CS100A Lecture 22 17 November 1998 Prelim 3 Statistics Maximum 94 Median 68 Mean 66.6 Minimum 12 Standard deviation 14.9 Question 2 was poorly written. It could not be easily understood. We apologize. We are exploring alternatives to rectify the situation. CS100A, Lecture 23, 19 November 1998

Introduction to the programming language C Goals: Acquire a reading knowledge of C Concepts: Basic C control structures, data, I/O Program organization Pointers and parameters Reference: The stand reference is The C Programming Language, by Kernighan & Ritchie (2nd ed., 1988). But this is written for experienced programmers and can be quite terse for beginners. History: C was developed in the lat 1960’s early 1970s, as a language for two people to use. It blossomed because the operating system UNIX was written in it, and everyone wanted UNIX. CS100A, Lecture 23, 19 November 1998

Extensive standard class libraries (for I/O, GUIs, networking, etc.). Java vs. C Java advantages Direct support of classes, objects, and other tools for effectively organizing medium-to-large programs. Extensive standard class libraries (for I/O, GUIs, networking, etc.). Automatic memory management (garbage collection). Portable (write once, run everywhere), because of the Java Virtual Machine. Used for writing Applets for the world wide web Disadvantages Portability, garbage collection, and other features impose execution time and space overhead. Java runtime environment keeps Java program at a distance from the underlying machine. Therefore, Java programs are much slower than C programs. CS100A, Lecture 23, 19 November 1998

Not objected oriented (but C++ is) Java vs. C (continued) C advantages Data types and constructs are close to those provided by the underlying hardware (microprocessor), so: Good for writing programs that squeeze maximum performance from hardware. Possible to have absolute control over machine resources --good for writing software that directly manipulates hardware devices. C is often thought of as a high-level assembly language. C Disadvantages Not objected oriented (but C++ is) No garbage collection. Programmer must manage the allocation and freeing of storage. This is highly prone to error. Care needed to write programs that are portable. As will be seen, programmer must manage “pointers” or references themselves, an extremely confusing and error-prone issue for many. CS100A, Lecture 23, 19 November 1998

Standard types similar to those in Java int, long (integers) Basic C Data Types Standard types similar to those in Java int, long (integers) float, double (floating point) char (characters, normally ASCII) No type boolean 0 represents false non-0 represents true relations (<. <=, ==, !=, >= >) and logical operations (!, &&, ||) yield 0 of false, 1 if true Size of arithmetic types depends on machine/compiler. Examples: int: usually 16 or 32 bits long: at least as many bits as int, usually 32, sometimes 64. Need to be careful if you want portable code. CS100A, Lecture 23, 19 November 1998

variable = expression ; Loops and conditionals just like in Java. Basic C statements Assignment as in C: variable = expression ; Loops and conditionals just like in Java. if (condition) if (condition) statement statement else statement while (condition) for (initialize; test; step) Curly braces used to group statements, as in Java. But declarations cannot appear in a block, only in a method body. For example, the following is not legal C: if (x < y) {int tmp= x; x= y; y= tmp;} CS100A, Lecture 23, 19 November 1998

/* Yield maximum of x and y */ int max (int x, int y) { if (x >= y) C functions A C program is a collection of functions. The syntax is much like Java, but,since there are no classes, function declarations appear freely in a source program. There is no explicit notion of public or private access in C. Example: /* Yield maximum of x and y */ int max (int x, int y) { if (x >= y) return x; else return y; } CS100A, Lecture 23, 19 November 1998

A procedure is a function with type void /* Print n lines of stars; line i, for 1<= i <= n, has i starts on it */ void print_stars (int n) { int j, k; /* invariant: first k-1 lines have been written */ for (i= 1; k <= n; k++) { /* Print line k, with k stars */ for (j= 1; j <= k; j++) printf(“*”); println(“\n”); } CS100A, Lecture 23, 19 November 1998

.c Source files C function definitions are placed in source files that have names ending in “.c”. All functions normally may be accessed (called) by any other function in any file. A function definition may be preceded by “static” to restrict access so that it may be called only by other functions defined in the same file. External (“global”) variables may be defined in a .c file outside any function. External variables are created when program execution begins; they retain their values until execution of the program terminates. External variables may also be defined static in order to restrict access to them to the file containing the variable definition. CS100A, Lecture 23, 19 November 1998

#include “xyz.h” (for user files) or .h Source files Every function (and external variable) must be defined exactly once by giving a full definition in some .c source file. A function (or external variable) may be called (or referenced) in other files provided that there is a declaration of the function (or external variable) in the other files. The declaration of a function gives the function name and parameters, but not the function body. Example: int max(int x, int y); Related declarations are often grouped in header files (name ending in “.h”). The declarations in a head file xyz.h can be incorporated in another file by the directive #include “xyz.h” (for user files) or #include <xyz.h.> (for standard C libraries) CS100A, Lecture 23, 19 November 1998

Example: Standard Math Library File math.h contains declarations of the standard C math library routines: double sqrt (double x); double sin (double x); double cos (double x); etc. … A client program can access the definitions using the appropriate include directive: #include <math.h> /* yield twice the square root of x */ double sqrt2 (double x) { return 2 * sqrt(x); } CS100A, Lecture 23, 19 November 1998

Example: Print Fahrenheit-Celsius table #include <stdio.h> Standard Output The library <stdio.h> includes basic routines to read formatted data from the standard input (usually the keyboard) and print on the standard output (usually a window on the monitor). The basic output routine is printf. A call to printf has one or more arguments. The first is the “format string”, which contains literal text to be printed interspersed with formatting instructions for printing the remaining arguments. Example: Print Fahrenheit-Celsius table #include <stdio.h> /* Print a table of Fahenehit-Celsius values from 0 to 300 in increments of 20 */ void main (void) { int d; printf(“Fahrenheit Celsius\n”); for (d= 0; d <= 300; d= d+20) printf(“%3d %6.1f\n”, d, (5.0/9.0) * (d-32) ); } CS100A, Lecture 23, 19 November 1998

Parameters and arguments As in Java, when a C function is called, a frame is allocated, with space for the functions parameters and local variables, and the arguments of the call are assigned to the parameters. Example: Consider execution of #include <stdio.h> void f (int x, int y) { int tmp; tmp= x; x= y; y= tmp; } void main(void) { int j= 17; int k= 42; f(j, k); printf(“j= %d, k= %d\n”, j, k); CS100A, Lecture 23, 19 November 1998

Pointers and References In Java, one can’t write a procedure to swap the values of two int variables x and y. The following method and call does not change x and y: // Try to swap p1 and p2 public void notaSwap(int p1, int p2) {int t= p1; p1= p2; p2= t;} int x= 5; int y= 6; notaSwap(x,y); But one can change the values of fields of a class: public class Coord {int x; int y;} // Swap x fields of p1 and p2 public static void Swap(Coord p1, Coord p2) {int t= p1.x; p1.x= p1.y; p1.y= t;} Swap(p1, p2); CS100A, Lecture 23, 19 November 1998

Pointers and References C provides operator & to create a pointer to a variable int x= 5; int & p= &x; int & x; The type of x is “int &”, read as “address of int) C provides operator a “dereferencing” operator If p is a pointer to an int variable, then * is the variable. * p = * p + 1; int * x; The type of x is “int *”, read as “pointer to int” 5 x p 6 x p CS100A, Lecture 23, 19 November 1998

Procedure to swap two integers void swap(int & x, int & y) { int tmp; tmp= * x; *x= *y; *y= tmp; } The parameters are no long of type int, but instead “point to int”. Therefore, to reference the actual variables, use &x and &y CS100A, Lecture 23, 19 November 1998

Standard input Function scanf in library <stdio.h> reads from the standard input device and deciphers the input characters according to the format string given as the first argument of a call. The remaining arguments of a call to scanf are pointers to variables where the input values should be stored. As with printf, C compilers generally can’t (or don’t) check that the types of the variables match the format codes in the format string. A mismatch, usually a mistake, can lead to mysteriously scrambled bytes in memory or a program or machine crash. CS100A, Lecture 23, 19 November 1998

Standard input Example This program uses the earlier written function (see slide 12)to read in two integers and print their maximum. void main(void) { int j, k; printf(“Please type in two integers”); // read integers into j and k. scanf(“%d &d”, &j, &k); printf(“the larger of the two is\n %d”, max(j,k)); Note that scanf f requires pointers to variables as arguments! CS100A, Lecture 23, 19 November 1998