C Basics

Last time: Unix processes and redirection

Prof. Brian D. Davison

Computer Science & Engineering, Lehigh University

Announcements

• Short Quiz #2 today
• Reminder: Programming assignment #1 is due Monday. Questions?
• New: Homework #2 is posted, due Wednesday.

C versus C++

• C++ is mostly a superset of C
• So what are the major additions?

Major C differences from C++

• No classes or objects -- all code is in functions
• C structures cannot have methods
• I/O in C is based on library functions
• No function overloading
• No new or delete (use library functions instead)
• No reference variables (aliases)

Other Differences

• Variables must be declared at the beginning of a block, typically at the beginning of the function
• This is no longer true for C99
• No bool datatype
• No << and >> operators for I/O
• Cannot substitute and, or, and not for boolean operators &&, ||, and !
• Different approach to strings

Basic C Data Types

• Pretty much the same as in C++
  • int (%d, -40), short int, long int, long long int, unsigned int
  • float (%f, 3.14), double, long double
  • char (%c, 'a'), unsigned char, signed char
  
  
• Variable names are also similar to C++
  • made of letters, digits, underscore
  • cannot start with a number

Operators

• Arithemetic operators
  • Typical binary operators: + - * /
  • Unary + and -
  • Modulus % (e.g., 25 % 4 = 1)
• Logical (Boolean) operators
  • &&, ||, !
• Bitwise operators
  • &, |, ^, ~, <<, >>
• Relational operators
  • <, <=, >, =>, ==, !=
• Conditional operator

  expression ? expression1 : expression2 ;

Assignment Operators

• Assignment = (e.g., a = 4;)
• Modify and assign
• Increment ++ and Decrement -- (e.g., a++; --b;)
• Combinations *=, /=, %=, +=, -=, &=, ^=, |=, <<=, >>=

  a += 4;
  b <<= 1;
  

Type Conversions

• C permits (and performs) many kinds of type conversions
• e.g., implicitly (and explicitly) within assignments
• includes automatic type promotion and demotion

int i = 4.3;
float f = 3;
int g = 0;
int h = 0;

f = 3.7;
g = i + f + .4;
h = (int) i + (int) f + (int) .4;

• -Wall will warn you about some conversions if they are not explicitly cast (but not those above)

Type Conversions continued

• When a float converted to an int, the value is truncated (demotion)
• Integer arithmetic generates integer results (even when the result has a fractional value)
• If any value in an expression is of type float (or double, etc.) then the result will also be of that type
• Use of the type cast operator explicitly tells the compiler to do the conversion (and has higher precedence than anything but unary +/-)

  (int) 29.55 + (int) 21.99 is the same as 29 + 21 in C
  

Control Flow

• Loops

while (expression)
   statement

for (expr1; expr2; expr3)
   statement

do
   statement
while (expression);


The break statement drops control out of the innermost loop while continue moves on to the next iteration.
• Conditionals

if (expression)
   statement1
else
   statement2

switch (expression) {
   case const-expression: statements
   case const-expression: statements
   default: statements
}

C Arrays

• Assuming we know the number of elements, we can easily create and manipulate arrays of items.

  int i[5];
  
  i[0] = i[1] = 1;
  i[2] = 2;
  i[i[2] + 1] = i[2] + 2;
  i[i[2] + i[2]] = i[3] + i[2];
  

• Unfortunately, we cannot set all values of an array at once, nor assign one array to another. Instead, we must iterate through each item.

  int i, a[10];
  
  // clear or copy all values
  for ( i = 0; i < 10; i++ ) {
     a[i] = 0;  or   a[i] = x[i];
  }
  

Array Initialization

• We can, however, initialize the elements of an array when declared.
• All of these are equivalent:
  
  

  unsigned short int a[10] = {1, 2, 3, 4, 5, 6, 0, 0, 0, 0};
  unsigned short int a[10] = {1, 2, 3, 4, 5, 6};
  unsigned short int a[] = {1, 2, 3, 4, 5, 6, 0, 0, 0, 0};
  unsigned short int a[] = {[0]=1, [1]=2, [2]=3, [3]=4, [4]=5, 
                             [5]=6, [9]=0};
  

  
  
• The number of elements can be inferred from the initializers, and missing initializers are always set to 0.

Multidimensional Arrays

• You can have arrays of any type, including arrays of arrays.

  int i, j;
  float a[3][10] = { {14.2, 1, 0, 9.4},
                     {0, 144, 7, 2.5},
                     {2, 3, 109, 42.0} };
  

• Could print out contents:

  for (j = 0; j < 10; j = j + 1)
     printf("\t%d:", j);
  printf("\n");
  
  for (i = 0; i < 3; i = i + 1) {
     printf("%d:", i);
     for (j = 0; j < 10; j = j + 1)
          printf("\t%f", a[i][j]);
     printf("\n");
  }
  

• Note that we could have gone in the opposite order (2nd subscript first).

Character Arrays

• An array of characters works the same. We can create and initialize them with the same syntax.

  char hello[] = {'h', 'e', 'l', 'l', 'o', '!'};
  

• There are many characters that are not letters, numbers, or keyboard symbols. Such characters are represented by an escape sequence using the backslash.
• Common characters include:

    \n    a "newline" character (linefeed, ctrl-j)
    \b    a backspace
    \r    a carriage return (without a line feed, ctrl-m)
    \'    a single quote (e.g., in a character constant)
    \"    a double quote (e.g., in a string constant)
    \\    a single backslash
  

• Character constants always use single quotes.

Looking Ahead

• Monday's lab will be in PL502 and we will cover more unix commands.
• Continue with mycat (if time)
• Previously we wrote a simple mycat program.
• Let's revise it to include line numbers, ala cat -n.
Demonstrate error handling!