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decision-making

Desicion Making

Very often while writing code, there is a need for performing different actions for different decisions. Conditional statements are used to meet those needs. Javascript offers the following conditional statements
if condition to decide to execute a block of code when the specified condition is true.
else condition to decide to execute a block of code when the condition specified in if is false.
else if to specify a new condition to execute a new block when the first condition is false
switch to jump between many alternative blocks of code based on the requirement

The if Statement

if condition to decide to execute a block of code when the specified condition is true.

Syntax:

if (condition) {
block of code to be executed if the condition is true
}

Example

<!DOCTYPE html>
<html>
<body>
<p id="demo">a is greater than b</p>
<script>
var a=10;
var b=20;
if (a<b) {
document.getElementById("demo").innerHTML = "a is less than b";
}
</script>
</body>
</html>

Output

a is less than b


    
        <script>
  /*
    JavaScript program to check if the input year is a leap year or not by using following conditions
      The year can be evenly divided by 4, unless;
      The year can be evenly divided by 100, it is NOT a leap year, unless;
      The year is also evenly divisible by 400. Then it is a leap year.
    Input
    Enter year:
    1992
    Output
    1992 is a leap year
  */
  msg = 'Enter year:';
  var year = prompt(msg);
  //integer conversion
  year = parseInt(year)
  //first divisible by 4 and not by 100 or second divisible by 400
  if ((year % 4 == 0 && year % 100!=0) || year%400==0){
    document.write(`${year} is a leap year`);}
  else{
    document.write(`${year} is a leap year`);}
</script>

        Try

The if else statement:

The 'if...else' statement is the next form of control statement that allows JavaScript to execute statements in a more controlled way.

Syntax:

if (expression){
Statement(s) to be executed if expression is true
}
else{
Statement(s) to be executed if expression is false
}

Example:

<html>
<body>
<script>
var a=10;
var b=20;
if (a>b) {
document.write("a is less than b");
}
else{
document.write("a is greater than b");
}
</script>
</body>
</html>

Output:

a is greater than b


    
        <script>
  /*
  JavaScript program to print the grade of person using the following conditions
    ->A if percentage >85
    ->A- if %<=85 and >80
    ->B if %<=80 and >70
    ->C if %<=70 and >60
    ->D if %<=60 and >40
    ->E if %<=40 and also should print 'candidate failed' if %<=35
  Input
  enter the percentage of student:
  86
  Output
  Grade - 'A'
*/
//finding grades
//message variable
var msg = 'enter the percentage of student:';
//printing message for user input
var a = prompt(msg);
if(a>85)
    {//if percent > 85 --A
      document.write('Grade - A');}
else if(a<=85 && a>80)
    {//80<percent80=85  --A
      document.write('Grade - A-');}
else if(a>70 && a<=80)
    {//70<percent<=80 --B
      document.write('Grade - B');}
else if(a>60 && a<=70)
    {//60<percent<=70  --C
      document.write('Grade - C');}
else if(a>40 && a<=60)
    {//40<percent<=60  --D
      document.write('Grade - D');}
else if(a<=40)
    {//percent<=40  --E
      if(a<=35)
        {//failed condition
          document.write('Grade - E Candidate failed');}
      else
        {document.write('Grade - E');}}
</script>

        Try

The else if Statement

else if statement is used to specify a new condition which will be assesed when the first condition is false.

Syntax:

if (condition1) {
block of code to be executed if condition1 is true
} else if (condition2) {
block of code to be executed if the condition1 is false and condition2 is true
} else {
block of code to be executed if the condition1 is false and condition2 is false
}

Example:

<!DOCTYPE html>
<html>
<body>
<script>
var greeting;
var time = new Date().getHours();
if (time < 10) {
greeting = "Good morning";
} else if (time < 20) {
greeting = "Good day";
} else {
greeting = "Good evening";
}
document.write(greeting);
</script>
</body>
</html>

Output:

Good day

The Switch Statement

switch statement is used to select one of many blocks of code to be executed.

Syntax:

switch(expression) {
case n:
code block
break;
case n:
code block
break;
default:
code block
}

This is how it works:

The switch expression is evaluated once.
The value of the expression is compared with the values of each case.
If there is a match, the associated block of code is executed.

Example:

<!DOCTYPE html>
<html>
<body>

<script>
var i=2;
switch(i){
case 0:
document.write("i equals to 0");
break;
case 1:
document.write("i equals to 1");
break;
case 2:
document.write("i equals to 2");
break;
default:
document.write("i equals to 0 or 1 or 2");
}
</script>
</body>
</html>

Output:

i equals to 2