To Lecture Notes

IT 238 -- Apr 13, 2026

Review Exercises

1. Find the errors in the source code files contained in this project:
        meters-to-ft-in.zip.
   Answer: here is a zipfile of the corrected project.
2. What are the four primitive JavaScript datatypes that we saw last week?
   Answer: string, boolean, number, and undefined. We also saw that null, bigint, and symbol are JavaScript datatypes, but you only need to know the first four for now.
3. Name two non-primitive JavaScript datatypes?
   Answer: object and function
4. What is the JavaScript ** operator?
   Answer: ** is the exponentiation operator; 5 ** 3 means 5 raised to the 3rd power = 5 * 5 * 5 = 125.
5. How do the Math.floor, Math.ceil, and Math.round methods work? Look at the Some Math Methods section of the Mar 8 Notes to answer this question. Answer:
   
   Math.floor returns the largest integer that is ≤ to its input.
   Math.ceil returns the smallest integer that is ≥ to its input.
   Math.round returns the integer that is closet to its input.
6. How can you use Math.round to round a floating point number to three digits after the decimal point? Answer:
   

   var amount = 23.5425643;
   var amtRounded = Math.round(amount * 1000) / 1000;
   // Details of calculation
   // Original amount: 23.5425643;
   // Amount after multiplying by 1000: 23542.5643
   // Amount after taking the floor:    23543
   // Amount after dividing by 1000:    23.543
   

7. Use the JavaScript Math.random method, which returns a floating point number in the interval [0, 1), to return a random number in the set {1, 2, 3, 4, 5, 6}. Answer:
   

   var randomValue = Math.floor(Math.random( ) * 6) + 1
   // JS Expression       Random Output Range
   // Math.random( )      [0, 1)
   // Math.random( ) * 6  [0, 6)
   // Math.floor(Math.random( ) * 6)     {0, 1, 2, 3, 4, 5}
   // Math.floor(Math.random( ) * 6) + 1 {1, 2, 3, 4, 5, 6}
   

8. What were the control structures mentioned in Edsgar Dykstra's seminal letter Go To Considered Harmful sent to the editor of the journal of the Association of Computing Machinery (ACM) in 1968?
   Answer: The two control structures are if B then A (decision) and while B repeat A (repetition). These are in addition to sequence, which means listing statements in order.
9. This Road Runner cartoon shows two versions of the while loop.
   
   
10.  Explain how a while loop is related to a traditional for loop.
    Answer: The traditional for loop header has three parts: initialization, condition, and iteration. Here are three examples:
    

    // Display the numbers from 1 to 20:
    for(var n = 1; n <= 20; i++) {
        document.writeln(n + " ");
    }
    // Output: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
    // Corresponding while loop, which still has the iteration,
    // condition, and iteration statements, but not as conveniently
    // displayed on the same line:
    var n = 1;
    while(n <= 20) {
        document.writeln(n + " ");
        n++;
    }
    
    // Display the odd numbers less than 20:
    for(var n = 1; n < 20; i += 2) {
        document.writeln(n + " ");
    }
    // Output: 1 3 5 7 9 11 13 15 17 19
    // Corresponding while loop:
    var n = 1;
    while(n <= 20) {
        document.writeln(n + " ");
        n += 2;
    }
    
    // Start with 1 and keep doubling
    // the number while the number is
    // less than 100.
    for(var n = 1; n < 100; i *= 2) {
        document.writeln(n + " ");
    }
    // Output: 1 2 4 8 16 32 64
    // Corresponding while loop:
    var n = 1;
    while(n <= 20) {
        document.writeln(n + " ");
        n *= 2;
    }
    

Expression Interpolation

• Expression interpolation means inserting variables or expressions into a string.
• Strings that use expression interpolation are called f-strings in Python.
• A JavaScript string that uses expression interpolation is delimited with backquotes.
• Try out this example script:
  

  // ExpressionInterpolation Example
  var name = "Larry";
  var amt = "347.81";
  document.writeln(`${name} owes me $${amt}.`);
  // Output:
  Larry owes me $347.81.
  

• Change the third line of these statements to use expression interpolation.
  

  var feet = 13;
  var inches = 8;
  var output = feet + "\' " + inches + "\"";
  

Comparisons for Equality

• You are familiar with the comparison operator ==, used in JavaScript, Python, Java, and C#.
• This operator acts somewhat differently in each language.
• The JavaScript == operator is liberal in its comparisons.
• x == y returns true if x equals y after conversion to the same datatype.
• For example,
  

  5 == 5, "5" == "5", 5 == "5", 0 == false, 1 == true
  

  all return true.
• The === operator is strict in its comparisons.
• x === y returns true if the values x and y are equal and also are of the same datatype.
• For example 5 === "5", 0 === false, and 1 === true all return false because, in each case, the operands are not of the same datatype.
• Predict the output of these statements:

  document.writeln(`${5 == 5} ${5 == 3} ${"5" == "5"}<br>`);
  // The string "5" is converted to a number before
  // performing the comparison:
  document.writeln(`${5 == "5"}`);
  // Output:
  true false true
  true
  

• When performing a comparison, to convert a number to boolean (true or false), the rule is that zero converts to false, one converts to true; any other value does not convert to boolean.
  Predict the output of this statement:
  

  document.writeln(`${0 == false} ${5 == true} ${0.00000001 == false}`);
  // Output:
  true false false
  

• When performing a comparison, to convert a string to boolean, the rule is that the strings "", "0", and "0.0" convert to false, "1" and "1.0" convert to true. Any other string does not convert to boolean.
  Predict the output of these statements:
  

  document.writeln(`${"1" == true} ${"" == false}<br>`);
  document.writeln(`${"0" == false} ${"1" == true} ${"5" == true}<br>`);
  document.writeln(`${"0.00" == false} ${"1.00" == true} ${"5.00" == true}`);
  // Output:
  true true false
  true true false
  true true false
  

• The === comparison operator only returns true if the datatypes of the operands are the same without converting.
  Predict the output of these statements:
  

  document.writeln(`${5 === "5"}<br>`);
  document.writeln(`${"" === false} ${"1" === true}<br>`);
  document.writeln(`${"0" === false} ${"1" === true}<br>`);
  document.writeln(`${"0.00" === false} ${"1.00" === true}`);
  // Output
  false
  false false
  false false
  false false
  

• Practice Problem: Predict the output:
  

  var n = 1.0 - 0.8;
  var m = 0.3 - 0.1;
  document.writeln((n == m) + " ");
  // Output:
  false
  

  Why? Because of differing roundoff errors due to the binary representation of the double precision floating point numbers, 1.0 - 0.8 and 0.3 - 0.1 are not represented with exactly the same values. To see this, look at the output of this statement:
  

  document.writeln(n - m);
  // Output:
  -2.7755575615628914e-17 
  

  Instead of checking for equality with floating point values, we should check that n and m are very close to each other:
  

  document.writeln((Math.abs(n - m) < 1.0e-15) + " ");
  // Output:
  true
  

Arrays

• An array is a list of values accessed by index. What is the output?
  

  var a = [2, 3, 5, 7, 11, 13];
  document.writeln(a + "<br>");
  document.writeln(a[4]);
  // Output:
  2,3,5,7,11,13
  11
  

• Without arrays, multiple variables would be needed to hold a list of data values:
  

  a0 = 2;
  a1 = 3;
  a2 = 5;
  a3 = 7;
  a4 = 11;
  a5 = 13;
  

• Also, the index of an array can be a variable, which facilitates processing array items in a loop. For example:
  

  for(var i = 0; i < 5; i++) {
      document.write(a[i] + " ");
  }
  // Output:
  2 3 5 7 11 13
  

  or

  for(var n of a)) {
      document.write(n + " ");
  }
  // Output:
  2 3 5 7 11 13
  

• An array can contain mixed types:
  

  var a = [45.3, 'apple', false, null];
  

• What is the output of these statements (surprise)?
  

  var a = [2, 3, 5, 7];
  document.writeln(typeof a);
  // Output:
  object
  // Reason: an array is not a primitive datatype, it
  // is a composite datatype called an object type.
  

Project 2a

• Look at the Project 2a Description.