Operators, conversions and parsing input

Arithmetic operators

Integer division: the most common surprise

When both sides of / are whole-number types, C# does integer division: it keeps only the whole part and throws away the rest. This is useful (how many full boxes of 12 can I pack from 50 mangoes? 50 / 12 is 4) but it surprises people working out averages.

int total = 250;
int subjects = 3;
Console.WriteLine(total / subjects);            // 83
Console.WriteLine(total % subjects);            // 1
Console.WriteLine((double)total / subjects);    // 83.33333333333333

Shortcuts: ++, --, +=

int count = 5;
count++;        // 6
count += 10;    // 16
count -= 1;     // 15
count *= 2;     // 30
Console.WriteLine(count);

Joining strings and numbers

If either side of + is a string, + joins (concatenates) instead of adding. C# works left to right, which can give surprises.

Console.WriteLine("Total: " + 5 + 3);    // Total: 53
Console.WriteLine(5 + 3 + " items");      // 8 items
Console.WriteLine($"Total: {5 + 3}");     // Total: 8

Implicit and explicit conversions

Converting is changing a value from one type to another. When no information can be lost, C# does it for you: this is an implicit conversion, for example int to long or int to double. When information might be lost, you must ask with a cast: put the target type in brackets. This is an explicit conversion.

int marks = 87;
double asDouble = marks;        // implicit: 87
double price = 9.99;
int rupees = (int)price;        // explicit cast: 9 (decimals cut off, not rounded)
Console.WriteLine(rupees);

Parsing: turning text into numbers

Console.ReadLine always gives a string. "42" is text, not a number. To do maths you must parse it: read the text and build a number from it.

string input = "42";
int n = int.Parse(input);           // 42
decimal amount = decimal.Parse("1299.50");
int bad = int.Parse("forty");       // crashes: FormatException

TryParse is the safe version. It returns true or false to say whether it worked, and gives the number through an out variable (explained in module 2). If parsing fails, the number is 0 and your program keeps running.

Console.Write("How many tickets? ");
string? text = Console.ReadLine();
if (int.TryParse(text, out int tickets))
{
    Console.WriteLine($"Booking {tickets} tickets");
}
else
{
    Console.WriteLine("Please type a whole number.");
}

Rounding money correctly

Math.Round uses banker's rounding by default: a value exactly halfway goes to the nearest EVEN number. So Math.Round(2.5) is 2 and Math.Round(3.5) is 4. Bills usually round halves up, so say so explicitly.

Console.WriteLine(Math.Round(2.5));                                  // 2
Console.WriteLine(Math.Round(2.5, MidpointRounding.AwayFromZero));   // 3
Console.WriteLine(Math.Round(10.125m, 2, MidpointRounding.AwayFromZero)); // 10.13

Overflow: when a number gets too big

An int can hold up to 2,147,483,647 (int.MaxValue). By default, going past it at run time does not crash; the number wraps around to a large negative value. This is called overflow. Wrap the code in checked to get an OverflowException instead.

int big = int.MaxValue;
big = big + 1;
Console.WriteLine(big);          // -2147483648

checked
{
    int x = int.MaxValue;
    x = x + 1;                   // throws System.OverflowException
}

Dividing by zero

Dividing an int by zero throws a DivideByZeroException. Dividing a double by zero does not crash; it gives infinity (or NaN, "not a number", for 0.0 / 0.0). Check the divisor first.