C Programming class week 2

C by Discovery 4th edition by Foster and Foster

C By Discovery (4th Edition)

Chapter 2 – Gaining Control

1. Expressions and Statements
   Expressions are sequences of tokens that can be evaluated to a numeric quantity.  A statement is a sequence of tokens that end with a semicolon and can be recognized by the compiler. Statements may not have values.
   Expressions in C are lvalues, an expression that has a location in memory, or rvalues, which can be evaluated by not changed.  X would be a lvalue, X + 3 would be an lvalue. 
2. Blocks and Compound Statements
   A compound statement is a sequence of statements.  A block can contain compound statements as well as declarations. It must have opening and closing braces ( { } ).  It does not necessarily have to be a function but can reside in a function. If a variable is declared in the block, when the block is exited it does not exist.  This makes it a local variable.  The block can access any variables that are in the block that it exist in. 
   1. A word about style
      Not good programming style to
      A) use variables with the same name close together.
      B) All variables inside and outside blocks should be given different names
      C) A program is generally more readable if all declarations for a function appear together in one place.
3. The if and if-else statements
   A conditional statement allows for a program to test a condition and then go in a certain direction depending on if it is true or false.  The if statement is one of these:
   
   if (expression)
   statement
   
   Expression will be a test to determine true or false, statement can be one line or a block.  Style says that after the test for the if, the line(s) that you would execute should be on the next line and indented slightly to show they are part of the if.  Along with the if statement the if else is used. 
   
   if (expression)
       statement
   else
        statement
   
   1. Compound if-else statement
      The if can be used as a series of tests as well.
      
      if (expression)
         statement
      else if (expression)
           statement2
      else if (expression)
           statement3
      ……….
      else
           last_statement
      
      When this is coded properly only one statement will be run. 
      It is possible to nest an if..else statement inside another one.  This should be done with braces to identify the statements that go with each other.  You are basically checking for the truth of one statement and the truth of another.
      
4. Relational operators and expressions
   It is possible to test for relationships between variables.  They will either return a true or a false.  The relationship expressions in C are:
   

   Expression	what it compares
   <=	less than or equal to
   >=	greater than or equal to
   ==	is equal to
   !=	is not equal to
   >	greater than
   <	less than

   
   x == 3 would return a true id x was equal to 3.

4.1 A word of warning
It is easy to make the mistake of assigning a variable in place of testing it. For example


   1: invar =3;






   2: if (intvar = 4)






   3:     printf("the number is 4\n);







will print the “number is 4” because you are assigning the number not comparing it.

4.2 Operator precedence

  Each side of the relationship will be calculated before it will be compared using the normal mathematical relationships tested before.

5. The while loop

Sometimes we may want to do a certain step several times (called  iterations). To do this we will use a while loop. 

   1: while (expression)

   2:     statement(s);

The first time it runs, the expression is evaluated.  If it is true the statements will run.  There will be some change of the value as part of the statements and then it will be tested again and process repeated.  Eventually it will test false and then drop out of the loop. 

5.1 using getchar() and putchar() in a loop.

getchar() and putchar() are special inputs for single characters.  This can be used in the while loop by using and EOF or end of file character. This will vary by operating systems but the popular ones are cntrl-d for Linux/Unix and ctrl-z for windows.  You can compare the input received to the EOF and terminate if it receives it.  The variable is of type int even though we are getting a character.

6. Logical operators and expressions

Another set if ways to test are:

!	logical not	true if statement is false, false if statement is true
&&	logical and	true if both are true
||	logical or	true if one side or both are true

They are evaluated in the order shown and in a left to right progression.

7. The for loop

   1: for (initialize_variable; test_to_perform; how_change_initial_value)

   2:     statements

 

8. Making C readable – programming style

8.1 Choice of identifiers

X, y and z may be legal choices, but having the variable describe what it does helps to make the code make sense later. Also a good idea to make constants all upper case.

8.2 Indentation

Indenting lines of codes in if/then and for loops make the code easier to understand.

8.3 Placement of braces

Two styles here. One is to place the open brace right after the test and the close brace as first character on line after it ends. The other is to put the open and close as the first characters on the first and last line of the block. 

8.4 Block structuring

Short blocks of code that do a single task are easier to read than larger ones.  It also makes larger programs easier when the get large and complex.

8.5 Use of parentheses

While the code may be right for the compiler, adding parentheses to math equations may not help the compiler but it may help the person trying to read it later (and that may be you). 

8.6 Comments

Use of comments is necessary to explain to a future person maintaining it what is going on. 

8.7 Paragraphing

It helps in reading the program if you break the parts of it that are similar in nature into manageable chunks. For example, the code that initializes variables might have a few blank lines after it and before the while loop to get input.

8.8 Whitespace

Whitespace, blank characters that add nothing to the code, can make it much easier to read.

Introduction to C