This document introduces the C++ programmer to the rudiments of C.   For those who wish to learn more about C, refer to Appendix B of  the COSC220 text: Data Abtraction & Structures Using C++ for a comparison of C and C++.   An excellent reference on C is THE C PROGRAMMING LANGUAGE, 2nd ed (or later?)  by Kernighan and Ritchie, Prentice Hall.

This document looks only at:

Formatted I/O
File processing
Parameter passing
Structs
A look at some C libraries

scanf and printf require the stdio.h library.

scanf
          Reads from keyboard, ignoring blank space.

Except for strings, actual params must be preceded by '&'.
    That is, args must be pointers!
Must specify argument types, in order, in a string using '%'.
 
%d              (decimal) integer, base 10)
%i                integer     (may be octal (leading 0) or hex (leading 0x)).
%u               unsigned integer
%c               character
%s               string         Puts '\0' at end for you.
%e,%f,%g   Optional sign; opt decimal point; optional exponent.
Numerics can be preceded by 'h' to indicate short, 'l' to indicate long, as in
          %li for long integer
d,i,u, etc are called "conversion chars".

Examples:
 

To read in 25 Feb 1999 with declarations int day, year; char monthName[20];
    ...
    scanf("%i %s %i", &day,monthName,&year);

To read in 02/25/99 with month as type int, you can embed single characters (such as
the slash / ) in the string, as in --
    ...
    scanf("%i/%i/%i", &month,&day,&year);

printf
The output function printf translates data values into strings of characters to be displayed at the screen.
If only a literal string is to be displayed, no conversion characters are needed.
Each conversion specification begins with a '%' and ends with a conversion character, just as scanf does.
Between the '%' and the conversion character there may be, in this order:
 
. A minus sign to specify left justification
. A number to specify field width
. A period to separate field width from precision
. A number for precision to specify:
 
Max # of charas to be printed from a string OR
Number of digits after a decimal point OR
A floating point value OR
The minimum # of digits for an integer.
 
conversion char             argument             type

d,i                                 int                      integer  (base 10)
u                                   int                       unsigned integer
c                                   int                       single character
s                                   char*
f                                   double                 double floating point
e,E                               double                 double floating point         Ex: -5.43234e+3
g,G                              double                 double floating point in exponent notation

                                                            Trailing zeroes or decimal points not printed.
                                                            Use %e or %E if exponent < -4 or >= the precision;
                                                            otherwise use %f.
x,X                                int                      Unsigned hexadecimal (no leading 0x) with a-f or A-F.

Ex:  Let age=29 and ht=70:
            printf("His age is %i and his height is %i inches.", age, ht);
 
            Output:    His age is 29 and his height is 70 inches.

See printf.c and tstio.c on the k drive and in mlmalone's demos directory on ssu037.

See TstFiles.c on the k drive and in mlmalone's demos directory on ssu037.

stdio.h must be included.
A file variable is a pointer to the type FILE, as in FILE* inputFile;

To OPEN a file:
 

inputfile = fopen("filName", "r");
 
"r"     open text file for reading
"w"    open text file for writing (discards previous contents)
"a"     open text file for appending (creates new if doesn't exist already)
"r+"   open text file for update (read and write)
"w+"  open text file for update(discards existing contents)
"a+"   open text file for update; writing at the end.
 
If the file name (logical, not physical) is NULL after you've tried to open it, that means that the fopen failed.
            ...
inputFile = fopen("filName","r");
if (inputFile == NULL)
    ...     /* print error message*/
 
To CLOSE a file:
 
fclose(inputFile);
 
To read from a file, use fscanf:
 
Use the same formatting as for reading from the keyboard (standard input) but use the filename as the first parameter to fscanf, as in--
          fscanf(inputFile,"%s %i", name, &age);        where name is a string of characters.

You can use fscanf as a function, returning an integer which can be tested for the end-of-file constant (as defined in stdio.h, typically -1), as in--

          status = fscanf(inputFile,"%s %i", name, &age);
          while (status != EOF)
                ...         /* continue processing the file */
 

To write to a file, use fprintf:
 
Use the same formatting as for printing to the screen (standard output) but use the filename as the first parameter to fprintf, as in--
        fprintf(outputFile,"%15s%6i\n", name, age);

Pass-by-value:     Same as in C++.

Pass-by-reference:
        Nothing required for strings of characters (as in C++) or other arrays!
        For all other data:
 

Pass in the address of the value:
        Ex of a call:         Swap (&num1,&num2);

The formal parameter is expressed as a pointer:
        Ex:                      void Swap (int* num1, int* num2);

The prototype:
        Ex:                      void Swap (int*, int*);

Every reference to the pass-by-reference parameter inside the procedure must use pointers:
        Ex:

void Swap (int* num1, int* num2)
{
        int temp;
        temp = *num1;                 <--Except for arrays,  you must explicitly dereference
        *num1 = *num2;                   a pass-by-reference parameter in the body of the subroutine
        *num2 = temp;
}
 
See TestSubs.c on the k drive and in mlmalone's demos folder on ssu037.   It shows the syntax for passing a pass-by-reference parameter in a subroutine to another subroutine as pass-by-reference.

See the TstStruc.c demo on the k drive and in mlmalone's demos folder on ssu037.

The only difference betwen C and C++ here is that once you've defined a struct data type, when you declare a variable to be of that struct data type you must still use the word "struct" at the beginning of the declaration. C++ does not require that you do that.

Ex:     typedef  struct  someStruct
                                {
                                ...                 <-- the fields declared here (same as in C++)
                                };

          struct someStruct s1,s2;                        <-- declares s1 and s2 as type someStruct.
          struct someStruct ListOfStructs[10];     <-- declares an array of 10 structs, each of type someStruct

Library         Purpose                                     Exs of functions and procedures                                    

stdio.h             I/O functions                         fopen, fclose, rename, printf, scanf, fprintf, fscanf, sscanf,
                                                                    fflush, remove, sprintf, vprintf,  fgetc, fgets, fputc, fputs,
                                                                    getc, getchar, gets, purc, putchar, puts, ungetc, fseek, ftell,
                                                                    rewind, fgetpos, fsetpos, clearerr, feof, ferror, perror, ...

ctype.h             Testing chars (Similar to C++)
                                                                    isalnum, isalpha, iscntrl, isdigit, isgraph, islower, isprint,
                                                                    ispunct, isspace, isupper, isxdigit, toupper, tolower, ...

string.h             For strings (Same or very close to C++)
                                                                    strcpy, strncpy, strcat, strncat, strcmp, strncmp, strchr,
                                                                    strrchar, strspn, strcspn, strpbrk, strstr, strlen, strerror, strtok, ...

math.h             Math functions (Same or very close to C++)
                                                                    sin, cos, tan, asin, acos, atan, atan2, sinh, cosh, tanh, exp,
                                                                    log, log10, pow, sqrt, ceil, floor, fabs, ldexp, frexp, modf, fmod, ...

stdlib.h             Utility functions                     atof, atoi, atol, strtod, strtol, strtoul, rand, srand, calloc, malloc,
                                                                    realloc, free, abort, exit atexit, system, getenv, bsearch, qsort,
                                                                    abs, labs, div, ldiv, ...

time.h               Day, time functions               tm_sec, tm_min, tm_hour, tm_mday, tm_mon, tm_year,
                                                                    tm_wday, tm_yday, tm_isdst, clock, time, difftime, mktime,
                                                                    asctime, ctime, gmtime, localtime, strftime, ...

limits.h, float.h     Max,min for integer and floating point types.

assert.h               Allows use of assertions. (Similar to C++)

signal.h                Functions for signal processing, handling interrupts.

           And others...