6.087 Lecture 4 – January 14, 2010Review Control flow I/O Standard I/O String I/O File I/O 1 Blocks• Blocks combine multiple statements into a single unit. • Can be used when a single statement is expected. • Creates a local scope (variables declared inside are local to the block).Blocks can be nested.• { int x=0; { int y=0; /∗ both x and y visible ∗ /}/∗ only x visible ∗ /} 1 Conditional blocksif ... else..else if is used for conditional branching of execution if ( cond ) { /∗ code executed if cond is true ∗ /} else { /∗ code executed if cond is false ∗ /} 2 Conditional blocksswitch..case is used to test multiple conditions (more efficient than if else ladders). switch ( opt ) { case ’A’ : /∗ execute if opt == ’A’ ∗ /break ;case ’B’ :case ’C’ :/∗ execute if opt == ’B’ || opt == ’C’ ∗ /default : } 3 Iterative blocks• while loop tests condition before execution of the block. • do..while loop tests condition after execution of the block. • for loop provides initialization, testing and iteration together. 4 6.087 Lecture 4 – January 14, 2010Review Control flow I/O Standard I/O String I/O File I/O 5 goto• goto allows you to jump unconditionally to arbitrary part of your code (within the same function). • the location is identified using a label. a label is a named location in the code. It has the same • form as a variable followed by a ’:’ start : { if ( cond ) goto outside ; /∗ some code ∗ /goto start ; } outside : /∗ outside block ∗ /5 Spaghetti codeDijkstra. Go To Statement Considered Harmful. Communications of the ACM 11(3),1968 • Excess use of goto creates sphagetti code. • Using goto makes code harder to read and debug. • Any code that uses goto can be written without using one. 6 error handling Language like C++ and Java provide exception mechanism to recover from errors. In C, goto provides a convenient way to exit from nested blocks. cont _flag =1; for (..) for ( . . ) { { for ( i n i t ; c o n t _ f l a g ; i t e r ) for ( . . ) { { i f ( error_cond ) i f ( error_cond ) { goto e r r o r ; c o n t _ f l a g =0; /∗ skips 2 blocks ∗ /break ; } } } /∗ i n n er loop ∗ /e r r o r : } i f ( ! c o n t _ f l a g ) break ; /∗ outer loop ∗ /} 7 6.087 Lecture 4 – January 14, 2010Review Control flow I/O Standard I/O String I/O File I/O 8 Preliminaries• Input and output facilities are provided by the standard library and not by the language itself. • A text stream consists of a series of lines ending with ’\n’. The standard library takes care of conversion from ’\r\n’−’\n’ • A binary stream consists of a series of raw bytes. • The streams provided by standard library are buffered. 8 Standard input and outputint putchar(int) • putchar(c) puts the character c on the standard output. • it returns the character printed or EOF on error. int getchar() • returns the next character from standard input.it returns EOF on error.• 9 Standard input and outputWhat does the following code do? int main () { char c; while ((c=getchar ())!= EOF) { if ( c>=’A’ && c<=’Z’ )c=c−’A’+’a’ ;putchar (c );}return 0;} To use a file instead of standard input, use ’<’ operator (*nix). • Normal invocation: ./a.out • Input redirection: a.out < file.txt. Treats file.txt as source of standard input.This is an OS feature, not a language feature. 10 Standard output:formattedint printf (char format[],arg1,arg2 ,...) • printf() can be used for formatted output. • It takes in a variable number of arguments. • It returns the number of characters printed. • The format can contain literal strings as well as format specifiers (starts with %). Examples: printf ( "hello world\n" ); printf ( "%d\n" ,10); /∗ format: %d (integer),argument:10 ∗ /printf ( "Prices:%d and %d\n" ,10 ,20); 11 printf format specificationThe format specification has the following components %[flags][width ][. precision][length] type: type meaning example d,i integer printf ("%d",10); /∗prints 10∗/x,X integer (hex) printf ("%x",10); /∗ print 0xa∗/u unsigned integer printf ("%u",10); /∗prints 10∗/c character printf ("%c",’A’); /∗prints A∗/s string printf ("%s","hello"); /∗prints hello∗/f float printf ("%f",2.3); /∗ prints 2.3∗/d double printf ("%d",2.3); /∗ prints 2.3∗/e,E float(exp) 1e3,1.2E3,1E−3 % literal % printf ("%d %%",10); /∗prints 10%∗/12 printf format specification (cont.)%[flags][width ][. precision][modifier] width: format output printf ("%d",10) "10" printf ("%4d",10) bb10 (b:space) printf ("%s","hello") hello printf ("%7s","hello") bbhello 13 printf format specification (cont.)%[flags][width ][. precision][modifier] flag: format output printf ("%d,%+d,%+d",10,−10) 10,+10,-10 printf ("%04d",10) 0010 printf ("%7s","hello") bbhello printf ("%-7s","hello") hellobb 14 printf format specification (cont.)%[flags][width ][. precision][modifier] precision: format output printf ("%.2f,%.0f,1.141,1.141) 1.14,1 printf ("%.2e,%.0e,1.141,100.00) 1.14e+00,1e+02 printf ("%.4s","hello") hell printf ("%.1s","hello") h 15 printf format specification (cont.)%[flags][width ][. precision][modifier] modifier: modifier meaning h interpreted as short. Use with i,d,o,u,x l interpreted as long. Use with i,d,o,u,x L interpreted as double. Use with e,f,g 16 Digression: character arraysSince we will be reading and writing strings, here is a brief digression • strings are represented as an array of characters • C does not restrict the length of the string. The end of the string is specified using 0. For instance, "hello" is represented using the array {’h’,’e’,’l’,’l’,’\0’}. Declaration examples: • char str []="hello";/∗compiler takes care of size∗/• char str[10]="hello";/∗make sure the array is large enough∗/• char str []={ ’h’,’e’,’l’,’l’,0}; Note: use \" if you want the string to contain ". 17 Digression: character arraysComparing strings: the header file provides the function int strcmp(char s[],char t []) that compares two strings in dictionary order (lower case letters come after capital case). the function returns a value <0 if s comes before t • the function return a value 0 if s is the same as t • the function return a value >0 if s comes after t • • strcmp is case sensitive Examples • strcmp("A","a")/∗<0∗/• strcmp("IRONMAN","BATMAN")/∗>0∗/• strcmp("aA","aA")/∗==0∗/• strcmp("aA","a")/∗>0∗/18 Formatted inputint scanf(char∗ format ,...) is the input analog of printf. • scanf reads characters from standard input, interpreting them according to format specification • Similar to printf , scanf also takes variable number ofarguments.• The format specification is the same as that for printf • When multiple items are to be read, each item is assumed to be separated by white space.It returns the number of items read or EOF.• • Important: scanf ignores white spaces. • Important: Arguments have to be address of variables (pointers). 19 Formatted inputint scanf(char∗ format ,...) is the input analog of printf. Examples: printf ("%d",x) scanf("%d",&x) printf ("%10d",x) scanf("%d",&x) printf ("%f",f) scanf("%f",&f) printf ("%s",str) scanf("%s",str) /∗note no & required∗/printf ("%s",str) scanf("%20s",str) /∗note no & required∗/printf ("%s %s",fname,lname) scanf("%20s %20s",fname,lname) 20 String input/outputInstead of writing to the standard output, the formatted data canbe written to or read from character arrays.int sprintf (char string [], char format[],arg1,arg2)• The format specification is the same as printf. • The output is written to string (does not check size). • Returns the number of character written or negative value on error. int sscanf(char str [], char format[],arg1,arg2) • The format specification is the same as scanf; • The input is read from str variable. • Returns the number of items read or negative value on error. 21 File I/OSo far, we have read from the standard input and written to thestandard output. C allows us to read data from text/binary filesusing fopen().FILE∗ fopen(char name[],char mode[])• mode can be "r" (read only),"w" (write only),"a" (append) among other options. "b" can be appended for binary files. • fopen returns a pointer to the file stream if it exists orNULL otherwise.• We don’t need to know the details of the FILE data type. • Important: The standard input and output are also FILE* datatypes (stdin,stdout). • Important: stderr corresponds to standard erroroutput(different from stdout).22 File I/O(cont.)int fclose(FILE∗ fp) • closes the stream (releases OS resources). • fclose() is automatically called on all open files when program terminates. 23 File inputint getc(FILE∗ fp) • reads a single character from the stream. • returns the character read or EOF on error/end of file. Note: getchar simply uses the standard input to read a character. We can implement it as follows: #define getchar() getc(stdin) char[] fgets(char line [], int maxlen,FILE∗ fp) • reads a single line (upto maxlen characters) from the input stream (including linebreak). • returns a pointer to the character array that stores the line (read-only)return NULL if end of stream.• 24 File outputint putc(int c,FILE∗ fp) • writes a single character c to the output stream. returns the character written or EOF on error. • Note: putchar simply uses the standard output to write a character. We can implement it as follows: #define putchar(c) putc(c,stdout) int fputs(char line [], FILE∗ fp) • writes a single line to the output stream. • returns zero on success, EOF otherwise. int fscanf(FILE∗ fp,char format[],arg1,arg2) • similar to scanf,sscanf • reads items from input stream fp. 25 Command line input• In addition to taking input from standard input and files, you can also pass input while invoking the program. • Command line parameters are very common in *nix environment. • So far, we have used int main() as to invoke the main function. However, main function can take arguments that are populated when the program is invoked. 26 Command line input (cont.)int main(int argc,char∗ argv[]) • argc: count of arguments. • argv[]: an array of pointers to each of the arguments • note: the arguments include the name of the program as well. Examples: • ./cat a.txt b.txt (argc=3,argv[0]="cat" argv[1]="a.txt"argv[2]="b.txt"• ./cat (argc=1,argv[0]="cat") 27 MIT OpenCourseWarehttp://ocw.mit.edu 6.087 Practical Programming in CJanuary (IAP) 2010For information about citing these materials or our Terms of Use,visit: http://ocw.mit.edu/terms.