1 / 35

CS1010: Programming Methodology comp.nus.sg/~cs1010/

CS1010: Programming Methodology http://www.comp.nus.edu.sg/~cs1010/. Week 6: Modular Programming. Objectives: Understand how to use function to pass back more than one value Understand how to use header files and separate compilation to make your program modular References:

zion
Download Presentation

CS1010: Programming Methodology comp.nus.sg/~cs1010/

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. CS1010: Programming Methodologyhttp://www.comp.nus.edu.sg/~cs1010/

  2. Week 6: Modular Programming • Objectives: • Understand how to use function to pass back more than one value • Understand how to use header files and separate compilation to make your program modular • References: • Chapter 5: Lessons 5.4 – 5.5 (Functions that “return” more than one value) • Chapter 8: Data Structures and Large Program Design CS1010 (AY2011/2 Semester 1)

  3. Outline 0. Week 5 Exercise 4: Prime Number • Functions Revisit • Pointer Variables • Functions with &parameters • Exercise #1: Code tracing • Modularization and Interfaces • Separate Compilation • Exercise #2: Modularize “Draw Stick Figures” problem with separate programs Topics 5 – 7 are non-examinable. CS1010 (AY2011/2 Semester 1)

  4. 0. Week 5 Exercise #4: Prime Number • Primality test is a classic programming problem • Given a positive integer, determine whether it is a prime • A prime number has two distinct factors (divisors): 1 and itself. Examples: 2, 3, 5, 7, 11, ... (Note: 1 is not a prime!) • Write a program Week5_PrimeTest.c. You should include a function is_prime(int). • Sample runs: Enter a positive integer: 131 131 is a prime. Enter a positive integer: 713 713 is not a prime. CS1010 (AY2011/2 Semester 1)

  5. 1. Functions: Revisit (1/3) • In week 3, we learned about functions • In C, a function maps some input values to zero or more output values • Zero output through “void func ( … ) { … };” • One output through, e.g., “double func ( … ) { …; return value; };” • More outputs through changing input values • ‘&’– ‘address of’ operator • ‘*’– ‘indirection’ operator; go to the address stored in the variable following the * to get the/put a value at that address • We have done void functions and functions that return a single value, how about functions that return more than one value? CS1010 (AY2011/2 Semester 1)

  6. 1. Functions: Revisit (2/3) • But a function can return only one value! • What is the output of this program? #include <stdio.h> int f(int, int, int); int main(void) { int a = 9, b = -2, c = 5; printf("Result = %d\n", f(a,b,c)); return 0; } int f(int x, int y, int z) { return x; return y; return z; } CS1010 (AY2011/2 Semester 1) 

  7. 1. Functions: Revisit (3/3) • What if we really need to “return” more than one value? • 2 possible approaches • Returning a collection of data (eg: array, structure) – we will discuss this another time • Use variables to hold these values, and pass the addresses of these variables to the functions • This involves the use of & (address operator) and * (indirection operator) (also known as the dreaded pointer) CS1010 (AY2011/2 Semester 1)

  8. 2. Pointer Variables (1/4) • A pointer variable stores the address of another variable • C provide two unary pointer operators • Address operator & • Indirection operator * • Example: i j p 10 20 inti = 10, j = 20; int*p; // p is a pointer to an integer variable p = &i; // p now stores the address of variable i printf("value of i is %d\n", *p); value of i is 10 Now *p is equivalent to i CS1010 (AY2011/2 Semester 1)

  9. 2. Pointer Variables (2/4) 12 12 inti = 10, j = 20; int *p; // p is a pointer to an integer variable p = &i; // p now stores the address of variable i printf("value of i is %d\n", *p); • Example (cont.): i j p 10 20 // *p accesses the value of pointed/referred variable *p = *p + 2; // increment *p (which is i) by 2 p = &j; // p now stores the address of variable j *p = i; // value of *p (which is j now) becomes 12 CS1010 (AY2011/2 Semester 1)

  10. 2. Pointer Variables: Demo #1 (3/4) • Week6_Pointers.c What is the output? #include <stdio.h> int main(void) { double a, *b; b = &a; *b = 12.34; printf("%f\n", a); return 0; } CS1010 (AY2011/2 Semester 1) 

  11. 2. Pointer Variables: Common Mistake (4/4) #include <stdio.h> int main(void) { int *n; *n = 123; printf("%d\n", *n); return 0; } • Week6_Pointers_Common_Mistake.c What’s wrong with this? CS1010 (AY2011/2 Semester 1) 

  12. 3. Functions with &parameters (1/6) // Week6_FunctionDemo1.c #include <stdio.h> void f(int, int, int); int main(void) { int a = 9, b = -2, c = 5; f(a, b, c); printf("a = %d, b = %d, c = %d\n", a, b, c); return 0; } void f(int x, int y, int z) { x = 3 + y; y = 10 * x; z = x + y + z; printf("x = %d, y = %d, z = %d\n", x, y, z); } a b c 9 -2 5 CS1010 (AY2011/2 Semester 1) 

  13. 3. Functions with &parameters (2/6) // Week6_FunctionDemo2.c #include <stdio.h> void f(int *, int *, int *); int main(void) { int a = 9, b = -2, c = 5; f(&a, &b, &c); printf("a = %d, b = %d, c = %d\n", a, b, c); return 0; } void f(int *x, int *y, int *z) { *x = 3 + *y; *y = 10 * *x; *z = *x + *y + *z; printf("*x = %d, *y = %d, *z = %d\n", *x, *y, *z); } a b c 9 -2 5 CS1010 (AY2011/2 Semester 1) 

  14. 3. Functions with &parameters (3/6) // Week6_FunctionDemo3.c #include <stdio.h> void f(int *, int *, int *); int main(void) { int a = 9, b = -2, c = 5; f(&a, &b, &c); printf("a = %d, b = %d, c = %d\n", a, b, c); return 0; } void f(int *x, int *y, int *z) { *x = 3 + *y; *y = 10 * *x; *z = *x + *y + *z; printf("x = %d, y = %d, z = %d\n", x, y, z); } CS1010 (AY2011/2 Semester 1) 

  15. 3. Functions with &parameters (4/6) // Week6_FunctionDemo4.c #include <stdio.h> void f(int *, int *, int *); int main(void) { int a = 9, b = -2, c = 5; f(&a, &b, &c); printf("a = %d, b = %d, c = %d\n", a, b, c); return 0; } void f(int *x, int *y, int *z) { *x = 3 + *y; *y = 10 * *x; *z = *x + *y + *z; printf("x = %p, y = %p, z = %p\n", x, y, z); } CS1010 (AY2011/2 Semester 1) 

  16. 3. Functions with &parameters (5/6) #include <stdio.h> void swap(int, int); int main(void) { int a = 9, b = -2; swap(a, b); printf("a = %d, b = %d\n", a, b); return 0; } void swap(int x, int y) { int temp; temp = x; x = y; y = temp; } • One useful application: swapping of 2 variables Does this work? Why? CS1010 (AY2011/2 Semester 1)

  17. 3. Functions with &parameters (6/6) • Can you correct the previous program? (Answer will be shown in class.) CS1010 (AY2011/2 Semester 1) 

  18. 4. Exercise #1 void f(int w, double x, int *y, double *z) { printf("w = %d, x = %f, y = %p, z =%p\n", w, x, y, z); w = 2 * w; x = 3 * x; *y = *y * 4; *z = 5 * *z; } • Trace the code manually. What are the outputs? #include <stdio.h> void f(int, double, int *, double *); int main(void) { int a = 5; double b = 7.1; int c = 12; double d = 22.3; printf("a = %d, b = %f, c = %d, d =%f\n", a, b, c, d); printf("&a = %p, &b = %p\n", &a, &b); f(c, d, &a, &b); printf("After returning from function f:\n"); printf("a = %d, b = %f, c = %d, d =%f\n", a, b, c, d); return 0; } CS1010 (AY2011/2 Semester 1) 

  19. Warning! Global Variables • We do not encourage the use of global variables. • Variables that are declared outside all functions • Use of global variables will be heavily penalized. #include <stdio.h> #define PI 3.14159 intvalue; double sum; int main(void) { . . . } This is a constant, not a global variable. These are global variables. CS1010 (AY2011/2 Semester 1)

  20. 5. Modularization and Interfaces (1/3) • So far we have compiled our programs directly from the source into an executable: • For the development of large programs with teams of programmers this is not suitable • Need to be able to “break” the program into multiple modules (files) • Need to be able to separately compile modules • Need to be able to link all modules into an executable Executable code produces Compiler a.out eg: gcc welcome.c CS1010 (AY2011/2 Semester 1)

  21. 5. Modularization and Interfaces (2/3) • Header Files and Separate Compilation • Problem is broken into sub-problems and each sub-problem is tackled separately – divide-and-conquer. • Such a process is called modularization. • The modules are possibly implemented by different programmers, hence the need for well-defined interfaces. • The signature of a method (its return type, name and parameter list) constitutes theinterface (header file). The body of the function (implementation) is hidden – abstraction. • Good documentation (example: comment to describe what the method does) aids in understanding. double mean(double, double); // Returns the mean of two double floating-point values. CS1010 (AY2011/2 Semester 1)

  22. 5. Modularization and Interfaces (3/3) • Reasons for Modular Programming • Divide problems into manageable parts • Reduce compilation time • Unchanged modules do not need to be re-compiled. • Debug modules separately • Small test programs can be written to exercise the functions in one module. • Build libraries of useful functions • Code can be re-used in different projects. • Faster development. • Do not need to know how some functionality is implemented, e.g., image processing routines. • Example: OpenCV – a computer vision library. CS1010 (AY2011/2 Semester 1)

  23. 6. Separate Compilation • A module contains in most cases functions that are related, e.g., math functions. • A module consists of • A header file (e.g., f1.h). This file contains: • Constant definitions, e.g.: • #define MAX 100 • Function prototypes, e.g.: • double mean(double, double); • A source file (e.g., f1.c). This file contains: • The functions that implement the function prototypes in the header file (e.g., the code for the function mean(…)). • Other functions, variables, and constants that are only used within the module (i.e., they are module-local). f1.h f1.c CS1010 (AY2011/2 Semester 1)

  24. 6.1 Separate Compilation, Case 1 Case 1: All the source files are compiled and linked in one step. Sourcefiles.c & .h math.h Libraryfile(s) f1.h libm.a f1.c f2.h -lm f2.c a.out gcc f3.h f3.c Executablefile Compilation and Linking main.c CS1010 (AY2011/2 Semester 1)

  25. 6.1 Demo #2: Separate Module • Let’s re-visit our Freezer example. We will create a module that contains a function to calculate the freezer temperature: • Module header file: Week6_FreezerTemp.h • Module source file: Week6_FreezerTemp.c // Compute new temperature in freezer floatcalc_temperature (floathours_float); #include <math.h> // Compute new temperature in freezer floatcalc_temperature (floathours_float) { return ((4.0 * pow(hours_float, 10.0))/(pow(hours_float, 9.0) + 2.0)) - 20.0; } CS1010 (AY2011/2 Semester 1)

  26. 6.1 Demo #2: Main Module #include <stdio.h> #include "Week6_FreezerTemp.h" int main (void) { int hours, minutes; floathours_float; // Convert hours and minutes into hours_float float temperature; // Temperature in freezer // Get the hours and minutes printf("Enter hours and minutes since power failure: "); scanf("%d %d", &hours, &minutes); // Convert hours and minutes into hours_float hours_float = hours + minutes/60.0; // Compute new temperature in freezer temperature = calc_temperature(hours_float); // Print new temperature printf("Temperature in freezer = %.2f\n", temperature); return 0; } Now we can write aprogram which uses our new external function: File Week6_FreezerMain.c CS1010 (AY2011/2 Semester 1)

  27. 6.1 Demo #2: Compilation and Linking • Let’s compile and link our program • Case 1: One step compile-and-link • Here the compiler creates temporary object files (which are removed after linking) and directly creates a.out. • Hence you don’t get the chance to see the object files. • (Note: We have omitted the –Wall option above due to space constraint. Please add the option yourself.) $ gcc Week6_FreezerMain.c Week6_FreezerTemp.c -lm CS1010 (AY2011/2 Semester 1)

  28. 6.2 Separate Compilation, Case 2 Case 2: Source files are compiled separately and then linked. Sourcefiles.c & .h Objectfiles math.h Libraryfile Compilation f1.h Libm.a f1.c f1.o gcc -c f2.h -lm f2.c f2.o gcc -c a.out gcc f3.h f3.c f3.o Executablefile gcc -c Linking main.c main.o The compiler creates separate object files. gcc -c CS1010 (AY2011/2 Semester 1)

  29. 6.2 Demo #3: Compilation and Linking • Let’s compile and link our program • Case 2: 3 steps -> compile, compile, and link • Here we first create the Week6_FreezerMain.o and Week6_FreezerTemp.o object files. • Then, we link both object files into the a.out executable. • (Note: We have omitted the –Wall option above due to space constraint. Please add the option yourself.) $ gcc –c Week6_FreezerMain.c $ gcc –c Week6_FreezerTemp.c $ gcc Week6_FreezerMain.o Week6_FreezerTemp.o -lm CS1010 (AY2011/2 Semester 1)

  30. 6.3 Notes (1/2) • Difference between • #include < … > and #include " … " • Use " … " to include your own header files and < … > to include system header files. The compiler uses different directory paths to find < … > files. • Inclusion of header files • Include *.h files only in *.c files, otherwise duplicate inclusions may happen and later may create problems: • Example: Week6_FreezerTemp.h includes <math.h> Week6_FreezerMain.c includes <math.h> and “Week6_FreezerTemp.h”Therefore, Week6_FreezerMain.c includes <math.h> twice. CS1010 (AY2011/2 Semester 1)

  31. 6.3 Notes (2/2) • ‘Undefined symbol’ error • ld: fatal: Symbol referencing errors. • The linker was not able to find a certain function, etc., and could not create a complete executable. • Note: A library can have missing functions  it is not a complete executable. • Usually this means you forgot to link with a certain library or object file. This also happens if you mistype a function name. CS1010 (AY2011/2 Semester 1)

  32. 7. Exercise #2 • Take the Week 3 Exercise #4 of drawing stick figures to the screen and put all the drawing functions into a separate module. • Create a separate module • Week6_DrawingFunctions.c (and Week6_DrawingFunctions.h) • Change the main module to use the external module functions. Call it Week6_DrawFigures.c • Compile, link and test your program. CS1010 (AY2011/2 Semester 1)

  33. Summary for Today • Today’s most important lessons • Functions that “return” more than one value, through passing the addresses of variables into the functions • Modularization and Interfaces • Structure your program into separate modules, each containing a set of related functions and data structures. CS1010 (AY2011/2 Semester 1)

  34. Announcements/Things-to-do • Read Chapter 8 Data Structures and Large Program Design • Do Take-home Lab #3 (to be released later this week) • PE1 this Saturday! • Refer to module website (“Labs” page) for more information, such as venues, seating plan, etc. • Next week: • Recess  No class • Next lecture (after the recess): • Revision and Arrays (Chapter 6 Numeric Arrays) CS1010 (AY2011/2 Semester 1)

  35. End of File CS1010 (AY2011/2 Semester 1)

More Related