modular programming chapter 6. 2 6.1 value and reference parameters t function declaration: void...
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Modular Programming
Chapter 6
2
6.1 Value and Reference Parameters
Function declaration:
void computesumave(float num1, float num2, float& sum, float& average) {
…}
Function call:computeSumAve (x, y, sum, mean)
ACTUAL FORMAL x num1(input) y num2(input)
sum sum(output) mean average(output)
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computeSumAve.cpp
// File: computeSumAve.cpp
#include <iostream>
using namespace std;
void computeSumAve (float, float, float&,
float&);
int main ()
{
float x,
y,
sum,
mean;
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computeSumAve.cpp
cout << "Enter 2 numbers: ";
cin >> x >> y;
computeSumAve (x, y, sum, mean);
cout << " Sum is " << sum << endl;
cout << " Average is " << mean << endl;
return 0;
}
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computeSumAve.cpp
// COMPUTES THE SUM AND AVERAGE OF NUM1 AND NUM2
// Pre: num1 and num2 are assigned values.
// Post: The sum and average of num1 and num2
// are computed and returned as function outputs.
void computesumave(float num1, float num2,
float& sum,
float& average)
{
sum = num1 + num2;
average = sum / 2.0;
}
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Before Execution
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After Execution
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Call-by-Value and Call-by-Reference Parameters
Call by Value– Local function copy of actual argument values
Call by Reference (&)– memory address of actual argument
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Call computeSumAve8
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computeSumAve
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Call-by-Value and Call-by- Reference
& Call by Reference– Formal Argument in function heading– Argument list in function proto-type
Used to modify values in a function– Input– Output– Input/Output
Void functions that return results!
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Call by Value
Local copy of argument made at time of the function call
Local copy used in function Modified local copy not actual value When finished local copy destroyed Actual value not changed
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Call by Reference
Memory address of the actual argument is what is passed to the function
Because it is the address in memory of the actual argument you can modify its value
Data can flow into a function and out of the function
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Protection and Usage of Value and Reference Parameters
Value arguments not changeable (or changeable without affecting caller)
Reference use could create a side effect If one return value is enough use value
arguments with a return If more than one return is needed use
reference arguments for the ones needing return values
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Protection and Usage of Value and Reference Parameters
Typically use reference arguments in getData() type functions
Value arguments used with printing type functions
When a function must return more than one value a reference argument must be used
Avoid using reference arguments because of side effects (Large Projects)
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Value and Reference Parameters
Expressions can be passed to functions– Always passed by value
– Only variables can be passed by reference Unallowed calls for: void f(int x, int y, float& b, int& a)
– f(10, m, b, m+n)
m+n can not be an output parameter!
– f(n+m, m, 12.6, m)
12.6 can not be an output parameter!
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Function Syntax & Arguments
Correspondence between actual and formal arguments is determined by position in their respective argument lists. These lists must be the same size. The names of corresponding actual and formal arguments may be different
Formal arguments and corresponding actual arguments should agree with respect to type
For reference arguments, an actual argument must be a variable. For value arguments, an actual argument may be a variable, a constant or an expression
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6.2 Functions with Output and Input Parameters
Now examine functions that have only output or inout (input/output) parameters.
testGetFrac.cpp– Data items are entered at the keyboard
sort3Numbers.cpp– Demonstrate multiple calls to a function with
inout parameters
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testGetFrac.cpp
// File: testGetFrac.cpp
// Tests the fractions.
#include <iostream>
using namespace std;
getFrac(int&, int& );
int main()
{
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testGetFrac.cpp
int num,denom;
cout << "Enter a common fraction "
<< "as 2 integers separated by a slash: ";
getFrac(num, denom);
cout << "Fraction read is " << num << " / " << denom << endl;
return 0;
}
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testGetFrac.cpp
// Reads a fraction.
// Pre: none
// Post: numerator returns fraction numerator,
// denominator returns fraction denominator
void getFrac(int& numerator, int& denominator)
{
char slash;
// Read the fraction
cin >> numerator >> slash >> denominator;
}
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testGetFrac.cpp
Program Output
Enter a fraction as 2 integers separated by a slash :
3 / 4
The Fraction is : 3 / 4
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sort3Numbers.cpp
// FILE: sort3Numbers.cpp
// READS THREE FLOATING POINT NUMBERS AND SORTS
// THEM IN ASCENDING ORDER
#include <iostream>
using namespace std;
// SORTS A PAIR OF NUMBERS
void order(float&, float&);
int main ()
{
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sort3Numbers.cpp
// Local data ...
float num1, num2, num3;
// Read and sort numbers.
cout << "Enter 3 numbers to sort:";
cin >> num1 >> num2 >> num3;
order (num1, num2);
order (num1, num3);
order (num2, num3);
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sort3Numbers.cpp
// Display results.
cout << "The three numbers in order are:" <<
endl;
cout << num1 << " " << num2 << " " << num3 <<
endl;
return 0;
}
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sort3Numbers.cpp
// SORTS A PAIR OF NUMBERS REPRESENTED BY x AND y
void order(float& x, float& y)
// Pre: x and y are assigned values.
// Post: x is the smaller of the pair and y is
// the larger.
{
// Local data ...
float temp;
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sort3Numbers.cpp
// Compare x and y and exchange values if not
// properly ordered.
if (x > y)
{
temp = x;
x = y;
y = temp;
}
}
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sort3Numbers.cpp
Program Output
Enter 3 numbers to be sorted separated by spaces:
7.5 9.6 5.5
The three numbers in order are:
5.5 7.5 9.6
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Sort3Numbers.cpp
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6.3 Stepwise Design with Functions
Use functions as building blocks in design Start small and add functions compiling as
you go Case study the sum and average problem Classic Stepwise design steps
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Stepwise Design with Functions
Problem statement Problem analysis Program design Program implementation Test and verification
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Case Study Structure Chart
C a se S tu d y S truc tu re C h a rt
R ea d then u m b er o fd a ta item s
C o m p u teth e sum
C o m p u teth e a ve ra ge
P rin t th e suma nd thea vera ge
C o m p ute an d p rin t th e suma n d a ve rag e o f a co lle ctiono f flo a tin g p o in t d a ta ite m s
ComputeSum ComputeAve PrintSumAve
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computeSumAve.cpp
// File: computeSumAve.cpp
// Computes and prints the sum and average of
// a collection of data.
// File: computeSumAveFunctions
// Computes the sum and average of a collection
// of data
#include <iostream>
using namespace std;
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computeSumAve.cpp
// Functions used ...
// Computes sum of data
float computeSum (int);
// Computes average of data
float computeAve (int, float);
// Prints number of items, sum, and average
void printSumAve (int, float, float);
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computeSumAve.cpp
int main()
{
// Local data . . .
int numItems;
float sum;
float average;
// Read the number of items to process.
cout << "Enter the number of items to
process:";
cin >> numItems;
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computeSumAve.cpp
// Compute the sum of the data.
sum = computeSum(numItems);
// Compute the average of the data.
average = computeAve(numItems, sum);
// Print the sum and the average.
printSumAve(numItems, sum, average);
return 0;
}
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computeSumAve.cpp
// Insert definitions for functions computeSum,
// computeAve, and printSumAve here.
// Computes sum of data.
// Pre: numItems is assigned a value.
// Post: numItems data items read; their sum
// is stored in sum.
// Returns: Sum of all data items read if
// numItems >= 1; otherwise, 0.
float computeSum (int numItems)
{
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computeSumAve.cpp
// Local data ...
float item;
float sum;
// Read each data item and accumulate it in
// sum.
sum = 0.0;
for (int count = 0; count < numItems; count++)
{
cout << "Enter a number to be added: ";
cin >> item;
sum += item;
} // end for
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computeSumAve.cpp
return sum;
} // end computeSum
// Computes average of data
// Pre: numItems and sum are defined; numItems
// must be greater than 0.
// Post: If numItems is positive, the average is
// computed as sum / numItems;
// Returns: The average if numItems is positive;
// otherwise, 0.
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computeSumAve.cpp
float computeAve (int numItems, float sum)
{
// Compute the average of the data.
if (numItems < 1)
{
cout << "Invalid value for numItems = " <<
numItems << endl;
cout << "Average not computed." << endl;
return 0.0;
} // end if
return sum / numItems;
} // end computeAve
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computeSumAve.cpp
// Prints number of items, sum, and average of
// data
// Pre: numItems, sum, and average are defined.
// Post: Displays numItems, sum and average if
// numItems > 0.
void printSumAve (int numItems, float sum,
float average)
{
// Display results if numItems is valid.
if (numItems > 0)
{
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computeSumAve.cpp
cout << "The number of items is " <<
numItems << endl;
cout << "The sum of the data is " <<
sum << endl;
cout << "The average of the data is " <<
average << endl;
}
else
{
cout << "Invalid number of items = " <<
numItems << endl;
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computeSumAve.cpp
cout <<
"Sum and average are not defined." << endl;
cout <<
"No printing done. Execution terminated." <<
endl;
} // end if
} // end printSumAve
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computeSumAve.cpp
Program OutputEnter the number of items to be processed: 3
Enter a number to be added: 5
Enter a number to be added: 6
Enter a number to be added: 17
The number of items is 3
The sum of the data is 28.00
The average of the data is 9.3333
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6.4 Using Objects with Functions
Two ways to use functions to process objects Member function modifies the objects
attributes– testString.remove (0, 5);
Pass object as a function argument– Passing string object in function doReplace.cpp
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moneyToNumberTest.cpp
// File: MoneyToNumberTest.cpp
// Tests function moneyToNumberString
#include <string>
#include <iostream>
using namespace std;
// Function prototype
void moneyToNumberString(string&);
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moneyToNumberTest.cpp
int main()
{
string mString;
cout << "Enter a dollar amount with $
and commas: ";
cin >> mString;
moneyToNumberString(mString);
cout << "The dollar amount as a number is " <<
mString << endl;
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moneyToNumberTest.cpp
return 0;
}
// Removes the $ and commas from a money string.
// Pre: moneyString is defined and may contain
// commas and begin with $ or -$.
// Post: $ and all commas are removed from
// moneyString.
void moneyToNumberString (string& moneyString)
{
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moneyToNumberTest.cpp
{ // Local data . . . int posComma; // position of next comma
// Remove $ from moneyString if (moneyString.at(0) == '$') moneyString.erase(0, 1); else if (moneyString.find("-$") == 0) moneyString.erase(1, 1);
// Remove all commas posComma = moneyString.find(",");
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moneyToNumberTest.cpp
while (posComma >= 0 && posComma <
moneyString.length())
{
moneyString.erase(posComma, 1);
posComma = moneyString.find(",");
}
} // end moneyToNumberString
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6.5 Debugging and Testing a Program System
Top-Down testing– Stub, for example, a dummy function instead of the
function itself– 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
Bottom-Up testing – Driver used by developer to test full functionality of
their function
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Debugging and Testing a Program System
Debugging Tips for Program Systems– Carefully document each function parameter and
local variable using comments as you write the code. Also describe the function’s purpose using comments.
– Create a trace of execution by displaying the function name as you enter it.
– Trace or display the values of all input and input/output parameters upon entry to a function. Check that these values make sense.
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Debugging a Program System
Debugging Tips for Program Systems– Make sure that the function stub assigns a value to
each output parameter.
Identifier Scope and Watch Window Variables
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Testing
Static methods:– Code inspection
• Goals: e.g. is specification “=“ implementation?, were standards used?
• Expert team examines program system thoroughly and using a formal plan (e.g. moderator, authors, experts)
– Review• Same as inspection, less formal
– Walkthrough• Less structured; e.g. author reads and experts ask questions
spontaneously
– Pair programming• Two programmers from the beginning
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Testing
Dynamic methods:– White box testing
• Tester (e.g. developer) has the source code
• Examples of policies based on flow chart (FC)– Statement coverage: Number of executions (NE) a any node in FC >= 1
– Branch coverage: NE of any branch in FC >= 1
– Condition coverage: Any condition >= 1 time “true” AND 1 time “false”
– Path coverage: Each path in FC should have been executed at least 1 time
– Black box testing• Tester (e.g. from test department) does not have the source code, only object
code (as a black box)
• Specification of program system is needed
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Testing
Examples (White Box Testing)– Program segment for maximum:
cin >> x; cin >> y; max = 0;
if(x <= y) max = y;
cout << “Maximum is “ << max;
– Statement coverage:• Test set = {(2, 5)} satisfies
policy
• However, program is faulty !!!
– Condition coverage: Maximum of 3 numberscin >> x; cin >> y; cin >> z; max = 0;if(x >= y) max = x;else max = y; // right portion for two numbersif(z >= max) then max = z;else max = y; // here error, “else” not needed!cout << “Maximum is “ << max;
– Test set = {(2,1,3), (4,10,6)}
satisfies condition policy– But program is faulty for x > max(y,z)
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6.7 Common Programming Errors
Argument inconsistencies with Call by Reference Arguments– Side effects
Forgetting & Argument type mismatch Argument positional errors or missing
arguments