Search and filter
- C course
- Intermediate
- Lesson 68/116
- 6 min
Write search and filtering with function pointers.
Search by title
To find a book by title, write find_by_title. Signature: Book *find_by_title(Library *lib, const char *title). Return type Book *; the found book's address or NULL.
Body: loop with for (int i = 0; i < lib->count; i++); check if (strcmp(lib->books[i].title, title) == 0) return &lib->books[i];. strcmp compares two strings; 0 means equal. After the loop, return NULL;. This pattern is the standard form of search functions in C; a pointer return carries both result and failure.
Filtering patterns
To filter a collection, use a predicate function: typedef int (*BookPred)(Book *);. BookPred is a function pointer taking a Book and returning int. A filter function takes this pointer and prints the matching books.
Example predicate: int is_borrowed(Book *b) { return b->borrowed; }. Inside filter loop with for and call the predicate with if (pred(&lib->books[i])). This is a real use of function pointers; the filter stays the same even when the predicate changes. This technique is C's equivalent of the strategy pattern in non-OO languages.
Practice Add find_by_title
Add Book *find_by_title(Library *lib, const char *title) to the existing code. Body: loop with for (int i = 0; i < lib->count; i++); check if (strcmp(lib->books[i].title, title) == 0) return &lib->books[i];; after the loop return NULL;. In main add a book, call Book *found = find_by_title(&lib, "C Tutorial"); and if (found) print_book(found);.
Starter code
#include <stdio.h>
#include <string.h>
typedef struct {
char title[64];
int year;
int borrowed;
} Book;
typedef struct {
char name[64];
int id;
} Member;
typedef struct {
Book books[10];
int count;
} Library;
void library_init(Library *lib) {
lib->count = 0;
}
void library_add(Library *lib, Book b) {
lib->books[lib->count++] = b;
}
void print_book(Book *b) {
printf("Book: %s (%d)\n", b->title, b->year);
}
int checkout(Book *b) {
if (b->borrowed) return 0;
b->borrowed = 1;
return 1;
}
int return_book(Book *b) {
if (!b->borrowed) return 0;
b->borrowed = 0;
return 1;
}
int main(void) {
Library lib;
library_init(&lib);
Book b1 = {"C Tutorial", 2024, 0};
library_add(&lib, b1);
checkout(&lib.books[0]);
printf("%d\n", lib.books[0].borrowed);
return_book(&lib.books[0]);
printf("%d\n", lib.books[0].borrowed);
return 0;
}
// Write find_by_title function
// In main: find_by_title for "C Tutorial" and call print_bookSample solution
#include <stdio.h>
#include <string.h>
typedef struct {
char title[64];
int year;
int borrowed;
} Book;
typedef struct {
char name[64];
int id;
} Member;
typedef struct {
Book books[10];
int count;
} Library;
void library_init(Library *lib) {
lib->count = 0;
}
void library_add(Library *lib, Book b) {
lib->books[lib->count++] = b;
}
void print_book(Book *b) {
printf("Book: %s (%d)\n", b->title, b->year);
}
int checkout(Book *b) {
if (b->borrowed) return 0;
b->borrowed = 1;
return 1;
}
int return_book(Book *b) {
if (!b->borrowed) return 0;
b->borrowed = 0;
return 1;
}
Book *find_by_title(Library *lib, const char *title) {
for (int i = 0; i < lib->count; i++) {
if (strcmp(lib->books[i].title, title) == 0) return &lib->books[i];
}
return NULL;
}
int main(void) {
Library lib;
library_init(&lib);
Book b1 = {"C Tutorial", 2024, 0};
library_add(&lib, b1);
Book *found = find_by_title(&lib, "C Tutorial");
if (found) print_book(found);
return 0;
}Expected output
Book: C Tutorial (2024)
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match search tools
| Term | Meaning |
|---|---|
strcmp |
String comparison |
Book * |
Return type |
NULL |
Not found |
lib->count |
Loop bound |
2 What does find_by_title return when not found?
- Empty Book
- NULL Correct answer
- 0
- -1
Explanation: When not found it returns NULL; the caller checks for NULL.
3 Complete the strcmp comparison
if (____(lib->books[i].title, title) == 0) return &lib->books[i];if (strcmp(lib->books[i].title, title) == 0) return &lib->books[i];Explanation: strcmp compares two strings; 0 means equal.