Bits, bytes and memory

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Bits, bytes and memory

• Your computer's memory can be seen as a sequence
  of cells.
• Each cell is 8 bits (one byte) large. Data is
  stored by setting these bits to 1s and 0s.
• Each cell has an address. You don't need to know
  (or care to know) what this address is. Your
  system uses the address to locate the data stored
  there.

0
1
2
Address ? ? ?
storedbytes
3
4
5
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Variables

• We need to be able to store data in memory,
  during the execution of our program.
• We also need to be able to access and even modify
  this data.
• Solution variables
• A variable is a reserved location in memory that
• has a name
• has an associated type (for example, integer)
• holds data which can be modified

3
Variables

• In order to use a variable in our program we must
  first declare it.
• HOW?
• A declaration statement has the format type
  variable_name
• type what kind of data will be stored in that
  location (integer? character? floating point?)
• variable_name what is the name of the variable?
• semi-colon this is a statement!
• WHERE?
• At the beginning of a function, right after the
  opening brace.

4
Variable names

• In C, variable names are built from
• the letters of the alphabet
• the digits 0 through 9
• the underscore
• A variable name must start with a letter or an
  underscore
• A variable name must not be the same as a
  reserved word used by the C language.
• Only the first 31 characters of a variable name
  are significant. The rest are ignored.

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Variable names

• Selecting good variable names is important for
  program readability.
• A variable name must be descriptive of the data
  that will be stored in the variable.
• It must not be too long.
• It must not be a single character (there is one
  allowed exception to this rule, which we will
  talk about later)

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Variable names

• Good, legal variable names
• totalArea temp_in_F
• counter1 isEmpty
• num_trees pNuts
• Legal, but bad variable names
• l11 (is it L11, L1L, LL1, or LLL?)
• x (what does it mean?)
• maximum_number_of_students_in_my_class
• a23456789_123456789_123456789_12345678
• Illegal variable names
• product main not-this
• total 3rd

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Variable types

• We've already seen a type, int, which is used for
  integers (more types coming up...)
• Here's a sample program that declares a variable
  that will hold an integer

/ program that demonstrates a variable
declaration. / include ltstdio.hgt int main ()
int num_students return 0
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Variable values

• After a variable has been declared, its memory
  location contains randomly set bits. In other
  words, it does not contain valid data.
• The value stored in a variable must be
  initialized before we can use it in any
  computations.
• There are two ways to initialize a variable
• by reading its value from the keyboard using
  scanf
• by assigning a value using an assignment
  statement (more on that later)

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Keyboard input scanf()

• scanf() will scan formatted input from the
  keyboard.
• It uses special format specifiers that specify
  the type of the variable whose value is to be
  read from the keyboard.
• To read an integerint num_studentsscanf("d",
  num_students)

Place value into this variable
Formal specifier for read an integer value
VERY IMPORTANT special symbol
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Printing variable values

• printf() will print formatted output to the
  screen.
• To print a message printf("This is a
  message\n")
• How do we print the value of a variable?
• Answer Use format specifiers depending on the
  type of the variable (similar to scanf)

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Printing variable values

• To print an integer

printf("The temperature is d degrees.",
degreesF)
Specifier for print an integer value
Read value from this variable
IMPORTANT degreesF MUST be initialized so that
it contains a valid value.
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Example
/ sample program that demonstrates declaring a
variable, assigning a value to it using scanf
and printing that value using printf
/ include ltstdio.hgt int main () int
num_students printf("How many students are
there? ") scanf("d", num_students)
printf("There are d students.\n") return 0