error. Will be reported on line 4 when % is used. Line 2: ";" can be a semantic or syntax error: the statement is a legal function prototype so you could call it semantic; but the following code block is not legal in this case, so you could call it syntax if viewed with the following code. Line 3: "=" is a logic error. It will result in a dynamic (run-time) semantic error (division by zero). Not detected by compiler! Line 6: syntax error. No return type for main(). ** Line 9: forgot "&" -- syntax or semantic error? Line 10: static semantic error; caught by the linker, not the compiler. Line 11: missing return value is a semantic error. The semantics of main() is that it returns an int. ** ** Many compilers will permit this, even though it violates standard.

C Prototype /* function prototype */ int sum(int count, int array[ ] ) ; int average(int n, int x[]) { /* compute average of x[] */ return sum(n, x)/n; } int sum(int n, int x[]) { int i, s = 0; for(i=0; i<n; i++) s += x[i]; return s;