Dart control flow
last modified January 28, 2024
In this article we talk about the flow control in Dart. We define several keywords that enable us to control the flow of a Dart program.
When a Dart program is run, the code is executed from top to bottom. The flow of the program can be altered with various keywords, including if/else, for, while, and switch.
The control flow structures can be used to executed code conditionally or multiple times.
Dart if & else
The if statement specifies the conditional execution
of a block. If the expression evaluates to true, the block is executed.
If the else statement is present and the if statement evaluates
to false, the block following else is executed.
There can be multiple if/else statements.
Dart if/else examples
The following examples demonstrate conditional execution of blocks with if/else.
void main() {
final num = 4;
if (num > 0) {
print("The number is positive");
}
}
In the example we have a simple condition; if the num variable
is positive, the message "The number is positive" is printed to the console.
Otherwise; nothing is printed.
$ dart main.dart The number is positive
The message is printed since value 4 is positive.
void main() {
final num = -4;
if (num > 0) {
print("The number is positive");
} else {
print("The number is negative");
}
}
Now we have added the second branch. The else statement specifies
the block that is executed if the if condition fails.
$ dart main.dart The number is negative
For the -4 value, the "The number is negative" is printed.
import 'dart:math';
void main() {
const int MAX = 10;
final num = new Random().nextInt(MAX) - 5;
if (num > 0) {
print("The number is positive");
} else if (num == 0) {
print("The number is zero");
} else {
print("The number is negative");
}
}
In this example, we add additional branch with if else. We generate
random values between -5 and 4. With the help of the if &
else statement we print a message for all three options.
Dart switch statement
The switch statement is a selection control flow statement. It creates multiple
branches in a simpler way than using the combination of if/else
statements.
The switch keyword is used to test a value from the variable or the
expression against a group of values. The values are presented with the
case keyword. If the values match, the statement following the case
is executed. There is an optional default statement. It is executed if no other
match is found.
Each case can be terminated with break. If it is not,
it falls through.
void main() {
final dayOfWeek = DateTime.now().weekday;
switch (dayOfWeek) {
case DateTime.sunday:
print("dies Solis");
break;
case DateTime.monday:
print("dies Lunae");
break;
case DateTime.tuesday:
print("dies Martis");
break;
case DateTime.wednesday:
print("dies Mercurii");
break;
case DateTime.thursday:
print("dies Jovis");
break;
case DateTime.friday:
print("dies Veneris");
break;
case DateTime.saturday:
print("dies Saturni");
break;
}
}
The example determines the current day of week and prints its Latin equivalent.
switch (dayOfWeek) {
...
}
In the round brackets, the switch keyword takes a value from an expression, which is going to be tested. The body of the switch keyword is placed inside a pair or curly brackets. In the body, we can place multiple case options. Each option is ended with the break keyword.
case DateTime.sunday:
print("dies Solis");
break;
With the case statement, we test the value of the matching
expression. If it is equal to DateTime.sunday, we print the Latin
dies Solis.
$ dart main.dart dies Veneris
Dart while loop
The while statement is a control flow statement that allows code to
be executed repeatedly based on a given boolean condition.
This is the general form of the while loop:
while (expression)
{
statement;
}
The while keyword executes the statements inside the block enclosed
by the curly brackets. The statements are executed each time the expression is
evaluated to true.
void main() {
int i = 0;
int sum = 0;
while (i < 10) {
i++;
sum += i;
}
print(sum);
}
In the code example, we calculate the sum of values from a range of numbers.
The while loop has three parts. Initialization,
testing and updating. Each execution of the statement is called a cycle.
int i = 0;
We initiate the i variable. It is used as a counter.
while (i < 10) {
...
}
The expression inside the round brackets following the while
keyword is the second phase: the testing. The statements in the body are
executed until the expression is evaluated to false.
i++;
This is the last, third phase of the while loop: the updating. We
increment the counter. Note that improper handling of the while
loops may lead to endless cycles.
$ dart main.dart 55
Dart classic for loop
The classic for loop was taken from the C programming language. A for loop has also three phases: initialization, condition and code block execution, and incrementation.
void main() {
var sum = 0;
for (int i = 0; i < 10; i++) {
sum += i;
}
print(sum);
}
In this example, we sum values 0..9 and print the result to the console.
for (int i = 0; i < 10; i++) {
sum += i;
}
In the first phase, we initiate the counter i to zero. This phase
is done only once. Next comes the condition i < 10. If the
condition is met, the statement inside the for block is executed.
In the third phase the counter is increased. Now we repeat the 2, 3 phases until the condition is not met and the for loop is left. In our case, when the counter i is equal to 10, the for loop stops executing.
Dart for in loop
The for/in form enables us to loop over an iterable easily.
It is similar to the forEach function.
void main() {
final words = ['sky', 'load', 'cup', 'tea', 'rock', 'plate'];
for (final word in words) {
print(word);
}
}
In the example, we define a list of words. With for/in we loop
over the list and print all words.
$ dart main.dart sky load cup tea rock plate
Dart break statement
The break statement can be used to terminate the execution of a loop created
by while and for statements.
import 'dart:math';
void main() {
const int MAX = 30;
while (true) {
var num = new Random().nextInt(MAX);
print("$num");
if (num == 22) {
break;
}
}
print("\n");
}
We define an endless while loop. We use the break statement to get
out of this loop. We choose a random value from 1 to 30. We print the value. If
the value equals to 22, we finish the endless while loop.
$ dart main.dart 21 4 20 20 22
Source
In this article we have covered control flow in Dart.
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