Topic 5: A Quick Intro To C. Reading From K&R Text. "goto Considered Harmful" History

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1 Topic 5: A Quick Intro To C Reading From K&R Text Assumption: All of you know basic Java. Much of C syntax is the same. Also: Some of you have used C or C++. Goal for this topic: you can write & run a simple C program basic functions with parameters & results very basic arrays no pointers or structures no string variables (just literal strings) Chapter 1 of C text: overview, don't have to know all details Chapter 2: types, operators & expressions may skip enum constants bitwise operators may be new: please note Chapter 3: control flow may skip: switch, do-while loops Chapter 4: functions (skip section 4.7) Section 7.2 & 7.4: printf & scanf 1 2 "goto Considered Harmful" History Text mentions goto statements in Chapter 3. DO NOT USE!!! Automatic style deduction even on quizzes & final exam. Programming languages evolve! BCPL B C C++ Java object-oriented 3 4

2 Versions of C C is an older language & has evolved. Many versions. ANSI C: a standard Most compilers are more flexible than ANSI Rule for this term: Your programs must work using the GNU C compiler. No special flags required. Caution: Make sure your C programs work on the CASLab system. Other systems may have different compilers or compiler settings. Differences Between C and C++ 1. Obvious big difference: no classes in C. Structures only (we'll discuss later) 2. Some syntax rules are more restrictive in C 3. I/O is different. 4. Different libraries to include 5. Memory allocation is different (later) 6. No bool type 5 6 Compiling and Running a C Program (1) Compiling and Running a C Program (2) For now: one-file programs. File extension must be.c Suppose my program is hello.c To get a nicer name for executable: gcc -o greeting hello.c Now executable file is called greeting (no extension) gcc command: GNU Compiler Collection File extension tells compiler which language. Simplest way to compile & run: gcc hello.c Produces an executable file called a.out. 7 8

3 Compiling and Running a C Program (3) Two-step method: gcc -c hello.c Compiles only. Does not link. Produces hello.o ("object code") To get an executable (link object code with C library): gcc -o greeting hello.o Warnings Recommended flag for gcc: -Wall Enables many helpful warnings. Examples: single = inside a conditional wrong number of arguments for printf or scanf forgetting to return a value from a function that's not void unused variables Not required for CISC 220, but very helpful in debugging. Suggestion: alias gcc="gcc -Wall" 9 10 One-Line Comments Control Structures Strict ANSI C: no one-line comments (//) All comments are /*... */ Just like Java: if, while, for One difference with for loops: gcc compiler allows one-line comments, so OK to use for CISC 220. In C++ & Java, OK to declare for loop counters in loop: for (int i = 1; i <= 10; i++)... Standard C does not allow this. Instead: int i; for (i = 1; i <= 10; i++)

4 Booleans Signed vs. Unsigned No bool/boolean type in C. Results of tests are integers. 0 = false non-zero = true Be very careful about "=" vs "==": int x = 2; if (x = 3) printf("yes"); else printf("no"); printf("%d\n",x); Integers may be specified signed or unsigned. Default is signed. Integers on CASLab: 4 bytes (32 bits) Signed integer on CASLab: to Unsigned integer on CASLab: 0 to Bitwise Operations in C Example: File Protections Logical operators on sequences of bits. View an integer as a string of booleans -- each bit is true (1) or false (0). &: bitwise and : bitwise or ~: bitwise not ("one's complement") = & 1100 = 0100 ~0110 = 1001 Goal: store read/write/execute protection for a file in one integer (instead of three separate integers) #define READ 4 #define WRITE 2 #define EXECUTE 1 int main() { int file1perm = READ WRITE; int file2perm = READ EXECUTE; int common = file1perm & file2perm; // remove write permission from file1 file1perm = file1perm & ~WRITE; 15 16

5 Integer Division Rules: arithmetic between two ints: result is an int (division is truncated) arithmetic between int & float or two floats: result is a float (no truncation) use "(float)" to cast an int into a float printf C++ programmers: The >> operator is part of C++, not C. To write characters to the screen (standard output) in C, use the printf function. Simple use to write a message: printf("hello, world\n"); Ending with \n is important: no equivalent of Java's println printf Conversion Specifications printf Field Widths: Integer To include the value of a variable in output, use special conversion codes in first parameter: %d: integer (d for "decimal", not "double"!) %u: unsigned int %ld: long %lu: unsigned long %f: float or double %c: single character %s: string Example: int x =...; float y =...; printf("the answers are %d and %f\n", x, y); May put minimum field width after a %: printf("%5d\n", num); pads with left blanks if necessary 19 20

6 printf Field Widths: floating point For float and double, may also specify a precision: printf("%8.2f\n", x); Minimum of 8 characters total Exactly two digits after decimal point scanf Reads from the standard input. Conversion specifications similar to print, not exactly the same! type printf scanf char %c %c int %d %d float %f %f double %f %lf Return value from scanf: EOF if hit end of file before reading values otherwise, number of values read Portability Issues Floating-Point Types Sizes of numeric types differ, depending on compiler & target machine. C arithmetic is not portable! sizeof(type): number of bytes used by the type <limits.h>: contains constants giving max & min values for the integer types <float.h>: constants relating to floating-point types Look at sizes.c... Basic floating-point type: float Other types: double, long double All floating point numbers are signed

7 Arrays (quick preview) To create an array of 20 integers: int nums[20]; Things to note: no "new" array elements are not initialized no ".length" C Functions and Program Structure Reading: Chapter 4 of C text You may skip Section 4.7 (register variables) Included in this sub-topic: functions, parameters and return results multi-file programs the C preprocessor (no macros with arguments) example: array1.c Things to remember array parameters passed by reference (address of array) type of array parameter doesn't include size example: array2.c Functions main Function Functions in C very similar to Java/C++ All parameters are passed by value (no "ref" parameters in C) A C program must have a main function. Optional parameters to main: later (command-line args) Return type of main is int: return status. callbyvalue.c Good practice when writing C programs that may be run in Linux: return 0 for success, non-zero for error aborts To abort a program from a function that's not main: exit(1) (exit function is in <stdlib.h>) Add an exit to callbyvalue.c

8 Order of Declarations In C, every function & variable must be declared before it is used. In smallfuncs.c, defined functions in convenient order: each function fully defined before it is called. Not always possible or convenient: mutual recursion preferences about order of functions in files separate compilation Declaration vs. Definition Declaration of function tells you: name of function return types number and types of parameters A specification: not how the function works but how to use it. Declaration == header only Sometimes called a "forward declaration" Definition of function: full definition, including the body. All details of how the function works. Includes repetition of information from declaration. Definition = header + body Global Variables The C Pre-processor Variables defined outside any functions. Must be declared before use. Use with caution: can make program much less readable! Example: add global count & accuracy to smallfuncs.c Two phases of C compilation: source code pre-processor modified source code compiler object code 31 32

9 Pre-Processor Language Every line starting with "#" is a command to the pre-processor. Important: Pre-processor syntax C syntax! To run pre-processor by itself: cpp P <filename> #define (note: We're skipping macros with arguments section 6.3.3) #include #undef #if #ifdef #ifndef #else #elif #endif Pre-Processor Directives examples Notes and Reminders Pre-Processor Definitions with gcc #include's can nest to any depth Preprocessor will complain if you define something that's already defined use #undef first (or check with #ifdef) #if/#ifdef useful to comment out a block of code temporarily (avoids problems with nested comments) #if & #ifdef also useful to implement different "modes" for example debugging and non-debugging mode, or linux and Windows mode arithmetic in #if expressions is integer arithmetic #include <filename>: look in system directories #include "filename": look in current directory gcc -DNAME=VALUE program.c Caution: if NAME is already defined in program.c you'll get a warning

10 Separating Programs Into Files Java: very simple, by classes. C: program is a sequence of definitions (mostly functions) separate definitions into multiple files no automatic scheme for where to find functions Why Multiple Files? 1. Large programs: faster compilation *** 2. Logical organization related definitions grouped together 3. Working in groups each programming editing a different file 4. Library files definitions useful in more than one program *** Goal: be able to compile each file separately. Later, link compiled portions of program together module.h: void func1(int x); int func2(float y); program.c: #include "module.h"... int main() {.. func1(17); z = func2(3.4); } /* end main */ Usual Style module.c: #include module.h void func1(int x) {... (function body) } int func2(float y) {... (function body) } Demonstration Split smallfuncs.c into several files: one file contains input function another file contains square root function and helpers other file contains main function 39 40

11 To Compile One File a Partial Program To Link a Program Together gcc Wall c module.c Can use gcc to link several object files together: Output is module.o: object code machine code with some pieces unspecified (i.e. references to other parts of the program) gcc o program file1.o file2.o file3.o executable file created by combining all the object code & resolving references between them Possible Changes to Demo Program 1. Fix bug in square root function 2. Change square root function (use different algorithm) 3. Change detail in main function 4. Add a second parameter to the square root function 5. Add cube root function (used by main) 6. Change ACCURACY constant 43