//* some variations on IF and SWITCH *

int ifLeapYear(int y) {
    return ( y%4==0 && ( y%100!=0 || y%400==0 ) );
}

int getMonthDays1 (int m, int y) {
    int days;
    if (m==1) days=31;
    else if (m==2) days=28+ifLeapYear(y);
        else if (m==3) days=31;
            else if (m==4) days=30;
                else if (m==5) days=31;
                    else if (m==6) days=30;
                        else if (m==7) days=31;
                            else if (m==8) days=31;
                                else if (m==9) days=30;
                                    else if (m==10) days=31;
                                        else if (m==11) days=30;
                                            else if (m==12) days=31;
                                                else days=-1;

// it is important to check for each month value and return "illegal"
// number of days for illegal month number or signal the error in some
// other way. Another good way is print an error message.

// it is also important to use if-else to prevent further checking
// of the value of m when one already was found and days set to correct value
                                                
    return(days);
}

int getMonthDays2 (int m, int y) {
    if (m==1) return(31);
    if (m==2) return(28+ifLeapYear(y));
    if (m==3) return(31);
    if (m==4) return(30);
    if (m==5) return(31);
    if (m==6) return(30);
    if (m==7) return(31);
    if (m==8) return(31);
    if (m==9) return(30);
    if (m==10) return(31);
    if (m==11) return(30);
    if (m==12) return(31);
    return(-1);
// if you use return you do not have to use else because further tests
// would not be performed anyway. return exits the procedure
}

int getMonthDays3 (int m, int y) {
    int days;
    if (m==1||m==3||m==5||m==7||m==8||m==10||m==12)
        days=31;
    else if (m==4||m==6||m==9||m==11) 
        days=30;
    else if (m==2) 
        days=28+ifLeapYear(y);
    else 
        days=1;
// you can simplity the function by using more complex IF condition
    return(days);
}

int getMonthDays4 (int m, int y) {
    int days;
    switch (m) {
    case  1: days=31; break;
    case  2: days=28+ifLeapYear(y); break;
    case  3: days=31; break;
    case  4: days=30; break;
    case  5: days=31; break;
    case  6: days=30; break;
    case  7: days=31; break;
    case  8: days=31; break;
    case  9: days=30; break;
    case 10: days=31; break;
    case 11: days=30; break;
    case 12: days=31; break;
    default: days=-1;
    }
// switch construction is perfect for this kind of multiple if-else
// it looks well organized and can be easily checked for correctness
    return(days);
}

int getMonthDays5 (int m, int y) {
    int days;
    switch (m) {
    case  1:
    case  3:
    case  5:
    case  7:
    case  8:
    case 10:
    case 12: days=31; break;
    case  2: days=28+ifLeapYear(y); break;
    case  4:
    case  6:
    case  9:
    case 11: days=30; break;
    default: days=-1;
    }
// switch can be optimized so taht you do not have to repeat multiple 
// instructions, just reorganize cases. Make sure that the default case
// is always the very last one (just in case the compiler is not smart
// enough to reorganize the list of values on its own)
    return(days);
}

int getMonthDays6 (int m, int y) {
    switch (m) {
    case  1:
    case  3:
    case  5:
    case  7:
    case  8:
    case 10:
    case 12: return(31);
    case  2: return(28+ifLeapYear(y));
    case  4:
    case  6:
    case  9:
    case 11: return(30);
    default: return(-1);
    }
// you can use immediate return to avoid using break
}

int getMonthDays7 (int m, int y) {
    const char mdays[13] = {-1, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    // a look-up table is faster than several if-else statements and should be used
    // if values to pick from are integer and are in ranges
    // Note that we chose to start the array at [1] to avoid using slower [m-1]
    int days = -1;
    if (m<0 || m>12)    days=-1;
    else if (m==2)      days=mdays[m]+ifLeapYear(y);
    else days=mdays[m];
    return(days);
}