Decisions and Looping

Code Blocks

A code block is a group of JS statements that are enclosed in between curly braces ({}). Certain code-blocks allow for modification of the execution sequence in interesting ways -- decision making and iteration.

if(){}/else(){}

The if/else construct lends itself to implementing binary decision trees in JS. An if/else block has the following syntax:

if( <CONDITION> ){
  // execute statements in block
  // if the <CONDITION> is true
}
else{
  // execute statements in block
  // if the <CONDITION> is false
}

The <CONDITION> can be any valid JS expression. Unlike other languages the <CONDITION> does not have to strictly evaluate to a Boolean true or false. The Interpreter will typecast the <CONDITION> to the nearest Boolean value (Either true or false).

Consider the following example where different entry prices apply depending on age.

var entryPrice, moviePrice;
if(age < 3){
  entryPrice = 0;
  moviePrice = 0;
}
else if(age < 16){
  entryPrice = 10;
  moviePrice = 2;
}
else{
  entryPrice = 100;
  moviePrice = 20;
}

switch(){}/case

This construct allows implementation of decision trees and is best suited for more than 2 decison options. Consider the following example of assigning grades:

// score variable holds the percent score earned by student
var grade;
if(score < 60){
  grade = "D";
}
else if(score < 70){
  grade = "C";
}
else if(score < 90){
  grade = "B";
}
else{
  grade = "A";
}

This can also be implemented using switch/case:

switch (score){
  case (score < 60):
    grade = "D";
    break;
  case (score < 70):
    grade = "C";
    break;
  case (score < 90):
    grade = "B";
    break;
  default:
    grade = "A";
    break;
}

while(){} and for(){}

The two constructs allow for iteration. This will be explained by iterating over an array to find the minimum and maximum value from a list of values.

Consider the following list of values:

var values = [3, 13, 14, 234, 12, 4, 13, 54, 64, 76, 65];

This list can be iterated using an index variable i and the while() loop as follows:

var i = 0; // define an initialize the index variable
// JS arrays are zero-indexed

while(i < values.length){
  console.log(values[i]);
  i++; //equivalent to i = i + 1;
}

This same while loop can be written as a for loop as follows:

for(var i = 0; i < values.length; i++){
  console.log(values[i]);
}

As you can observe the for loop combines the index initialization and increment together with the conditional.

Now let us look how we can find the minimum and maximum value of an array using while and for.

// Using while
var values = [3, 13, 14, 234, 12, 4, 13, 54, 64, 76, 65];
var min = Infinity;
var max = 0;
var i = 0;
while(i < values.length){
  if(values[i]>max){
    max = values[i];
  }
  if(values[i]<min){
    min = values[i];
  }
  i++;
}

// Using for
var values = [3, 13, 14, 234, 12, 4, 13, 54, 64, 76, 65];
var min = Infinity;
var max = 0;
for(var i = 0; i < values.length; i++){
  if(values[i]>max){
    max = values[i];
  }
  if(values[i]<min){
    min = values[i];
  }
}

var values = [-1, -5, 35, 43, 234, -67, 76, 64, 34, 77];
var sum = 0;
for (var i = 0; i < values.length; i++){
  if(values[i] < 0){
    continue;
  }
  sum = sum + values[i];
}
var avg = sum/values.length;