Control Statements (Iteration-II)

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Control Statements (Iteration-II)

• Lesson 7

DEPARTMENT OF COMPUTER SCIENCE FACULTY OF SCIENCE
AND TECHNOLOGY ASSUMPTION UNIVERSITY
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Agenda

• Understanding scope of variables
• Nested loops

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Variables

• All variables have two important attributes which
  are scope and storage
• The scope of a variable is the area of the
  program where the variable is valid
• The storage of the variable explains how long
  does the variable will stay in the memory

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Blocks

• A block is a section of code enclosed in curly
  braces ( )
• e.g.
• int main()
• //something here
• return 0

Block
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Local Variables

• Local Variables are variables that are declared
  in a block

int main() int n //n is a local
variable return 0
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Local Variables (cont.)

• A local variables scope is limited to the block
  where it is declared and cannot be accessed
  outside the block
• int main()
• int no //no is a local variable to this block
• no //LEGAL!
• no //ILLEGAL!
• no //ILLEGAL!

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Local Variables (cont.)

• Notice that in for loops, local variables can be
  created and used, and you have to be careful
  about its scope

for (int count 0 count lt MAX count)
sum count // count is out of scope from
here on. cout ltlt count //ILLEGAL!
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Local Variables (cont.)

• Local Variables that are not static are
  considered as temporary variables
• Each time the block is entered, the temporary
  variables are initialized if required
• Temporary variables are allocated from a section
  of memory called the stack at the beginning of
  the block
• The space used by the temporary variables is
  returned to the stack at the end of the block

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Global Variables

• Global Variables are special variables that are
  valid from declaration until the end of the
  program
• int global //global variable
• int main()
• global //can be used everywhere

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Global Variables (cont.)
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Global Variables (cont.)

• It is possible to declare a local variable with
  the same name as a global variable
• This is called naming clash
• However the order which C/C understands the
  name is to pick names from the most local space
  first and gets back until it reaches global

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Global Variables (cont.)

• Observe count in the next example which one is
  the global, another a local variable

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Global Variables (cont.)

• Global Variables are always permanent in storage
• They are created and initialized before the
  program starts and remain until it terminates

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Static Variables (Optional)

• By adding an optional static keyword in front of
  your variable will cause an additional define
  about how your variable will behave in your
  program.
• The table in the next slide will explain how it
  would behave in different situations?

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Nested loops

• When one loop structure is completely contained
  in the body of another, the loops are said to be
  nested.
• That is, in the body statement of a loop can be
  another loop.
• A nested loop compose by outer loop and inner
  loop (s).

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Design nested loops

• Begin with an outer loop
• In the body of the outer loop, where the inner
  loop appears, make it as a separate module.

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Nested while loops

• In nested while loops, the entire loop body is
  executed for each of the values of the outer loop
  control variable.
• Thus for each value of the outer loop control
  variable, the inner while loop will run through
  all of its value.

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Nested while loops

• //initialize outer loop
• while ( outer loop condition )
• //outer loop statement
• . . .
• //initialize inner loop
• while ( inner loop condition )
• //inner loop statement
• . . .

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Nested while loops Example
include ltiostreamgt using namespace std int
main() int n cout ltlt "Input
times" while(cin gtgt n) while(n gt 1)
cout ltlt "Hello\n" --n
cout ltlt "Input times"
exit(1) system("PAUSE") return 0
Outer loop
Inner loop
End of Inner loop
End of outer loop
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Nested for loops

• In nested for loops, the entire loop body is
  executed for each of the values of the outer loop
  control variable.
• Thus for each value of the outer loop control
  variable, the inner for loop will run through all
  of its value.

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Nested for loops

• //initialize outer loop
• for(LCV value test update)
• for(LCV value test update) //inner loop
• //inner loop statement(s)

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Nested for loops Example
include ltiostreamgt using namespace std int
main() for ( int k 1 k lt 5 k)
for ( int j 1 j lt 3 j) cout ltlt k
j ltlt endl system ("PAUSE") return
0
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Class work

• Write program that print out the table of
  multiplication by using nested loop like below .
• 1 2 3 4 5
• 2 4 6 8 10
• 3 6 9 12 15
• 4 8 12 16 20
• 5 10 15 20 25

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Class work

• Printout the following patterns (a) to (d), by
  asking the user to key in the number of rows
  (columns)
• (a) (b) (c) (d)

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Problem solving using loops and conditionals

• Display both biggest and smallest numbers from a
  set of user inputs until -1 by the user (-1 is
  omitted from the final result).

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Class Exercise

• Write a program to print out all prime numbers
  including 2-1000.

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Loop Testing and Debugging

• Test data should test all sections of the
  program.
• Beware of infinite loops -- program doesnt stop
• Check loop termination condition, and watch for
  off-by-1 problem.
• Use get function for loops controlled by
  detection of \n character
• Use algorithm walk-through to verify pre- and
  post conditions.
• Trace execution of loop by hand with code
  walk-through
• Use a debugger to run program in slow motion or
  use debug output statements