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CNSP - Lecture 6 Chapter 6�Part II

BY PROF. RAFAEL ORTA

Disclosure: this presentation includes slides that were provided by the textbook publisher and Dr. Hnatyshin, some are adaptation and some are an exact replica.

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This week we will cover

  • Review about how to return function results through function arguments
  • Differences between call-by-value and call-by-reference
  • Understand difference between input, input / output, and output parameters.
  • How to modularize programs system and pass information between system modules.
  • How to document the flow of information using structure charts.
  • Learn testing and debugging techniques appropriate for a program system with several modules.

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What are the 3 repetition flow control structures we covered last week?

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While, For and Do- While

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6.1 Value and Reference Parameters

  • Functions can
    • return no value
      • type void
    • return exactly one value
      • return statement
      • function type
    • return more than one value
      • type void
      • reference parameters

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Listing 6.1 Function to compute sum and average

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Notes on Call-by-Reference

  • Place the & also in the prototype:

void computeSumAve(float, float, float&, float&);

  • Note that this is a void function

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Call-by-Value and Call-by-Reference Parameters

  • & between type and identifier defines a parameter as output mode (pass by reference)
  • no & in a parameter’s declaration identifies input mode (pass by value)
  • Compiler uses info in parameter declaration list to set up correct argument-passing mechanism

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When to Use a Reference or a Value Parameter

  • If information is to be passed into a function and doesn’t have to be returned or passed out of the function, then the formal parameter representing that information should be a value parameter (input parameter)
  • If information is to be returned to the calling function through a parameter, then the formal parameter representing that information must be a reference parameter (output parameter).
  • If information is to be passed into a function, perhaps modified, and a new value returned, then the formal parameter representing that information must be a reference parameter (input/output parameter)
  • Note: C++ does not distinguish between output parameters and input/output parameters

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Program Style

  • Formal parameter list often written on multiple lines to improve readability.
  • Value parameters first, followed by reference parameters
  • Comment each parameter to identify its use

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Summary of Value and Reference Parameters

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Argument/Parameter List Correspondence

  • Number: number of actual arguments used in function call must be the same as the number of formal parameters listed in the function header and prototype
  • Order: Order of arguments in the lists determines correspondence
  • Type: Each actual argument must be of a data type that can be assigned to the corresponding formal parameter with no unexpected loss of information
  • For reference parameters, an actual argument must be a variable. For value parameters, an actual argument may be a variable, a constant, or an expression

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How do you declare a reference parameter?

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A/ You place an & after the variable type.

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6.2 Functions with Output and Input Parameters

  • Function getFrac reads a common fraction typed in by a user and returns the numerator and denominator through two type int output parameters.

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Listing 6.5 Function to order three numbers

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Notice that the output of one function is the input of the other

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Does C++ makes any distinction between output and input – output parameters?

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A/ No

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6.3 Stepwise Design with Functions

  • Arguments are used to pass information to and from functions
  • Use functions to encode complex tasks

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Figure 6.4 Structure chart for general sum and average problem

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What is an alternative to returning values from a function other than passing arguments by reference in the function call?

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A/ To use the return clause of the function.

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In a function where you are passing to it all the parameters by value, can you return the output or result of it without using the return clause or global variables?

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A/ No, it must be either passed by reference, using a global variable, or using the return clause.

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6.5 Debugging and Testing a Program System

  • Keep each function to a manageable size
    • errors less likely
    • easier to read
    • simplifies testing
  • Two kinds of testing
    • Top-down
    • Bottom-up

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Why?

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Top-Down Testing and Stubs

  • Useful for large projects
  • Stubs for all functions not finished (substitute for a specific function) - just a heading without any details other than some type of message

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Listing 6.12 Stub for function computeSum

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Bottom-Up Testing and Drivers

  • Unit testing tests each function individually
  • Driver used by developer to test full functionality of their function
    • contains only sufficient declarations and executable statements to test a specific function
  • System integration tests combine functions for additional testing, in other words testing of the entire system.

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Listing 6.13 A driver to test computeSum

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Debugging Tips for Program Systems

  • Use comments to
    • document each function parameter and local variable
    • describe the function’s purpose
  • Create a trace of execution by outputting the function name as the function is entered
  • Trace (display) the values of all input and inout parameters upon function entry.
  • Trace the values of all function outputs after returning from function. Verify. Make sure all inout and output parameters are declared as reference parameters (&),
  • Make sure function stub assigns a value to each output parameter.
  • Make sure function driver assigns a value to each input parameter

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Debugging Tips for Program Systems

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Black-Box Testing

  • Assumes the program tester has no information about the code inside the function or system.
  • Tester’s job is to
    • verify that the function or system meets specifications.
    • for each function, ensure that post-conditions are satisfied whenever its preconditions are met
    • check for function crashing due to invalid input values
    • check boundaries of system

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White-Box Testing

  • Tester has full knowledge of function code
  • Must ensure each section of code has been thoroughly tested.
    • Check all possible paths
    • Determine that for each test correct path is taken
    • For loops, make sure correct number of iterations
    • Check boundary values

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What is the difference between black-box testing and white-box testing?

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A/ Black-box implies no knowledge about the program by the tested, white-box has knowledge and focuses in testing all possible paths?

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6.6 Recursive Functions

  • Recursive function calls itself, with different argument values for each call
  • Must identify a stopping case - a situation that stops the recursion.

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Template for a Recursive Function

1. If the stopping case is reached

1.1 Return a value for the stopping case

Else

1.2 Return a value computed by calling the function again with different arguments

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Listing 6.14 Recursive function factorial

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Recursion

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6.7 Common Programming Errors

  • Parameter inconsistencies with call-by-reference parameters
  • Forgetting to use & with call-by-reference parameters
  • Warning messages from compiler should not be ignored

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