Taking input from a user through the console

In Dart programming language, you can take standard input from the user through the console by the use of .readLineSync() function. To take input from the console you need to import a library, named dart:io from libraries of Dart.

Example

Taking string input from user

The code below asks a user to enter the name then a welcome to Flutter Moble App. Development course is display to the username.

// importing dart:io file
import 'dart:io';
void main()
{
    print("Enter your name");
    // Reading name of the user
    String name = stdin.readLineSync();
    // Printing the name
    print("Hello, ${name}! \nWelcome toFlutter Mobile App. Development. Hope you are having fun!!");
}

Result

Enter your name

Godwin Ashong

Hello, Godwin Ashong!

Welcome to Flutter Mobile App. Development. Hope you are having fun!!

NOTE: The \n is added to the text to make the text that follows move to the next line. 

Taking integer input from user

To take an integer value from the user, you have to convert the string to integer using the parse function. For integer values int.parse(string comes here) is used. In this case our string is the stdin.readLineSync()

Example

The sample code below ask a user to enter the age and displays the entered age to the user

// Importing dart:io file
import 'dart:io';
void main()
{
    // Asking for age
    print("Enter your age:");
    // Scanning number
    int age = int.parse(stdin.readLineSync());
    // Printing the age
    print("You are ${age} yearsold");
}

Result

Enter your age:

17

You are 17 years old

Taking fraction/decimal (double) input

Taking double input is similar to the integer input except we use the double.parse function instead.

Example

// Importing dart:io file 
import 'dart:io'; 
  
void main() 
{ 
    // Asking for age 
    print("Enter your salary:"); 
  
    // Scanning number 
    double salary = double.parse(stdin.readLineSync()); 
  
    // Printing the age 
    print("You salary is Ghc ${salary}"); 
} 

Result

Enter your salary:

3500

You salary is Ghc 3500.0

Conditionals –Making Decisions

A computer can process a program in one of the following ways: in sequence; selectively, by making a choice, which is also called a branch; repetitively, by executing a statement over and over, using a structure called a loop; or by calling a function. The figure below illustrates the first three types of program flow.

If Statement

The if statement is in this format:

if(condition is true){

            Run this block of codes

}

If a block of code(s) needs to be run if the condition is false, the else is included as shown below:

if(condition is true){

            Run this block of codes

}else{

            Run this block of codes

}

Examples

1. if (score is greater than or equal to 90)

grade is A

2. if (hours worked are less than or equal to 40)

wages = rate *hours

else

wages = (rate *40) + 1.5 *(rate *(hours – 40))

3. if (temperature is greater than 70 degrees and it is not raining)

Go golfing!

Relational Operators

Code Examples

Let's us modify our input samples to make decisions.

We will modify the code that takes the age of a person. If the age is 18 or more, you inform the user that he is qualified to drink but he should drink wisely otherwise we tell the user not qualified to drink because he/she is under age

// Importing dart:io file
import 'dart:io';
void main()
{
    // Asking for age
    print("Enter your age:");
    // Scanning number
    int age = int.parse(stdin.readLineSync());
    // Check if user is 18 or more
    if(age>=18){
		print("You are ${age} years old. You are qualified to drink but please drink wisely");
	}else{
		print("You are ${age} years old. You are under age and not qualify to drink so you are not allowed enter the bar");
	}
    
}

Results

Enter your age:

24

You are 24 years old. You are qualified to drink but please drink wisely

Enter your age:

16

You are 16 years old. You are under age and not qualify to drink so you are not allowed enter the bar

Write a program which determines whether an integer entered is an even number or not

Sample Code

Concept.

An even number is a number divisible by 2 without are remainder. The operator for calculating a remainder of a number divided by another number is modulus (%) as explained earlier. If the remainder is 0,then it means the number is an even number. Hence the sample code below.

// Importing dart:io file
import 'dart:io';
void main()
{
    // Asking for number
    print("Enter an integer:");
    // Scanning number
    int num = int.parse(stdin.readLineSync());
    // Check if number is even or not
    if(num%2==0){
		print("${num} is an even number");
	}else{
		print("${num} is an odd number");
	}
    
    
}

Results

Enter an integer:

3

3 is an odd number

Enter an integer:

4

4 is an even number

Conditions and Meaning

Else if

The else if is used for multiple if statements. It is written in the following format.

if(statement 1 is true){

            run this block of code

}else if(statement 1 is false and statement 2 is true){

            run this block of code

}else if(statement 2 is false and statement 3 is true){

            run this block of code.

}

.

.

.

else if(statement n-1 is false and statement n is true){

            run this block of code.

}else{

     if none of the above statements is true, run this block of code

}

Thus after option 1, the other options before the last option will have else if and the last option will have else

Example

The following is the grading system in a school.

Write a program to take the percentage score and output the grade to a user.

// Importing dart:io file
import 'dart:io';
void main()
{
    // Asking for score
    print("Enter your percentage score:");
    // Scanning number
    double score = double.parse(stdin.readLineSync());
    // Calculate the grade
    if(score>=90){
		print("${score} is grade A");
	}else if(score>=80){
		print("${score} is grade B");
	}else if(score>=50){
		print("${score} is grade C");
	}else{
		print("${score} is Fail");
	}
    
}

Results

Enter your percentage score:

75

75.0 is grade C

Enter your percentage score:

49

49.0 is Fail

Switch

There are two selection, or branch. The first selection structure, which is implemented with if and if. . .else statements, usually requires the evaluation of a (logical)expression. The second selection structure, which does not require the evaluation of a logical expression, is called the switch structure.

switch structure gives the computer the power to choose from among many alternatives.

A general syntax of the switch statement is:

switch(expression or value)

{

case value1:

statements1

break;

case value2:

statements2

break;

.

.

.

case valuen:

statementsn

break;

default:

statements

}

switch, case, break, and default are reserved words. The default statement is run when none of the case values is the expression.

The switch statement executes according to the following rules:

 1.          

When the value of the expression is matched against a case value (also called a label), the statements execute until either a break statement is found or the end of the switch structure is reached.

 2.           

 If the value of the expression does not match any of the case values, the statements following the default label execute. If the switch structure has no default label and if the value of the expression does not match any of the case values, the entire switch statement is skipped.

3.            

A break statement causes an immediate exit from the switch structure.

This is illustrated in the figure below.

Example

Let's modify our age drinking program so this time we use a switch statement instead for the condition checking

// Importing dart:io file
import 'dart:io';
void main()
{
    // Asking for age
    print("Enter your age");
    // Scanning number
    int age = int.parse(stdin.readLineSync());
    // Check if number is even or not
    switch(age>=18){
	    case true:
        print("Your age is ${age}. You are an adult. You can drink");
        break;
        case false:
        print("Your age is ${age}. You are under age. You can't' drink");
        break;
        default:
        print("Invalid input");
	}
    
    
}

Results

Enter your age

14

Your age is 14. You are under age. You can't' drink

Nested If

When one control statement is located within another, it is said to be nested.

Nested if is therefore an if  statement in another if statement.

Example

Write a program that determine if a number is an even or not and if even, determine whether is divisible by 10 or not.

// Importing dart:io file
import 'dart:io';
void main()
{
    // Asking for number
    print("Enter an integer");
    // Scanning number
    int num = int.parse(stdin.readLineSync());
    if(num%2==0){
	     if(num%10==0){
			print("${num} is an even number and divisible by 10");
		 }else{
			print("${num} is an even number but not divisible by 10");
		}
	}else{
		print("${num} is not an even number");
	}
}

Results

Enter an integer

20

20 is an even number and divisible by 10

Enter an integer

5

5 is not an even number

Enter an integer

26

26 is an even number but not divisible by 10

LOOP

Branch statements allow you to direct the flow of a program’s execution down one path or another.

The While Statement

The simplest form of looping statement is the while loop.Here’s what the while loop looks like:

while(condition)

{

// ...repeatedly executed as long as condition is true

}

From the above state, the block of code in the while repeats over and over again until the condition in the while loop is false.

The variable in the condition that makes it true is updated at each cycle of the loop in order to be false at a particular point.

Example

Let's modify our even number determination program so that the program repeats over and over again until a user enters the letter q to quit the program.

// Importing dart:io file
import 'dart:io';
void main()
{
    
    String quit = "e";
    while(quit.toLowerCase() !="q"){
		print("Enter an integer");
    	// Scanning number
	    int num = int.parse(stdin.readLineSync());
	    if(num%2==0){
			print("${num} is an even number");
		}else{
			print("${num} is an odd number");
		}
		print("Enter q to quit or any other input to run again");
		quit = stdin.readLineSync();
		//quit is the control variable. Once the value is q, the loop ends
	}
}

Results

Enter an integer

4

4 is an even number

Enter q to quit or any other input to run again

a

Enter an integer

3

3 is an odd number

Enter q to quit or any other input to run again

p

Enter an integer

5

5 is an odd number

Enter q to quit or any other input to run again

q

Quit with Q Result

Enter an integer

4

4 is an even number

Enter q to quit or any other input to run again

b

Enter an integer

6

6 is an even number

Enter q to quit or any other input to run again

b

Enter an integer

9

9 is an odd number

Enter q to quit or any other input to run again

Q

The variable to control the loop is quit. In order for the while loop to run at least once, the variable that controls the loop should be initialized with a value that makes the condition to be true.

From the above code, quit is initialized to e to make the condition quit.ToLower() != "q" true.

quit is converted to lower case in order to match q in case user enters Q. We will consider more of these String functions in this week's lesson.

 NOTE: != means not equal to

After the program task is completed, the user is asked to enter q to quit or any other input to continue. If user enters q, the condition will then be false since q !=q is false since q is equal to q. Once the condition is false, the loop ends.

Decreasing or increasing an integer value

To increase an integer value, you can use any of these methods:

To increase an integer value, you can use any of these methods:

1.     variable= variable + 1;

2.     variable++;

1 is added to the old value of the variable and the result is assigned to the variable.

To decrease an integer value, you can use any of these methods:

1.     variable= variable - 1;

2.     variable--;

1 is subtracted from the old value of the variable and the result is assigned to the variable.

Write a program that prints out the numbers from 1 to 10

// Importing dart:io file
import 'dart:io';
void main()
{
    int i = 1;
    while(i<=10){
	   //Print out the value of i
      print("${i}");
      //Increase the value of i to quit the loop at a point in time
      i++;
    }
}

Result

1

2

3

4

5

6

7

8

9

10

The while loop will keep on repeating so far as the condition i<=10 is true. i needs to be initialized to make the condition true in order for the block of code in the while loop to start execution. The repetition ends when i<=10 is false. Thus when i is more than 10. i++; increases the i anytime that line of code is executed. i++; is the same as i = i+1;

Detail Explanation

In summary while loop is written in the form below:

//initialize the loop control variable(s). The expression must be true to start the loop.

while(expression)

{

.

.

.

//update the loop control variable(s). This will make the expression to be false at a point in time.

.

.

.

}

NOTE: If your loop control variable(s)update doesn’t make the expression false at certain point in time, you will end up with an endless loop.

The Do … while statement

A separate, less frequently used version of the while loop known as the do … while appears identical except the condition isn’t tested until the bottom of the loop:

do

{

// ...the inside of the loop

} while (condition);

Because the condition isn’t tested until the end, the body of the do … while is always executed at least once. Thus since the condition is tested at the bottom, the block of code in the do{} is run at least once before the condition is tested. If the condition is true, the loop continues and if not it is not repeated.

Example

// Importing dart:io file
import 'dart:io';
void main()
{
    int i = 1;
    do{
	   //Print out the value of i
      print("${i}");
      //Increase the value of i to quit the loop at a point in time
      i++;
    }while(i>2);
}

Result

1

Even i which at the time the loop begins has a value of 1 and is less than 2, the code in the block of the do while loop is executed until the while(i>2) line which is false since 1 is not greater than 2 and the loop ends.

A do...while loop can be used for input validation. Suppose that a program prompts a user to enter a test score, which must be greater than or equal to 0 and less than or equal to 100. If the user enters a score less than 0 or greater than 100, the user should be prompted to re-enter the score. The following do...while loop can be used to accomplish this objective:

// Importing dart:io file
import 'dart:io';
void main()
{
    // Grade calculator
    double score;
    do{
	    // Asking for the score
         print("Enter your percentage score:");
         // Scanning number
        score = double.parse(stdin.readLineSync());
        //Only calculate grade when input is valid
        if((score>0)&&(score<=100)){
		    if(score>=90){
				print("${score} is grade A");
			}else if(score>=80){
				print("${score} is grade B");
			}else if(score>=50){
				print("${score} is grade C");
			}else{
				print("${score} is Fail");
			}
		}
	}while((score<0)||(score>100)); //Repeat loop until input is valid
    
}

Results

Enter your percentage score:

-70

Enter your percentage score:

120

Enter your percentage score:

92

92.0 is grade A

The for loop

The most common form of loop is the for loop. The for loop is preferred over the more basic

while loop because it’s generally easier to read (there’s really no other advantage).

The for loop has the following format:

for (initialization; conditional; increment or decrement)

{

// ...body of the loop

}

The for loop is equivalent to the following while loop:

initialization;

while(conditional)

{

// ...body of the loop

increment or decrement;

}

Examples

Write a program that takes an integer from the user and output the multiplication table fo rthat user. For instance if a user enters 2, the output should look something like this:

2 x 1 = 2

2 x 2 = 4

2 x 3 = 6

2 x 4 = 8

2 x 5 = 10

2 x 6 = 12

2 x 7 = 14

2 x 8 = 16

2 x 9 = 18

2 x 10 = 20

2 x 11 = 22

2 x 12 = 24

// Importing dart:io file
import 'dart:io';
void main()
{
    print("Enter the number to compute the multiplication table");
    int num = int.parse(stdin.readLineSync());
    for(int i=1;i<=12;i++){
		print("$num x $i = ${i*num}");
	}
    
}

Results

Enter the number to compute the multiplication table

2

2 x 1 = 2

2 x 2 = 4

2 x 3 = 6

2 x 4 = 8

2 x 5 = 10

2 x 6 = 12

2 x 7 = 14

2 x 8 = 16

2 x 9 = 18

2 x 10 = 20

2 x 11 = 22

2 x 12 = 24

The continue and break statement

When the break command is encountered, it causes control to exit the current loop immediately. The control passes from the break statement to the statement immediately following the closed brace at the end of the loop.

The format of the break commands is as follows:

while(condition) // break works equally well in for loop

{

if (condition)

{

break; // exit the loop

}

. // control passes here

. // control passes here

. // control passes here

}

// The control moves immediately here (after the closed brace at the end of the loop)

Example

Write a program that sum positive integers entered by a user continuously until a negative number is entered.

// Importing dart:io file
import 'dart:io';
void main()
{
    int sum = 0;
    while(true){
		print("Enter a positive number");
    	int num = int.parse(stdin.readLineSync());
        if(num>=0){
			sum = sum+num;
		}else{
			break;
		}
	}
	print("Total: ${sum}");
    
}

Result

Enter a positive number

5

Enter a positive number

6

Enter a positive number

8

Enter a positive number

-2

Total: 19

If negative number is entered the condition, (num>=0) will be false and hence the break in the else curly braces will move the control to outside the loop;

Continue

The continue statement is used in while, for, and do. . .while structures. When the continue statement is executed in a loop, it skips the remaining syntax in the loop and proceeds with the next iteration of the loop.

Example

// Importing dart:io file
import 'dart:io';
void main()
{
    int sum = 0;
    while(true){
		print("Enter a positive number");
    	int num = int.parse(stdin.readLineSync());
        if(num>=0){
			sum = sum+num;
		}else{
			continue;
		}
		print("Total: ${sum}");
	}
    
}

Result

Enter a positive number

4

Total: 4

Enter a positive number

5

Total: 9

Enter a positive number

-2

Enter a positive number

4

Total: 13

Enter a positive number

-5

Enter a positive number

2

Total: 15

Enter a positive number

5

Total: 20

Enter a positive number

When the continue is executed, the print("Total: ${sum}"); part is skipped and the loop is repeated.

Nested loops

A loop in another loop.

Example

Write a program that outputs the multiple table as shown below:

Solution

NOTE: To print horizontally you have to use stdout.write() which is a method in your dart library you imported when taking input from the user. The print method only displays information on a new line.

// Importing dart:io file
import 'dart:io';
void main()
{
        //For the row values
        for(int i=1;i<=12;i++){
		//For the column values
                for(int j=1;j<=12;j++){
			stdout.write("${i*j}  ");
		}
		//After 12 columns begin a new row
		print("");
	}
}

Result

1  2  3  4  5  6  7  8  9  10  11  12  

2  4  6  8  10  12  14  16  18  20  22  24  

3  6  9  12  15  18  21  24  27  30  33  36  

4  8  12  16  20  24  28  32  36  40  44  48  

5  10  15  20  25  30  35  40  45  50  55  60  

6  12  18  24  30  36  42  48  54  60  66  72  

7  14  21  28  35  42  49  56  63  70  77  84  

8  16  24  32  40  48  56  64  72  80  88  96  

9  18  27  36  45  54  63  72  81  90  99  108  

10  20  30  40  50  60  70  80  90  100  110  120  

11  22  33  44  55  66  77  88  99  110  121  132  

12  24  36  48  60  72  84  96  108  120  132  144 

Numbers

Dart numbers can be classified as:

int 

Integer of arbitrary size. The int data type is used to represent whole numbers.

double 

64-bit (double-precision) floating-point numbers. The double data type is used to represent fractional numbers.

As you have seen to convert a string to an integer we use the int.parse and to convert a string to a double we use the double.parse method. 

Number Properties

The following table lists the properties supported by Dart numbers.

Number Methods

Given below are a list of commonly used methods supported by numbers −

Boolean

The keyword bool is used for the boolean data type.

The boolean data type value is either true or false.

To check if a boolean variable is true, we simply write:

 if(nameOfBooleanVariable){ 

statement comes here

}

To check if a boolean variable is not true (false) we simply start with the exclamation mark (!) which means not in programming so the statement will be:

if(!nameOfBooleanVariable){

statement comes here

}

We can also write an expression which result is true or false and assign the result in a boolean variable.

Example:

void main(){
    bool is_underage;
    int age = 20;
    is_underage = age < 18;
    print(is_underage);
}

Output

false

We can modify the above code to take the age from the user and check the value of is_underage and display on the screen whether a user can drink or not.

import 'dart:io';
void main(){
    print("Enter your age");
    int age = int.parse(stdin.readLineSync());
    bool is_underage;
    is_underage = age < 18;
    
    //If it is not true that the person is under age
    if(!is_underage){
      print("You can drink but drink wisely");
    }else{
      //If it is true that the person is under age
      print("You are under age. You are not allowed to enter the bar");
    }
}

Output

Enter your age

17

You are underage. You are not allowed to enter the bar