Syntax for lab 6

Key syntax elements for lab 6

The C++ features we will add for lab 6 are structs (including structs whose fields are arrays), plus arrays of structs. (Refer back to the course notes or to the previous labs for refreshers on the syntax used earlier.)

Structs and arrays of structs

Declaring a struct data type, typically in the global definitions section of our programs, after the global constant definitions but before function prototypes

// declare a data type to represent time,
//    with ints for hours and minutes,
//    and a float for seconds (0-59.9999)
struct Time {
   int hour;     // 0-23
   int minute;   // 0-59
   float second; // 0-59.9999
};

Initializing variables of a struct type

Time startTime = { 11, 30, 0.00 };  // sets hour, minute, and seconds

Accessing the fields of a struct variable using the variable.field notation

   startTime.hour = 9;
   startTime.minute = 20;
   startTime.second =  0.01;
   std::cout << "Start time is " startTime.hour << ":" << startTime.minute;
   std::cout << ":" << startTime.second);

Using a function to set the fields of a struct (i.e. pass by reference)

// sets time to 00:00:00
void setTime(Time &T);

int main()
{
   Time t;
   setTime(t);
}

void setTime(Time &T)
{
   T.hour = 0;
   T.minute = 0;
   T.second = 0;
}

Declaring and using an array of structs:

int main()
{
    const int size = 10;
    Time timevals[size]; // creating the array

    // filling it one element at a time with calls to setTime
    for (int i = 0; i < size; i++) {
        setTime(timevals[i]);
    }

    // accessing the elements directly through the array
    for (int j = 0; j < size; j++) {
        cout << "Hour: ";
        cout << timevals[j].hour; // the hour field of the j'th element
        cout << ", minute: ";
        cout << timevals[j].minute; // the minute field of the j'th element
        cout << endl;
    }
}

Passing an array of structs as a parameter works much like passing any other array, e.g. the function below fills an array of times:

void printTimes(Time alltimes[], int size)
{
   for (int t = 0; t < size; t++) {
        cout << "Hour: ";
        cout << alltimes[j].hour;
        cout << ", minute: ";
        cout << alltimes[j].minute;
        cout << endl;
   }
}

The main routine could call this (instead of the manual print above) as follows:

int main()
{
    const int size = 10;
    Time timevals[size]; // creating the array

    // filling it one element at a time with calls to setTime
    for (int i = 0; i < size; i++) {
        setTime(timevals[i]);
    }

    // have the function print all the times
    printTimes(timevals, size);
}

Structs can have arrays as fields, e.g. a struct to represent a student and their marks on a set of assignments, and have main keep an array of all the students:

const int NumAssigns = 5; // assuming this is the same for all students in a class

struct Student {
   string name;
   float marks[NumAssigns];
};

// get the name and marks for a student
void fill(Student& s);

// display the name and marks for a student
void print(Student s);

int main()
{
   const int MaxStudents = 32; // max number of students the program can handle
   Student students[MaxStudents];  // create an array big enough for the max

   int numStudents; // actual number of students for this run
   cout << "Please enter the number of students (max " << MaxStudents << "): ";
   cin >> numStudents;  // of course we'd want to add error checking/repetition here

   // get all the students' info
   for (int s = 0; s < numStudents; s++) {
       fill(students[s]);  // fill in the info for the student in position s
   }

   // print all the students' info
   for (int s = 0; s < numStudents; s++) {
       print(students[s]);  // display the info for the student in position s
   }
}
void fill(Student& s)
{
   cout << "Enter their name: ";
   cin >> s.name;  // just grabs the name as one word in this example
   for (int m = 0; m < NumAssigns; m++) {
       cout << "Enter their mark for assignment " << m << ": ";
       cin >> s.marks[m];  // again, we'd want to add error checking
   }
}

void print(Student s)
{
   cout << s.name << ":";
   for (int m = 0; m < NumAssigns; m++) {
       cout << " " << s.marks[m];  // print out the next mark for this student
   }
   cout << endl;
}

We could use functions that get/print the information for all the students (calling the earlier functions once per student). Just the 'printAll' is shown here:

void printAll(Student allstudents[], int size)
{
    for (int snum = 0; snum < size; snum++) {
        print(allstudents[s]);
    }
}

The accompanying call from main would then look like:

{
    ... the same as the earlier main prior to the final for loop ...
    printAll(students, numStudents);
}