first-time-dart

Conditionals and the If Statement

Programs would be pretty boring if they could only perform the identical calculation every time regardless of what their input data looked like.

For that reason, programming languages provide a way of conditioning some steps in the program based on properties of their input data.

To do that, you can use a Conditional Expression that will evaluate to a Boolean value; i.e., either true or false. Most conditional expressions use one of these comparison operators:

Operator	Meaning
a < b	`true` if a is “less than” b
a <= b	`true` if a is “less than or equal” to b
a > b	`true` if a is “greater than” b
a >= b	`true` if a is “greater than or equal” to b
a == b	`true` if a is “equal to” b
a != b	`true` if a is “not equal” to b
a \|\| b	`true` if either a “OR” b is `true`
a && b	`true` if both a “AND” b is `true`

The == operator uses two equal signs to differentiate it from the assignment (=) operator.

For example, suppose we want to print one statement for positive numbers, another statement for negative ones, and a last one for zeros.

void main() {
  // TRY CHANGING THIS VALUE TO SEE DIFFERENT RESULTS
  var x = 0;

  if (x > 0) {
    print('$x is a positive number.');
  } else if (x < 0) {
    print('$x is a negative number.');
  } else {
    print('$x is ZERO!');
  }
}

The if statement allows your program to take two distinct paths, depending on the result of a test. The expression in parentheses after the if keyword is a conditional expression. If the value of the expression is true, then the main block of the if statement will be executed. Otherwise, the block following the else keyword will be executed (if any).

Notice how a sequence of tests can be chained together by using else if between conditional test and code block.

Another way of testing an expression against a number of possible values is to use the switch statement.

void main () {
    // TRY CHANGING THIS VALUE TO SEE WHAT HAPPENS.
    var ch = 'a';

    switch (ch) {
        case '0':
        case '1':
            print('$ch is a digit');
            break;

        case 'a':
        case 'b':
            print('$ch is a letter.');
            break;

        case '+':
            print('$ch is a symbol.');
            break;

        default:
            print('$ch is unknown.');
            break;
    } 
}

There is also a shortcut expression that allows you to conditionally choose a value in the middle of an expression instead of using the bulkier if expression - it’s called the ternary-operator and it uses the characters ? and ::

void main() {
    // TRY CHANGING THIS VALUE AND OBSERVE THE RESULTS
    var x = 10;
    var s;

    if (x < 0) {
        s = "x is negative";
    } else {
        s = "x is non-negative";
    }
    print(s);

    // More compact code using the ternary operator
    s = x < 0 ? "x is negative" : "x is non-negative";
    print(s);
}

Next - Loops

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