Chapter 08- printf and scanf

1. Chapter 08- printf and scanf • format indicators • advanced format control • printf • scanf

2. printf revisited • general form: printf(“format string”,arg0,arg1,...,argn); • “format string” contains literal text and format indicators (signalled by % sign). • formatters and arguments are matched from left. • printf does not automatically insert a new line. • Normally doesn’t display any data until newline is issued by program!

3. Common printf errors • use the wrong format indicator: • e.g. use a %d with a float variable (what happens?) • use the wrong number of formatters: • too few formatters - rightmost argument(s) will be ignored • too many formatters - garbage is printed in the place(s) given by the rightmost formatter(s)

4. Integer formatters %d signed decimal %i signed decimal (not really necessary) %u unsigned decimal %o unsigned octal %x unsigned hexadecimal (w/ lower case a-f) %X unsigned hexadecimal (w/ upper case A-F)

5. notes on integer formatters • By default, leading zeros are not printed • “0x” is not printed for hexadecimal • chars and shorts are promoted to full ints if displayed via an integer formatter • width of field varies depending on size of integer • %ld, %lu, ... for long ints • (the ‘l’ is a modifier, as we will see soon)

6. Real number formatters %f real number [-]mmm.dddddd(6 fraction digits) %e scientific form [-]m.dddddd e[+/-] xx %E scientific form [-]m.dddddd E[+/-] xx 7 digits total, with one before decimal point %g general purpose formatter (real or scientific) %G General purpose formatter (real or scientific) will use %f unless value <1.e-04 or >=1.e+07 in which case it uses %e for %g or %E for %G. %Lf, %Le, %Lg for long doubles

7. Other formatters %c single character %s string (print until \0 hit) %n prints total number of characters printed with printf to this point %p prints pointer/address (machine dependent) %% just prints a % sign

8. Advanced control of printf format • The general form of a formatter is: %[flag][width][.precision][modifier]format indicator • All [fields] are optional • flag controls justification and other properties • Width gives total length of display field • precision gives # characters in precision field • modifier short, double, and long control

9. Format flags Flag Operation Default 0 leading zeros leading spaces - Output is left justified Right justified + Always display sign only - sign <space> prints space or minus sign only - sign Note: + overrides <space> if both are given # adds leading 0 to octal format adds 0x (0X) to %x (%X) format forces decimal point in real formats (even if integer)

10. Field width specifier • Insert total number of characters: • printf(“%20s”,st1); /* yields a width of 20 */ • the field width is overridden if it is too small to accurately display variable. • adjustable field width is indicated by an * • you must give an argument in the list before the variable to give the (integer) length • printf(“%*s”,20,st1); /* equivalent to 1st example */

11. .precision specification • Indicated by a leading period: %.5f • %f, %e indicates # of digits to right of . • %g indicates total # digits in mantissa • %d, ... indicates minimum # digits printed • %s indicates maximum # of characters displayed (the rest is truncated) • %c, %p no effect

12. Precision comments • When used with field width: %12.5f • (field width is NOT necessary) • Variable precision • printf(“%.*f”,num_prec,num); • printf(“%*.*f,num_width,num_prec,num);

13. Program 8.1 #include <stdio.h> #define PI 3.141592654f int main(void) { /*demonstrates advanced formatting */ inti; char test[]="Hi World"; for (i=1;i<16;i++) { printf("%*s %+0*f\t\t",i,test,i,PI); printf("%.*s %+0.*f\n",i,test,i,PI); } return 0; }

14. final printf comments • type modifiers • h short integer • l long integer • L long double • h modifier exists with printf only for symmetry with scanf; short ints automatically promoted to int. • returned value • printf returns an int with the number of characters successfully printed or EOF (usually -1) for an error

15. scanf revisited scanf(“format string”,&arg1,...,&argn); • argument addresses must be given • returns an int of the # of successful conversions made (NOT # characters) or EOF (usually -1) if there is an error • The burden of insuring correct input is up to user. • Initial white space ignored, but separates fields. • scanf stops as soon as data is read or error occurs. Any and all unread characters remain for next scan!

16. Advanced control of scanf format • many of the formatters are identical with printf formatters. We will only describe differences. • %i will recognize any integer format (dec, oct, hex) • %hd must be used to read short ints • %f is only for floats • %lf is for doubles /* common source of errors */

17. Effect of literal text in format • Except for white space, any literal text in format string must be matched exactly by input source or scanf aborts: • scanf(“Radius = %lf ”,&radius); • Sometimes useful for disk file input, but there is typically a better way to ignore some input

18. The %* format indicator • Inserting an asterisk between % and any specifier tells scanf to ignore that entry: • scanf(“%*s, %lf”,&radius); • is better than previous alternative

19. Matching character sets • one can replace %s with a text string that contains a subset of all characters. scanf stops reading when any character outside that string is encountered. • scanf(“%[abcdefghijklmnopqrstuvwxyz]”,name); • reads only lower case letters • can be used to read spaces • can be abbreviated with hyphen • scanf(“%[a-zA-Z 0-9]”,name);

20. Excluding character sets • to exclude characters, use: • %[^char set] • To reject lower case letters use: • scanf(“%[^a-z]”,name); • to accept all characters, use • scanf(“%[^\n]”,name);

21. Field width specification • Field width, but not precision, can be specified with the same approach as printf: • scanf(“%4d %6f”,&i, &x); • This fixed format input is most useful with disk data.

22. Example 8.2 #include <stdio.h> /*********************************** * * * Demonstration of scanfformatting * * * * History : Date Reason * * 12/01/14 Created by W. Lawson * * * ***********************************/ int main(intargc, char *argv[]) { char name[80]; int test; printf("Input a string after each prompt\n\n"); printf("First, an unformatted scanf\n\n"); scanf("%s",name); printf("Result: %s\n\n",name); fflush(stdin);

23. Ex 8.2 continued printf("Now, with: scanf(\" %[^\\n] \",name);\n\n"); scanf("%[^\n]",name); printf("Result: %s\n\n",name); fflush(stdin); printf("Now, with: scanf(\" %[^a-z] \",name);\n\n"); scanf("%[^a-z]",name); printf("Result: %s\n\n",name); fflush(stdin); printf("Now, with: scanf(\" %[a-z WL] \",name);\n\n"); scanf("%[a-z WL]",name); printf("Result: %s\n\n",name); fflush(stdin);

24. Ex 8.2 continued printf("Now, with: scanf(\" %[a-zWL] \",name);\n\n"); scanf("%[a-zWL]",name); printf("Result: %s\n\n",name); fflush(stdin); printf("Now, with: scanf(\" %%*s, %%i \"&test);\n\n"); scanf("%*s, %i",&test); printf("Result: %i\n\n",test); fflush(stdin); return 0; }