CSC 215

1

CSC 215 Introduction to Programming with
C Instructor Eric Sedgwick
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HW

• Distribute grades
• automated mailing
• disagree with grading? - email me
• wrong email? - email grader
• Comment must include your name.
• Submission
• to ctigrader1_at_hotmail.com
• cc esedgwick_at_cs.depaul.edu
• subject CSC215HWn
• Did you get a receipt?

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Last Time

• Calculations
• variables/types/memory
• type checking
• Branching
• if-else
• if
• nested if-else
• Looping
• while ()
• do while ()

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This time

• Programming goals (I.e., receiving a good score
  on your hw)
• More flow control (Chapter 7)
• Multiway branching - switch
• More loops - for()
• Functions (Chapter 3)
• predefined
• user defined?

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Programming goals

• Turn in assignment complete and on time
• Source code
• Perform task correctly and efficiently
• Code should compile w/o errors or warnings
• More than one right method.
• Test your program
• Source code should be (grader) friendly
• Object code should be (user) friendly
• I/O goals see end of Lecture 1

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Friendly source code

• Header comment
• Including a description of the program
• your contact info
• Comments
• Include comments with boolean/logical expressions
• Comment anything that is hard to understand
• Comment anything that is ambiguous
• Style (follow what the book does)
• Whitespace group related statements, separate
  unrelated ones
• Indentation indent loop and if-else bodies
• Braces put them on new lines and indent body
• Constants use them.
• Meaningful names for variables and constants

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Multiway branching

• (Chapter 7)

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switch statement

• To perform multiway branching
• switch (controlling_expression)
• case Constant_1
• Statement_Sequence_1
• break
• case Constant_2
• Statement_Sequence_2
• break
• case Constant_n
• Statement_Sequence_n
• break
• default
• Default_statement_sequence

• Computer
• Evaluates controlling expression
• Matches the value with the constants.
• Starts executing after matching constant.
• Terminates switch when a break statement is
  encountered
• Executes default if no constant is matched.

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switch statement

• weight3.cpp -gt weight4.cpp
• Change user input to use a switch

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switch statement

• controlling_expression should evaluate to a char
  or int (or enum or bool)
• Constants must be char or int constants (no
  variables allowed)
• Dont forget breaks, otherwise subsequent cases
  are also executed.
• Group cases without breaks, when constants are
  equivalent. E.g,
• Case y
• Case Y
• Statement_sequence
• break
• Include a default case.

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switch or if-else
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Exercise

• Using a switch, write a program that lets the
  user input any one of the first 4 letters of the
  alphabet (upper or lowercase) and informs the
  user which letter of the alphabet it is.
• Sample output
• Please enter a letter c
• c is the third letter of the alphabet.

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for loop

• for(initialization boolean_expression
  update_action)
• body_of_loop
• Explanation
• Initialization - initializes variables that are
  used in boolean_expression
• Boolean_expression test for executing loop
• Update_action executed at the end of the loop
  body
• Body_of_loop - statements to be executed if
  boolean_expression is truethis can be a single
  statement or a compound statement using .
• Example
• n5
• for(count1 count lt n count)
• cout ltlt count
• Output
• 1 2 3 4 5

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for loop

• Typically used for counting
• Maybe counting by 2s, 5s or in a more
  complicated way
• sum.cpp
• Modify to use a for-loop
• Should use for loops for your HW

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for loop pitfall

• What is the output of this code?
• for (count0countlt4count)cout ltlt count
• Between the for and the is an empty statement
• Watch out for extra semi-colons!!

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break

• The books says that you can use a break to end a
  loop.
• Do not do this in this class.
• Only rarely outside of this class.
• In this class, use only for ending cases in a
  switch.

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for, while or do-while ?

• for( )
• Typically used for counting with int variables
• 1,2,3, 5,8,11, 99,97,95,
• while ( )
• More general.
• do while( )
• More general, always executes at least once.
  Often used for user input.

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Exercises

• What is the output?
• int i,x0
• for(i5igt0i--)
• xii
• cout ltlt x
• Write a for loop that sums the even integers less
  than a given integer n.

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Nested for loops

• What is the output?
• n3
• sum 0
• for(i1 iltn i)
• for(j1 jlti j)
• sum j
• cout ltlt The sum is ltlt sum ltlt endl

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Exercise

• Write a program that allows the user to input the
  length of the side of square and then outputs a
  square of that size made up of s.
• Example
• Please enter the size of the square 5
• Here is your 5 by 5 square
• square.cpp

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Exercise

• Write a program that allows the user to input the
  length of the side of triangle and then outputs a
  triangle with of that size made up of s.
• Example
• Please enter the length of the triangle 5
• Here is your triangle
• triangle.cpp

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Exercise

• How do you modify the previous program so that it
  does this
• Please enter the length of the triangle 5
• Here is your triangle

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Top-down design

• Most programs are too complicated to sit down and
  write at the computer.
• First write the steps of your program in English
  and then refine the steps until it can be easily
  written in C.

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Class exercise Write a program that specifies
how to share pizzas

• How many slices does your pizza have? 8
• 1 people can eat 8 slices
• 2 people can eat 4 slices
• 4 people can eat 2 slices
• 8 people can eat 1 slices
• How many slices does your pizza have? 5
• 1 people can eat 5 slices
• 5 people can eat 1 slices
• Decide on the algorithm first !!

User input
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Top-down design

• First, specify the big tasks the program must
  accomplish.
• Then break each of these into subtasks, and these
  into subtasks,
• When you know how to accomplish each subtask, you
  have an Algorithm.
• Make the subtasks functions

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Functions

• Functions aid top-down design.
• Functions separate subtasks from the main
  program.
• Functions allow a section of code to be called
  repeatedly (and from different places in your
  program.)
• Example an algorithm for computing grades might
  look like
• For each student in the class
• Compute the hw average
• Compute the course average
• Compute the letter grade
• Output the final grade
• (The steps 1-4 can be made into functions.)

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Predefined functions

• root.cpp
• Example sqrt() the square root function
• double root
• rootsqrt(9.0) // root 3.0
• Terminology
• sqrt(9.0) is a function call (or invocation)
• 9.0 is an argument to the function
• The value computed (in this case 3.0) is called
  the value returned by the function.

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functions

• Arguments can be expressions. Argument is
  evaluated and then function is called. E.g.,
• hypotenuse sqrt(basebase heightheight)
• Functions can be used in expressions. Function
  is called and then expression evaluated.
• Temperature 99.0 sqrt(wind_chill)
• Be careful about types. Arguments and return
  values are of a specified type. The argument to
  sqrt() is a double and it returns a double.
• int int_var
• int_var sqrt(9.0) // compiler not happy

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Predefined functions

• Need an include directive and namespace. (This
  is different from 2nd edition) E.g.,
• include ltcmathgt
• include ltcstdlibgt
• using namespace std
• For more functions see p. 111 and Appendix 4.
• cmath
• x sqrt(9.0) // x 3.0
• x pow(2.0,3.0) // x 8.0
• x fabs(-23.0) // x 23.0
• cstdlib
• x abs(-23) // x 23

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Functions that return a value

• Todays functions all return a value. (Later we
  will see void functions that do not return
  values)
• You must do something with that value. For
  example
• root sqrt(7.0) // remember the value
• cout ltlt the square root of 7 is ltlt sqrt(7.0)
• // print it out
• salary 34 sqrt(7.0) // use it in a calc
• I.e., This is legal but useless
• sqrt(7.0) // value thrown away
• Calls to functions that return values should
  always have something on their left.

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Type casting

• C has type casting (type changing) functions
  that explicitly convert from one type to another.
  E.g.,
• x double(3) // x 3.0
• i int(3.7) // i 3

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Programmer designed functions

• A small program to perform a subtask. Aids in
  top-down design.
• Example sumsquares.cpp
• To define a function, supply
• Function prototype before main, specifies the
  types of arguments and return values
• Function definition after main, specifies how
  to computer the function
• To use function, must make function call

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function prototype

• Before main
• return_type function_name(parameter_list)
• // comment on what function does
• Example
• int square(int number)
• // computes the square of a number

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function definition

• After main, the definition. It computes and
  returns answer. return_type
  function_name(parameter_list)
• variable_declarations
• executable_statements // computations
• return answer // at least one
• The names in parameter list are called formal
  parameters, they are placeholders for values
  given to the function.

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Formal parameters, the return and statement and
executing functions by hand

• Function call substitutes value
• s square(5)
• for formal paramater. I.e., is like executing
• int number5
• The return statement
• return numbernumber
• replaces the function call with the returned
  value, I.e., like
• s square(5) 25

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volume.cpp
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Class Exercise

• Write a function that computes the number of
  divisors of a given number.
• Write a program that generates all prime numbers
  less than a user supplied number.
• Harder Write a program that generates the first
  n primes, where n is supplied by the user.

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Next time

• Functions and top down design
• more user defined functions
• void functions
• call by reference