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CS106L:Assignment 6:Explore Courses 作业

news 2026/6/17 5:42:13

作业要求

在CourseDatabase类中实现find_course,在成员变量courses中寻找与传入的字符串course_title相同的course,返回std::optional<Course>
实现main函数中，利用Monadic，将std::optional<Course>转化为std::string，Course包含值则返回"Found course: title,number_of_units,quarter\n"，否则返回"Course not found.\n"

问题及解决

and_then transform or_else的具体写法

std::optional<Course>course;course.and_then([](constCourse&crs)->std::optional<std::string>{return...});course.transform([](constCourse&crs)->std::string{return...});course.or_else([]()->std::optional<std::string>{return...});

注意各个lambda函数的传入的参数和返回值

format的使用

对于"Found course: title,number_of_units,quarter\n"这种字符串打印需求，可以使用std::format

std::format("Found course: {},{},{}\n",crs.title,crs.number_of_units,crs.quarter);

main.cpp代码

/* * CS106L Assignment 6: Explore Courses * Created by Haven Whitney with modifications by Jacob Roberts-Baca and Fabio * Ibanez. */#include<algorithm>#include<type_traits>#include<vector>#include<optional>#include"autograder/utils.hpp"/** * A course. This should be familiar from Assignment 1! */structCourse{std::string title;std::string number_of_units;std::string quarter;/** * You don't have to ignore this anymore! We're defining the `==` operator for * the Course struct. */booloperator==(constCourse&other)const{returntitle==other.title&&number_of_units==other.number_of_units&&quarter==other.quarter;}};classCourseDatabase{public:CourseDatabase(std::string filename){autolines=read_lines(filename);std::transform(lines.begin(),lines.end(),std::back_inserter(courses),[](std::string line){autoparts=split(line,',');returnCourse{parts[0],parts[1],parts[2]};});}/** * Finds a course in the database with the given title, if it exists. * @param course_title The title of the course to find. * @return You will need to figure this out! */std::optional<Course>find_course(std::string course_title){for(constauto&elem:courses){if(elem.title==course_title){returnelem;}}return{};}private:std::vector<Course>courses;};intmain(intargc,char*argv[]){static_assert(!std::is_same_v<std::invoke_result_t<decltype(&CourseDatabase::find_course),CourseDatabase,std::string>,FillMeIn>,"You must change the return type of CourseDatabase::find_course to ""something other than FillMeIn.");if(argc==2){CourseDatabasedb("autograder/courses.csv");autocourse=db.find_course(argv[1]);/******************************************************** STUDENT_TODO: Populate the output string with the right information to print Please pay special attention to the README here ********************************************************/std::string output=course.and_then([](constCourse&crs)->std::optional<std::string>{returnstd::format("Found course: {},{},{}\n",crs.title,crs.number_of_units,crs.quarter);}).or_else([]()->std::optional<std::string>{return"Course not found.\n";}).value();/******************************************************** DO NOT MODIFY ANYTHING BELOW THIS LINE PLEASE ********************************************************/std::cout<<output<<std::endl;return0;}returnrun_autograder();}