Learn C in 15 minutes
(if you already know C++)

C is essentially a subset of C++. Here are some differences between the two languages.

• C has no classes or anything to do with classes e.g.,
  • no constructors
  • no destructors
  • no inheritance
  • no operator overloading
  • no templates
  Although there is no concept of classes, there is a struct. A struct is like a class which can have data members, but no member functions, and everything is public. For example:

  struct Student {
       char firstname[32];
       char lastname[32];
       int ID;
       double GPA;
  };
  .....
  int main()
  {
     struct Student Suzy;
     strcpy(Suzy.lastname,"Creamcheese");
     Suzy.ID = 234567;
     ...
  

  Notice that whenever you declare a structure, you have to precede it with the word struct.

• Instead of using new, use the system call
  void* malloc(int n)
  This works like new in that it gets new memory from the heap and returns a pointer. The argument n is the number of bytes of memory. You have to cast the function to the correct type. Here are some examples;

  char *cptr;
  double *dblptr;
  struct Student *stuptr;
  
  cptr = (char *)malloc(100); /* equiv to cptr = new char[100]; */
  dblptr = (double *)malloc(sizeof(double) * n);
      /* equiv to dblptr = new double[n]; */
  stuptr = (struct Student *)malloc(sizeof(struct Student) * 47);
      /* equiv to stuptr = new Student[47]; */
  

• Input and output are done quite differently in C. You cannot use the << and >> operators and cin and cout are not defined. You must use the library functions scanf() and printf(). These functions take variable numbers of arguments. The first argument is a format string (char *), and the remaining arguments are variables to write from in printf or read to in scanf.

  In the format string for printf, all characters are printed as themselves except those following the percent character (%). The percent characters indicate that the value of a variable should be printed. The percent sign should be followed immediately by a letter indicating the data type, d for integer, c for a character, s for a character string, and f for a float or double. There should be one argument for each % in the format string.

  Here are some examples of printf

  printf("Hello World\n");  /* prints Hello World followed by a carriage
                               return, known in Unix as a newline char */
  int x, y,;
  char c = 'A';
  char *s="Hello World";
  x = 17;
  printf("The value of x is %d", x);  /* prints The value of x is 17 */
  y = 430;
  printf("The sum of %d and %d is %d\n",x,y,x+y);
                             /* prints The sum of 17 and 430 is 447 */
  printf("The ascii value of %c is %d\n",c,c);
                             /* prints The ascii value of A is 65 */
  printf("%s\n",s);          /* prints Hello World */
  

  The function scanf() reads data from the keyboard, and works the same way.

  char name[32];
  int age;
  printf("Please enter your name: ");
  scanf("%s",name);
  printf("Please enter your age: ");
  scanf("%d",&age); 
  

  Note that you must pass the address of the variable name except in the case of strings for scanf variables.

  There are also a number of functions which read from standard input (the keyboard by default)

  char *gets(char *buffer) reads a line into buffer up to a newline character. This is a dangerous function to use because it is easy to overflow the buffer if the line read in is longer than the buffer size.

  int getchar() reads a single character from standard input. It returns the character.

  int putchar(int c) writes the character c to standard output (the terminal by default)

  int puts(char *s) writes the string s to standard output

• There are no reference variables in function arguments in C. To simulate call by reference, pass the address of a variable to the function. Here is an example:

  C program

  C++ equivalent

  
  

  void fctn(int x, int *y)         void fctn(int x, int& y)
  {                                {
      x = 17;                           x = 17;
      *y = 46;                          y = 46;
  }                                }
  
  int main()                       int main()
  {                                {
      int a = 12;                       int a = 12;
      int b = 3;                        int b = 3;
      fctn(a,&b);                       fctn(a,b);
      printf("%d %d\n",a,b);            cout << a << ' ' << b << endl;
      return 0;                         return 0;
  }                                }
  

  
  Both of these examples print the same output 12 46
• The include file for I/O is stdio.h, not iostream.h
• The suffix on C source files is .c, not .cpp. This is important, because many compilers (including Microsoft Visual Studio) invoke a different compiler depending on the suffix of the course code.