15-213 C PrimerOutlineWhat we will coverLike Java, like CSimple Data TypesJava programmer gotchas (1)Java programmer gotchas (2)Java programmer gotchas (3)“Good evening”Breaking down the codeformat_stringC PreprocessorAfter the preprocessor (gcc –E)Conditional CompilationSlide 15Command Line Arguments (1)Command Line Arguments (2)Command Line Arguments (3)ArraysStructuresPointersMemory layout and addressesUsing Pointers (1)Pointers made easy (2)Function ParametersExample 1: swap_1Example 2: swap_2Example 3: scanfDynamic MemoryNot like Javamallocfree15-213 C PrimerOutline•Overview comparison of C and Java•Good evening•Preprocessor•Command line arguments•Arrays and structures•Pointers and dynamic memoryWhat we will cover•A crash course in the basics of C•You should read the K&R C book for lots more detailsLike 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;datatype size valueschar 1 -128 to 127short 2 -32,768 to 32,767int 4 -2,147,483,648 to 2,147,483,647long 4 -2,147,483,648 to 2,147,483,647float 4 3.4E+/-38 (7 digits)double 8 1.7E+/-308 (15 digits long)Simple Data TypesJava 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!$Breaking down the code•#include <stdio.h>–Include the contents of the file stdio.h•Case sensitive – lower case only–No semicolon at the end of line•int main(…)–The OS calls this function when the program starts running.•printf(format_string, arg1, …)–Prints out a string, specified by the format string and the arguments.format_string•Composed of ordinary characters (not %)–Copied unchanged into the output•Conversion specifications (start with %)–Fetches one or more arguments–For example•char %c•char* %s•int %d•float %f•For more details: man 3 printfC Preprocessor#define FIFTEEN_TWO_THIRTEEN \ "The Class That Gives CMU Its Zip\n"int main(int argc, char* argv[]){ printf(FIFTEEN_TWO_THIRTEEN); return 0;}After the preprocessor (gcc –E)int main(int argc, char* argv){ printf("The Class That Gives CMU Its Zip\n"); return 0;}Conditional Compilation#define CS213int main(int argc, char* argv){ #ifdef CS213 printf("The Class That Gives CMU Its Zip\n"); #else printf("Some other class\n"); #endif return 0;}After the preprocessor (gcc –E)int main(int argc, char* argv){ printf("The Class That Gives CMU Its Zip\n"); return 0;}Command Line Arguments (1)•int main(int argc, char* argv[])•argc–Number of arguments (including program name)•argv–Array of char*s (that is, an array of ‘c’ strings)–argv[0]: = program name–argv[1]: = first argument–…–argv[argc-1]: last argumentCommand Line Arguments (2)#include <stdio.h>int main(int argc, char* argv[]){ int i; printf("%d arguments\n", argc); for(i = 0; i < argc; i++) printf(" %d: %s\n", i, argv[i]); return 0;}Command Line Arguments (3)$ ./cmdline The Class That Gives CMU Its Zip8 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 bytesStructures•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.5 10 12.5 9. 8 c dint x = 5, y = 10;float f = 12.5, g = 9.8;char c = ‘c’, d = ‘d’;4300 4304 4308431243164317Memory layout and addresses? ?ff_addr4300 4304?any floatany address? 4300ff_addr4300 4304Using Pointers (1)float f; /* data variable */float *f_addr; /* pointer variable */f_addr = &f; /* & = address operator */Pointers made easy (2)*f_addr = 3.2; /* indirection operator */float g = *f_addr;/* indirection: g is now 3.2 */f = 1.3; /* but g is still 3.2 */ff_addr4300 43043.2 4300ff_addr4300 43041.3 4300Function 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 examplesExample 1: swap_1void 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_2void 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
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