An Overview of C

1. An Overview of C Kernighan/Ritchie: Kelley/Pohl: Chapter 1 Chapter 1

2. Lecture Overview • Writing and compiling C programs • Variables and expressions • The C preprocessor • Input and output • Flow of control • Functions • Arrays, strings and pointers • Files

3. A First C Program • As always, the first program that we will write in C prints 'Hello, world!': • This program should be in a file with a '.c' extension, such as 'hello.c' #include <stdio.h> main() { printf ("Hello, world!\n"); }

4. Compiling C Programs • To compile the previous program, you should pass it as a parameter to a C compiler, such as cc or gcc • This will create an executable file called 'a.out', which can then be run: gcc hello.c a.out Hello, world!

5. Compiling C Programs • By convention, the executable should have the same name as the C file, but withoutthe '.c' extension • The executable for 'hello.c' should be called 'hello' • We can specify the name of the generated executable using the compiler's '-o' option: gcc –o hello hello.c

6. Compiling C Programs • Normally, the compiler will only issue errors, but not warnings • However, warnings can be very useful for improving the quality of your code • The '-Wall' option will generate all possible warnings: gcc –Wall –o hello hello.c

7. Lecture Overview • Writing and compiling C programs • Variables and expressions • The C preprocessor • Input and output • Flow of control • Functions • Arrays, strings and pointers • Files

8. Variables and Expressions • In C, all variables must be declared at the beginning of a program or a block • A variable name (identifier) consists of a sequence of letters, digits, underscore, but may not start with a digit int miles, yards, pacific_sea; float kilometers; char c1, c2, c3; char 1c, 2c, 3c; /* wrong */

9. Variables and Expressions • The evaluation of expressions can involve conversion rules: • The 2 in the second expression is automatically converted to a double, and thus the value of the complete expression is also a double 7 / 2 /* the int value 3 */ 7.0 / 2 /* the value 3.5 */

10. Variables and Expressions /* The distance of a marathon in kilometers. */ #include <stdio.h> int main() { int miles, yards; float kilometers; miles = 26; yards = 385; kilometers = 1.609 * (miles + yards / 1760.0); printf ("\nA marathon is %f kilometers.\n\n", kilometers); return 0; }

11. Lecture Overview • Writing and compiling C programs • Variables and expressions • The C preprocessor • Input and output • Flow of control • Functions • Arrays, strings and pointers • Files

12. The C Preprocessor –#define • The C compiler has a built-in preprocessor • Lines that begin with a '#' are called preprocessing directives: • A '#define' directive can appear anywhere in the program • It affects only lines that come after it #define LIMIT 100 #define PI 3.14159

13. The C Preprocessor –#define • The contents of quoted strings are never changed by the preprocessor: • The second PI will be replaced, the first one will not • Constants in C programs are normally declared using '#define' statements printf ("PI = %f\n", PI);

14. The C Preprocessor –#include • Another important preprocessor directive is the '#include' directive • It is used to insert a file into another file during compilation • This is used mostly to include header files– files containing declarations and definitions used by the program

15. The C Preprocessor –#include • The contents of 'util.h': • The contents of 'my_prog.c': • After preprocessing: extern int a; #include <util.h> main() { ... extern int a; main() { ...

16. The C Preprocessor –#include • The C system provides a number of standard header files, that define the interface with functions in the C library • Some common header files: Header File stdio.h math.h string.h stdlib.h Use Standard input and output Math functions String manipulation functions Miscellaneous functions

17. Lecture Overview • Writing and compiling C programs • Variables and expressions • The C preprocessor • Input and output • Flow of control • Functions • Arrays, strings and pointers • Files

18. Input and Output • Program output is printed using the function printf() • Returns the number of characters printed • Similarly, input is read into the program using the function scanf() • Returns the number of values read • To use either of these functions, the header file 'stdio.h' should be included

19. Input and Output • Both functions accept a variable number of arguments • The first (mandatory) argument is the control string • The number of additional arguments required is defined by the contents of the control string

20. Conversion Specifiers • The control string contains plain characters, plus conversion specifiers, which begin with the symbol '%' • For example, to print the letters 'abc', we could use any of the following statements: printf ("abc"); printf ("%s", "abc"); printf ("%c%c%c", 'a', 'b', 'c') ;

21. Conversion Specifiers • Some common conversion specifiers: Character %c %d %e %f %s Corresponding argument Character Decimal integer Floating point – scientific notation Floating point String

22. Defining Field Width • With printf(), it is possible to define the field width for each of the printed arguments • This is done by adding a number before the conversion specification: printf ("Chars:%3c%3c%3c\n", 'a', 'b', 'c') ; printf ("Twelve:%6d\n", 12); printf ("Two thirds:%6.3f\n", (2.0 / 3)); Chars: a b c Twelve: 12 Two thirds: 0.667

23. Defining Field Width • Add a '-' before the field width for left aligned printing, or a '0' for zero padded printing: int a = 5, b = 12; printf ("%d,%d\n", a, b); printf ("%4d,%4d\n", a, b); printf ("%-4d,%-4d\n", a, b); printf ("%04d,%04d\n", a, b); 5,12 5, 12 5 ,12 0005,0012

24. Reading Input • The function scanf() is analogous to printf() but is used for input • The symbol '&' is the address operator • The '%d' tells scanf() to try to read an integer from the standard input • The '&x' tells it to store the integer in x scanf ("%d", &x);

25. Input and Output – Example #include <stdio.h> int main() { char c1, c2; int i; float x; printf ("Input two characters, " "an int, and a float:\n"); scanf ("%c%c%d%f", &c1, &c2, &i, &x); printf ("\nThe data that you typed in:\n"); printf ("%3c%3c%5d%17e\n", c1, c2, i, x); return 0; }

26. Input and Output – Example • scanf() will skip white space when reading in numbers, but not when reading characters Input two characters, an int, and a float: Db 45 2.9858 The data that you typed in: D b 45 2.985800e+00

27. Getting Help on C Functions • For a full list of field width and conversion specifiers, and for additional options: • The second parameter (3) tells man tolook in section 3 of the manual pages, which contains help on C library functions man 3 printf

28. Lecture Overview • Writing and compiling C programs • Variables and expressions • The C preprocessor • Input and output • Flow of control • Functions • Arrays, strings and pointers • Files

29. Flow of Control • Logical expressions have an integer value of either 0 (false) or 1 (true) • The result of any expression can be treated as a Boolean value: • 0 means false • Any non-zero value means true

30. Flow of Control • C supports the familiar control flowstructures – if, while and for: if (condition) statement1; else statement2; while (condition) statement; for (initialization; condition; increment) statement;

31. Flow of Control – Examples #include <stdio.h> int main() { int i = 1, sum = 0; while (i <= 5) { sum += i; i++; } printf ("sum = %d\n", sum); return 0; } sum = 15

32. Flow of Control – Examples #include <stdio.h> int main() { int i = 3; while (i--) printf ("%d ", i); printf ("\n"); if (0.0) printf ("0.0 is true\n"); else printf ("0.0 is false\n"); } 2 1 0 0.0 is false

33. Lecture Overview • Writing and compiling C programs • Variables and expressions • The C preprocessor • Input and output • Flow of control • Functions • Arrays, strings and pointers • Files

34. Functions • A C program consists of one or more functions in one or more files • Every C program must contain exactly one main() function • Other functions are called from main(),or from within each other

35. Function Prototypes • Functions should always be declared before they are used: • function declarations of this type are called function prototypes • A prototype defines the parameter types and the return type of the function float maximum (float x, float y); void print_info();

36. Function Prototypes #include <stdio.h> void print_num (int); int main() { print_num (5); return 0; } void print_num (int a) { printf ("The number is: %d\n", a); } The number is: 5

37. Function Prototypes • A function does not need a prototype if it is only used after it's definition #include <stdio.h> void print_num (int a) { printf ("The number is: %d\n", a); } int main() { print_num (5); return 0; }

38. Passing Parameters by Value • In C, arguments to functions are always passed by value • When an expression is passed as an argument, it is first evaluated, and onlythe result is passed to the function • The variables passed as arguments are not changed in the calling environment

39. Passing Parameters by Value – Example #include <stdio.h> int main() { int a = 1; void try_to_change_it (int); printf ("%d\n", a); /* 1 is printed */ try_to_change_it (a); printf ("%d\n", a); /* 1 is printed again */ return 0; } void try_to_change_it (int a) { a = 777; }

40. Lecture Overview • Writing and compiling C programs • Variables and expressions • The C preprocessor • Input and output • Flow of control • Functions • Arrays, strings and pointers • Files

41. Arrays • Arrays are used when many variables, all of the same type, are desired. For example: • This allocates space for 3 integers • Each element of the array is of type int, and is accessed as: a[0], a[1], a[2] • Arrays are zero based (indexes start at 0) int a[3];

42. Arrays – Example #include <stdio.h> #define CLASS_SIZE 5 int main() { int i, j, sum = 0, tmp; int score[CLASS_SIZE]; printf ("Input %d scores: ", CLASS_SIZE); for (i = 0; i < CLASS_SIZE; i++) { scanf ("%d", &score[i]); sum += score[i]; } ...

43. Arrays – Example ... for (i = 0; i < CLASS_SIZE - 1; i++) for (j = CLASS_SIZE - 1; j > i; j--) if (score[j-1] < score[j]) { tmp = score[j-1]; score[j-1] = score[j]; score[j] = tmp; } printf ("\nOrdered scores:\n\n"); for (i = 0; i < CLASS_SIZE; i++) printf (" score[%d] = %d\n", i, score[i]); printf ("\n%d%s\n%.1f%s\n\n", sum, " is the sum of all the scores", (double)sum / CLASS_SIZE, " is the average"); return 0; }

44. Arrays – Example • The program stores 5 integers in an array, and then sorts them using bubble sort Input 5 scores: 63 88 97 53 77 Ordered scores: score[0] = 97 score[1] = 88 score[2] = 77 score[3] = 63 score[4] = 53 378 is the sum of all the scores 75.6 is the average

45. Using Pipes for Program Input • When writing or debugging the previous program, all input numbers need to bere-typed every time the program is run • Instead, we can use a pipe: • The program 09_arrays will accept the 5 input numbers as if they were typed in echo 63 88 97 53 77 | 09_arrays

46. Pointers • Pointers are variables that contain the address of a variable • The above assigns the address of c to the variable p, and p is said to "point to" c • The dereferencing operation uses '*': char *p; char c = 'A'; p = &c; char d = *p;

47. Arrays and Pointers • Pointers and arrays are closely related • An array variable is simply a pointer to the first element of the array • Unlike a pointer, an array variable can not be modified • In many cases, array and pointer notations are interchangeable

48. Strings, Arrays and Pointers • In C, a string is an array of characters • Arrays of characters are not different from arrays of any other type • String elements may be accessed directly as variables of type char • Pointers can be used to manipulate strings

49. Strings, Arrays and Pointers – Example #include <stdio.h> int main() { char *p = "pointer to char"; char str[] = "string array"; printf ("%s\n%s\n", p, str); p = str; *str = 'S'; str[1] = 'T'; *(p + 2) = 'R'; p[3] = 'I'; printf ("%s\n", p); return 0; }

50. Strings, Arrays and Pointers – Example • Both p and str refer to strings • Although defined differently, each one can be used either as an array, or as a pointer • Characters can be accessed as array elements, or through pointer dereferencing • Program output: pointer to char string array STRIng array