C Syntax Review and Exercises

Next, let's play around with the syntax and observe what the compiler reports.

In-Class Exercise 0.11 Try to compile and run this program.

int main ()
{
  // Didn't declare "i"

  // Assign it a value. Note the assignment operator "=" 
  i = 5;

  // Print out its value to the screen. 
  printf ("%d\n", i);
}
 

In-Class Exercise 0.12 Try to compile and run this program.

int main ()
{
  // Declare an integer variable called "i". 
  int i;

  // Wrong assignment operator
  i := 5;

  // Print out its value to the screen. 
  printf ("%d\n", i);
}
 

• Good programming style requires that variable names be chosen for readability.

• Variable names like i are usually only appropriate for loop (control) variables.

• What kinds of variable names are allowed?
  The rules that apply to variable names also apply to method and parameter names, the so-called identifiers.

About identifiers:

• An identifier must begin with a letter or underscore

• An identifiers can contain any number of letters, digits, or underscores.

• Case (upper or lower) is significant.

• No C reserved word can be used as an identifier.

• Examples of legal identifiers:
  i
  really_important_integer_for_counting_lines_in_a_text_file (bad style)
  num_lines_in_file (acceptable, old C style)
  numLinesInFile (recommended, new C style)

More about variable declaration syntax:

• Variables can be assigned values in a declaration:

  int main ()
  {
    // Declare an integer variable called "i" and assign it a value.
    int i = 5;
  
    // Print out its value to the screen. 
    printf ("%d\n", i);
  }
    

• Declarations can occur anywhere in a program body in C99 but not in earlier versions.
• Multiple variables (of the same type) can be declared in a single statement:

  int main ()
  {
    // Declare multiple variables of the same type. 
    int i = 5, j = 6, k;
    
    k = 7;
  
    printf ("i=%d  j=%d  k=%d\n", i, j, k);
  }
    

  The preferred style is either:

  int main ()
  {
    int i = 5;  // i is the variable that .... 
    int j = 6;  // j ... 
    int k = 7;  // k ... 
  
    printf ("i=%d  j=%d  k=%d\n", i, j, k);
  }
    

  or

  int main ()
  {
    int 
      i = 5,  // i is the variable that .... 
      j = 6,  // j ... 
      k = 7;  // k ... 
  
    printf ("i=%d  j=%d  k=%d\n", i, j, k);
  }
    

More Data Types

While there are seven additional data types, we will first focus on a few:

• double
  • Use double to store floating point numbers, as in:

    int main ()
    {
      double 
        pi = 3.14159,      // The constant Pi 
        radius = 2.0E03,   // Circle radius in exponent format. 
        area = 0;          // Area, not yet computed. 
    
      area = pi * radius * radius;
    
      printf ("Area of circle with radius %lf is %lf\n", radius, area);
    }
     

  • double constants are decimal values (3.14159), or in exponent notation (2.0E03).

• char
  • A char variable holds a single character, e.g.,

    int main ()
    {
      char 
       initial1 = 'J',
       initial2 = 'F',
       initial3 = 'K';
    
       printf ("Initials: %c %c %c\n", initial1, initial2, initial3);
    }
     

  • "Backslash" combinations are used to represent special characters:

    int main ()
    {
      char initial1 = 'J';
      char initial2 = 'F';
      char initial3 = 'K';
      char newline = '\n';
      char tab = '\t';
    
      printf ("%c Initials: %c \" %c %c %c \" %c", newline, tab, initial1, initial2, initial3, newline);
    }
     

  • Notice the double quote character inside the string "Initials: %c \".

• Strings
  • Strings in C are really char-arrays, pointed to by a variable of type char *
  • Example:

    int main ()
    {
      char *str1 = "Hello";
      char *str2 = "World!";
    
      // String constants can be concatenated by whitepace:
      char *str3 = "Hello" " again";
    
      printf ("%s %s \n %s \n", str1, str2, str3);
    }
    
     

int main ()
{
  int i = 5, j = 6;
  double x = 2.718,  y = 3.141;

  // Implicit cast: 
  x = i;
  printf ("x = %lf\n", x);   // Prints 5.0 
    
  // Explicit cast: 
  j = (int) y;
  printf ("j = %d\n", j);   // Prints 3 
}
 

int main ()
{
  printf ("%lf\n", ( (double) (int) 3.141 ) );
}
 

int main ()
{
  int k = 13;
  int i;

  for (i=0; i < 8; i++) {
    printf ("%d ", (k % 2));
    k >>= 1;
  }
  printf ("%\n");
}
 

    if (i = 5)
      printf ("i is equal to 5");
 

   int i = 10;
   while (i >= 1)
     printf ("i=%d\n", i);
     i--;
 

int main ()
{
  int i = 5;
  printf ("The square of %d is %d\n", i, squareIt (i));
}

int squareIt (int inputInteger)
{
  return (inputInteger * inputInteger);

}
 

int squareIt (int inputInteger)
{
  return (inputInteger * inputInteger);
}
  
int main ()
{
  int i = 5;
  squareIt (i);    // Return value not used.
}