Lecture 3: Statements and Flow of Control

• Last time we learned how to create C expressions using a variety of operators. Today we are going to look at how to create statements out of those expressions and to use C's flow control constructs to write interesting programs.

C Statements

• A C statement is an expression followed by a semicolon OR
• A series of C statements enclosed in curly braces (also called a block or compound statement)
• Either:
  p = q;
• Or:
• The empty statement(;) does remarkably little but is very efficient.

The If Statement

• the first branch of the C ``if'' statement is executed if the condition in parens is true
• the second branch is executed otherwise
• the else clause is optional
• Example:

  $$\backslash$$ } else {

  $$\backslash$$ }

More Examples of If

$$\backslash$$ else {   printf(``The more fascinating '');

$$\backslash$$ }

• Remember, a compound statement must be surrounded by curly braces

$$\backslash$$

$$\backslash$$ else

$$\backslash$$

• Oops! The last program is incorrect! CodeWarrior will issue an ``expression syntax error'' message. Why?

Else if

• Often you want to perform a series of tests.
• Just follow an else with another if:

Whiling Your Time Away

while (condition)

  statement

• The while loop statement executes as follows:

  1.

  Check the expression in parens.

  2.

  (a)

  If the condition in parens is true, execute the loop statement and repeat (ie: goto step 1)

  (b)

  If the condition is false, exit the loop.

• An example, computing factorial:

$$\backslash$$   return 0; }

For Loops

• The for loop is a very useful, specialized while loop:

  for (init; condition; op)

    statement

• The for loop above is equivalent to the following while loop:
• Factorial again:

  $$\backslash$$   return 0; }

More For Loops

• Using for loops we can write very compact code:

  $$\backslash$$   return 0; }

• ... but you should always strive for readability. Writing obfuscated code is very easy in C. Don't do it!

And More For Loops

• Notice the comma operator in the for loop:
• Arguments to the comma operator are evaluated left to right (here, we multiply and then subtract)
• The result of the comma operator is the result of the rightmost argument.
• Be careful not to confuse semicolons and commas!
• The comma is used occasionally in for loops and seldom elsewhere (except by mistake).

• Another interesting loop:
• We can leave out components of the for loop if we choose!
• If we leave out the condition, loop will repeat infinitely.

Functions

$$\backslash \! *$$    Inputs: a witticism number(w_num)    Side Effects: Prints witticism given by w_num

$$* \! \backslash$$ void print_witticism(int w_num) {

$$\backslash$$

$$\backslash \! * * \! \backslash$$ } int main() {   int x = 17, y = 3;   print_witticism(x);   print_witticism(y);   print_witticism(42);   return 0; }

Functions Cont'd ...

• Comment every function precisely!
• C functions are defined as follows:
  

  
  
    type function_name( param_list ) { ... }
  

  
  
  

  where param_list is:
    type₁ var₁, ..., type_n var_n

  
  
  

• The type before the function name is the return type of the function.
• The param_list is the list of parameters to the function with their types.
• C functions end with a return statement
• The main function should return an integer.
  • Return 0 if your program exits normally.
  • Return another number if there is an error.

Casts Revisited

• In the following example, we use a cast to help round:
  
  
  
• However, C also does automatic type converversion for function arguments and results. round1 above is equivalent to round2:
  
  
  

... And More Casts

• If you pass trunc a float, it will cast the argument to an integer, truncating it, and will convert the result back to a float:
  
  
  

  $$\backslash \! * * \! \backslash$$   return 0; }

The Switch Statement

• Switch is a multi-way choice statement.
• The expression is an integer or character expression.
• The const-expressions in the cases must all be different and can be in any order.
• The default is optional.
• Switch is evaluated as follows:

  1.

  Evaluate the expression

  2.

  Match the expression against each const-expression. If there is a match, execute that branch.

  3.

  If there is no match, execute the default.

  4.

  If there is no match and no default, do nothing.

An Example - The calculator

$$\backslash$$

$$\backslash \! * * \! \backslash$$       break;     }

$$\backslash$$   }

$$\backslash$$   return 1; }

More on Switch

Watch Out!

• break at the end of the case is not required BUT you should (almost) always put it in. If you forget it, execution will fall through to the next case
• Consider:
• When the program should execute the same branch for several different constant expressions, ``fall through'' intentionally:

  $$\backslash$$       break;     ...     }

More About the Break Statements

• Another use of the break statement is in loops:

  $$\backslash$$     scanf(``%c'', &stop);     if (stop == 'y')       break;   } return 0; }

Summary

• A statement: an expression followed by a semi-colon or a series of statements surrounded by curly braces.
• The If statement: The basic choice statement in C.
• The Switch statement: A multi-choice statement.
• The While statement: A looping construct
• The For statement: A second looping construct with initialization, and testing the exit condition ``built in''
• We also learned about C functions.