chapter 6 modular programming j. h. wang ( 王正豪 ), ph. d. assistant professor dept. computer...

Chapter 6Modular Programming

J. H. Wang (王正豪 ), Ph. D.

Assistant Professor

Dept. Computer Science and Information Engineering

National Taipei University of Technology

Functions with Simple Output Parameters

• When a function call executes, the computer allocates memory space in the function data area for each formal parameter.

• The value of each actual parameter is stored in the memory cell allocated to its corresponding formal parameter.

Figure 6.1 Function separate()

Figure 6.2 Diagram of Function separate() with Multiple Results

Functions with Simple Output Parameters(Cont’d)

• Asterisks– before the parameter names– in front of the parameter names in the

assignment statements

• char *signp tells the compiler that signp will contain the address of a type char variable.

• pointer – a memory cell whose content is the address of

another memory cell

Figure 6.3 Program That Calls a Function with Output Arguments (cont’d)

• The calling function must declare variables in which function separate can store the multiple results it computes.

• Changing of the values of memory cells in the data area of the calling function is considered a side effect of the call to function separate.

• The addresses of the arguments sn, whl, and fr are stored in the corresponding output parameters signp, wholep, and fracp.

Figure 6.4 Parameter Correspondence for separate(value, &sn, &whl, &fr);

Figure 6.5 Comparison of Direct and Indirect Reference

• The formal parameter is preceded by the indirection operator, unary *.– *signp = '+';– *wholep = floor(magnitude);– *fracp = magnitude - *wholep;

Meanings of * Symbol

• Three distinct meanings of the symbol *– the binary operator meaning multiplication.– in the declarations, read as “pointer to”– the unary indirection operator, “follow the

pointer”

Figure 6.6 Program to Sort Three Numbers

Figure 6.6 Program to Sort Three Numbers (cont’d)

Multiple Calls to a Function with Input/OutputParameters

• sort – a rearrangement of data in a particular sequence (increasing or

decreasing)

• input/output parameters– the function uses the current actual argument values as inputs and

may return new values

Figure 6.7 Data Areas After temp = *smp; During Call order(&num1, &num3);

Scope of Names

• scope of a name – the region in a program where a particular

meaning of a name is visible

• All of the formal parameters and local variables are visible only from their declaration to the closing brace in which they are declared.

Figure 6.8 Outline of Program for Studying Scope of Names

Figure 6.9 Function scan_fraction (incomplete)

Figure 6.9 Function scan_fraction (incomplete) (cont’d)

Figure 6.10 Data Areas for scan_fraction and Its Caller

CASE STUDY Arithmetic with Common Fractions

• To perform computations with common fractions and get results that are common fractions in reduced form

• Add, subtract, multiply, and divide several pairs of common fractions.

CASE STUDY Arithmetic with Common Fractions (Cont’d)

• DATA REQUIREMENTS– Problem Inputs

• int n1, d1 /* numerator, denominator of first fraction */• int n2, d2 /* numerator, denominator of second fraction */• char op /* arithmetic operator + - * or / */• char again /* y or n depending on user's desire to continue */

– Problem Outputs• int n_ans /* numerator of answer */• int d_ans /* denominator of answer */

• INITIAL ALGORITHM– 1. Repeat as long as user wants to continue.– 2. Get a fraction problem.– 3. Compute the result.– 4. Display problem and result.– 5. Check if user wants to continue.

• Step 2 Refinement– 2.1 Get first fraction.– 2.2 Get operator.– 2.3 Get second fraction.

• Step 3 Refinement– 3.1 Select a task based on operator:– '+': 3.1.2 Add the fractions.– ‘-': 3.1.3 Add the first fraction and the negation of the second.– '*': 3.1.4 Multiply the fractions.– '/': 3.1.5 Multiply the first fraction and the reciprocal of the second.– 3.2 Put the result fraction in reduced form.

• Step 3.2 Refinement– 3.2.1 Find the greatest common divisor (GCD) of the numerator

and denominator.– 3.2.2 Divide the numerator and denominator by the GCD.

Figure 6.11 Structure Chart for Common Fraction Problem

Figure 6.12 Program to Perform Arithmetic Operations on Common Fractions

Figure 6.12 Program to Perform Arithmetic Operations on Common Fractions (cont’d)

Figure 6.13 Sample Run of a Partially Complete Program Containing Stubs

Debugging and Testing a Program System

• Not all functions will be ready at the same time

• use of stubs enables us to test and debug the main program flow and those functions that are available

• top-down testing – the process of testing flow of control between a

main function and its subordinate functions

Debugging and Testing a Program System (Cont’d)

• stub – a skeleton function that consists of a header

and statements that display trace messages and assign values to output parameters; enables testing of the flow of control among functions before this function is completed

Figure 6.14 Stub for Function multiply_fractions

• unit test – a test of an individual function– Perform a preliminary test of a new function– It is easier to locate and correct errors when dealing

with a single function rather than with a complete program system.

• Perform a unit test by writing a short driver function to call it.

• Once a function works properly, substitute it for its stub

Figure 6.15 Driver for Function scan_fraction

• bottom-up testing – the process of separately testing individual

functions of a program system

• system integration tests – testing a system after replacing all its stubs with

functions that have been pretested

Debugging Tips for Program Systems

• A list of suggestions for debugging a program system follows.– Carefully document each function parameter

and local variable using comments– Create a trace of execution by displaying the

function name as you enter it– Trace or display the values of parameters upon

entry to a function– Trace or display the values of all function

outputs

Common Programming Errors

• An actual output argument must be of the same pointer data type as the corresponding formal parameter.

• The C scope rules

chapter 6 modular programming j. h. wang ( 王正豪 ), ph. d. assistant professor dept. computer...

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