15 213 C Primer Outline Overview comparison of C and Java Good evening Preprocessor Command line arguments Arrays and structures Pointers and dynamic memory What we will cover A crash course in the basics of C You should read the K R C book for lots more details Like Java like C Operators same as Java Arithmetic i i 1 i i i 2 Relational and Logical Syntax same as in Java if else while do while for i 1 i 100 i switch case 1 continue break Simple Data Types datatype char short int long float double size 1 2 4 4 4 8 values 128 to 127 32 768 to 32 767 2 147 483 648 to 2 147 483 647 2 147 483 648 to 2 147 483 647 3 4E 38 7 digits 1 7E 308 15 digits long Java programmer gotchas 1 int i for i 0 i 10 i NOT for int i 0 i 10 i Java programmer gotchas 2 Uninitialized variables catch with Wall compiler option include stdio h int main int argc char argv int i factorial i return 0 Java programmer gotchas 3 Error handling No exceptions Must look at return values Good evening include stdio h int main int argc char argv print a greeting printf Good evening n return 0 goodevening Good evening Breaking down the code include stdio h Include the contents of the file stdio h Case sensitive lower case only No semicolon at the end of line int main The OS calls this function when the program starts running printf format string arg1 Prints out a string specified by the format string and the arguments format string Composed of ordinary characters not Copied unchanged into the output Conversion specifications start with Fetches one or more arguments For example char char int float c s d f For more details man 3 printf C Preprocessor define FIFTEEN TWO THIRTEEN The Class That Gives CMU Its Zip n int main int argc char argv printf FIFTEEN TWO THIRTEEN return 0 After the preprocessor gcc E int main int argc char argv printf The Class That Gives CMU Its Zip n return 0 Conditional Compilation define CS213 int main int argc char argv ifdef CS213 printf The Class That Gives CMU Its Zip n else printf Some other class n endif return 0 After the preprocessor gcc E int main int argc char argv printf The Class That Gives CMU Its Zip n return 0 Command Line Arguments 1 int main int argc char argv argc Number of arguments including program name argv Array of char s that is an array of c strings argv 0 program name argv 1 first argument argv argc 1 last argument Command Line Arguments 2 include stdio h int main int argc char argv int i printf d arguments n argc for i 0 i argc i printf d s n i argv i return 0 Command Line Arguments 3 cmdline The Class That Gives CMU Its Zip 8 arguments 0 cmdline 1 The 2 Class 3 That 4 Gives 5 CMU 6 Its 7 Zip Arrays char foo 80 An array of 80 characters sizeof foo 80 sizeof char 80 1 80 bytes int bar 40 An array of 40 integers sizeof bar 40 sizeof int 40 4 160 bytes Structures Aggregate data include stdio h struct name char name int age DO NOT FORGET the semicolon int main int argc char argv struct name bovik bovik name Harry Bovik bovik age 25 printf s is d years old n bovik name bovik age return 0 Pointers Pointers are variables that hold an address in memory That address contains another variable Memory layout and addresses int x 5 y 10 float f 12 5 g 9 8 char c c d d 5 10 4300 4304 12 5 4308 9 8 c d 4312 4317 4316 Using Pointers 1 float f float f addr f data variable pointer variable f addr any float 4300 4304 any address f addr f f address operator f addr 4300 4300 4304 Pointers made easy 2 f addr 3 2 f indirection operator f addr 3 2 4300 4300 4304 float g f addr indirection g is now 3 2 f 1 3 but g is still 3 2 f f addr 1 3 4300 4300 4304 Function Parameters Function arguments are passed by value What is pass by value The called function is given a copy of the arguments What does this imply The called function can t alter a variable in the caller function but its private copy Three examples Example 1 swap 1 void swap 1 int a int b int temp temp a a b b temp Q Let x 3 y 4 after swap 1 x y x y A1 x 4 y 3 A2 x 3 y 4 Example 2 swap 2 void swap 2 int a int b int temp temp a a b b temp Q Let x 3 y 4 after swap 2 x y x y A1 x 3 y 4 A2 x 4 y 3 Example 3 scanf include stdio h int main int x scanf d n x printf d n x Q Why using pointers in scanf A We need to assign the value to x Dynamic Memory Java manages memory for you C does not C requires the programmer to explicitly allocate and deallocate memory Unknown amounts of memory can be allocated dynamically during run time with malloc and deallocated using free Not like Java No new No garbage collection You ask for n bytes Not a high level request such as I d like an instance of class String malloc Allocates memory in the heap Lives between function invocations Example Allocate an integer int iptr int malloc sizeof int Allocate a structure struct name nameptr struct name malloc sizeof struct name free Deallocates memory in heap Pass in a pointer that was returned by malloc Example int iptr int malloc sizeof int free iptr Caveat don t free the same memory block twice
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