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CS113 Introduction to C

CS113 Introduction to C. Instructor: Ioannis A. Vetsikas E-mail: vetsikas@cs.cornell.edu Lecture 11 : September 18. The C preprocessor: Conditional Inclusion. #if , #elif , #else , #endif Allow us to define conditional statements for what the compiler should compile

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CS113 Introduction to C

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  1. CS113Introduction to C Instructor: Ioannis A. VetsikasE-mail: vetsikas@cs.cornell.edu Lecture 11 : September 18

  2. The C preprocessor:Conditional Inclusion • #if, #elif, #else, #endif • Allow us to define conditional statements for what the compiler should compile • #define name and #undef name • defined(name) returns 1 if name is defined with a #define statement and 0 otherwise • #ifdef and #ifndef are same respectively as #if defined and #if !defined #ifdef DEBUG printf(“Debug mode”); i++; #else printf(“Real mode”); #endif

  3. Streams • I/O done through streams; two kinds: text and binary • Text streams are sequences of lines, each of which is a sequence of characters terminated by a newline • Binary streams are sequences of characters corresponding to internal representation of data. • Streams created by opening files and referenced using stream pointers (FILE *) • Normally three standard streams are automatically open: • stdin (stream for standard input - from keyboard) • stdout (stream for standard output - to screen) • stderr (stream for standard error output - to screen)

  4. Functions for opening & closing • Open a file: • FILE *fopen(char *name, char *mode); • Opens file with name name using mode mode • Mode is taking values: “r” (open text for reading), “w” (open text for writing), etc… • For binary files add “b” at the end, e.g. “rb” (open binary for reading) • Usage: FILE *fp=fopen(“infile”, “r”); • Returns NULL if some problem occurs during the opening of the file (e.g. file is not found, or permissions are not set correctly etc.) • Close a file: • int flcose(FILE *fp);

  5. Some other functions • Defined in <stdio.h> • int fprintf(FILE *fp, char *format, …); • int fscanf(FILE *fp, char *format, …); • int fgetc(FILE *fp); • int fputc(int c, FILE *fp); • char *fgets(char *s, int n, FILE *fp); • int fputs(char *s, FILE *fp); • int feof(FILE *fp); • For more functions read appendix B of the K&R book

  6. Example (from Kelley&Pohl - modified) #include <stdio.h> #include <stdlib.h> void double_space(FILE *ifp, FILE *ofp) { int c; while ((c=getc(ifp)) != EOF) { putc(c, ofp); if (c==‘\n’) putc(‘\n’, ofp); /* duplicate Newline */ } int main(int argc, char **argv) { FILE *ifp = fopen(argv[1], “r”); FILE *ofp = fopen(argv[2], “w”); double_space(ifp, ofp); fclose(ifp); fclose(ofp); }

  7. Announcements • Wednesday: Mock quiz • Friday: Quiz (the real one) – Make sure you come • We are done with covering the K&R book • Read the appendices by yourselves and look especially at appendix B where the library functions are presented • Homework 4 is challenging and you should start as early as possible; I’ll announce extra office hours for the next week to allow you to come and talk to me about the homework • I will do some revision, answer questions and do miscellaneous programming (debugging) skills next

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