Javascript: Control Statements

Chapter 7: Javascript: Control Statements

CS 80: Internet Programming

Instructor: Mark Edmonds

Background and Terminology

Algorithm

• What is an algorithm?

– A procedure for solving a problem. Consists of:

1. The actions to be executed

2. The order in which the actions are to be executed

–

Notice: this definition has nothing to do with a programming language, program statements,

etc.

We are abstracting away code into problem solving

Background and Terminology

Pseudocode

• A way to express the essence of an algorithm without using a programming language

•

Informally, it is a way to express what and how the algorithm does something, but doesn’t specify

syntax

• Why is this useful?

–

Syntax is cumbersome, many trivial details of the code do not correspond to the overarching

problem the algorithm solves

Background and Terminology

Pseudocode

• Example: Leap year

1 // leap year pseudocode

2 if year is divisible by 400 then

3 is_leap_year

4 else if year is divisible by 100 then

5 not_leap_year

6 else if year is divisible by 4 then

7 is_leap_year

Mark Edmonds 1

Javascript: Control Statements

8 else

9 not_leap_year

• This is in between code and English!

• This can be directly converted to code, regardless of programming language used

Background and Terminology

Pseudocode

•

There are multiple ways of writing the same program, and we can write multiple versions of

pseudocode

• Example: Compound conditional leap year:

1 leapYear = false

2 if year % 400 == 0

3 leapYear = true

4 else if year % 100 != 0 and year % 4 == 0

5 leapYear = true

• Notice we are just describing the logic behind the program without consideration of syntax

Background and Terminology

Pseudocode

• Another example of the same logic encoded in dierent pseudocode:

1 leapYear = false

2 if year % 400 == 0 or (year % 100 != 0 and year % 4 == 0)

3 leapYear = true

Control Statements

• Prior to the if...else statement, our programs executed sequentially

– Programs executed from top to bottom

• if...else

introduced branching, or a conditional jump, meaning the program would "choose a

fork in the road" based on some boolean e valuation

–

Branching enables more powerful programs, since the state of particular variables can vary

with each execution

Mark Edmonds 2

Javascript: Control Statements

– if...else

gave us control over what program statements to executes for cases we cared

about

Control Statements

• History lesson: the goto statement

– Instructed a program to jump to a particular line in the program

–

Think of a function: "goto the starting line of the called function" and terminates with a

"goto to the next line of the calling function"

– This is a largely deprecated programming practice

–

Enabled a "spaghetti programming" where a program was like a whole bunch of spaghetti

laid out, with each piece of spaghetti representing a goto statement

Control Statements

•

Now, imperative languages (Javascript, C, C++, Java, Python, ...) use structed programming that

consists of three control structures

1. sequence structure - execute the program linearly, one right aer another

2. selection structure - select or ignore a program statement (if...else)

repetition structure - repeat the program statement(s) a certain number of times (

while,

do...while, for, for...in)

Control Statements

•

We can model the control structure of a program with graphical models that show the control

flow of the program

– A flowchart of the possible program executions

–

Useful for modeling sub components of a program, but infeasible to model an entire pro-

gram’s execution

Control Statements

Javascript Keywords

Mark Edmonds 3

Javascript: Control Statements

Figure 1: Javascript keywords

Control Statements

• if statement

– Already covered

– Important new terminology in terms of control flow:

is a single-selection statement. It selects or ignores a single action (program state-

ment(s))

We can think of an if statement as having a single entry point and single exit point

Control Statements

• if...else statement

– Already covered

– if...else is a double-selection statement

Selects among two actions

Control Statements

• Ternary Operator (Conditional Operator)

Mark Edmonds 4

Javascript: Control Statements

• We can shorthand the if else statement with a ternary operator of the following form:

1 cond ? true_action : false_action

• Example:

1 document.writeln( student_grade >= 60 ? "Passed" : "Failed" ); //

immediately using the result

2 var courseResult = student_grade >= 60 ? "Passed" : "Failed"; //

assignment to the result

Control Statements

Ternary Operator (Conditional Operator)

• Dierences with if...else

– Ternary operator returns a value

– This is how the above two examples work (collapse/evaluate the ternary to see...)

E.g. if

student_grade

was 50, this the same as calling

document.writeln("

Failed"); or assigning pass_fail = "Failed";

Dangling else’s

• We are permitted to write nested if...else statements

• We also don’t have to include the curly braces {}, which start a block statement

– Block statements are groupings of program statements

–

You can think of them like compound statements, in the same sense of compound condi-

tionals

Dangling else’s

Variable Scope

• Example block statement:

1 // javascript blocks and scope

2 var a1 = 3;

3 {

4 var a2 = 5;

5 }

6 console.log(a1 + a2);

Mark Edmonds 5

Javascript: Control Statements

• This behavior is dierent from C/C++!

– Block statements do not introduce scope

Dangling else’s

Variable Scope

• Scope is the "lifetime of a variable"

–

When a variable/function goes out of scope, it is not valid to use that variable or function

again

–

In the example above, an equivalent C/C++ code segement would fail to compile because

a2 would be out of scope

– The scope of a2 would be from its declaration to its closing curly brace

• Back to why this matters for if...else...

Dangling else’s

• If we don’t include the curly braces, we have an implicit block statement

– But what problems might we encounter with nested if...else’s?

Dangling else’s

•

Consider the following possibilities (hint: the indentation does not aect the semantic meaning)

1 // dangling else's

2 if ( x > 5 )

3 if ( y > 5 )

4 document.writeln( "<p>x and y are > 5</p>" );

5 else

6 document.writeln( "<p>x is <= 5</p>" );

1 // dangling else's

2 if ( x > 5 )

3 if ( y > 5 )

4 document.writeln( "<p>x and y are > 5</p>" );

5 else

6 document.writeln( "<p>x is <= 5</p>" );

Mark Edmonds 6

Javascript: Control Statements

Dangling else’s

• The first indentation reflects the semantics. Why?

1 // dangling else's

2 if ( x > 5 )

3 if ( y > 5 )

4 document.writeln( "<p>x and y are > 5</p>" );

5 else

6 document.writeln( "<p>x is <= 5</p>" );

1 // dangling else's

2 if ( x > 5 )

3 if ( y > 5 )

4 document.writeln( "<p>x and y are > 5</p>" );

5 else

6 document.writeln( "<p>x is <= 5</p>" );

Dangling else’s

• If there is no included block statements, a single statement is grouped to the if

– if...else is considered a single conditional statement

– This is part of JavaScript syntax, and is very common across programming languages

• What’s the solution?

–

Using block statements fixes this problem because it enforces which

the

else

belongs

Dangling else’s

• Fix:

1 // dangling else's

2 if ( x > 5 ){

3 if ( y > 5 )

4 document.writeln( "<p>x and y are > 5</p>" );

5 } else

6 document.writeln( "<p>x is <= 5</p>" );

• I (personally) recommend always wrapping conditionals, loops, etc. with block statements:

1 // dangling else's

Mark Edmonds 7

Javascript: Control Statements

2 if ( x > 5 ) {

3 if ( y > 5 ){

4 document.writeln( "<p>x and y are > 5</p>" );

5 }

6 } else {

7 document.writeln( "<p>x is <= 5</p>" );

8 }

Dangling else’s

• Consider another error-prone situation:

1 // dangling else's

2 if ( grade >= 60 )

3 document.writeln( "<p>Passed</p>" );

4 else

5 document.writeln( "<p>Failed</p>" );

6 document.writeln( "<p>You must take this course again.</p>" );

• Under what circumstances will "You must take this course again" be printed to the user?

Dangling else’s

1 // dangling else's

2 if ( grade >= 60 )

3 document.writeln( "<p>Passed</p>" );

4 else

5 document.writeln( "<p>Failed</p>" );

6 document.writeln( "<p>You must take this course again.</p>" );

• The Javascript interpreter does not read indentation for semantics

• The last line is not associated with the else

• Semantic version:

1 // dangling else's

2 if ( grade >= 60 )

3 document.writeln( "<p>Passed</p>" );

4 else

5 document.writeln( "<p>Failed</p>" );

6 document.writeln( "<p>You must take this course again.</p>" );

Mark Edmonds 8

Javascript: Control Statements

Dangling else’s

• Fix:

1 // dangling else's

2 if ( grade >= 60 )

3 document.writeln( "<p>Passed</p>" );

4 else {

5 document.writeln( "<p>Failed</p>" );

6 document.writeln( "<p>You must take this course again.</p>" );

7 }

Dangling else’s

• The main point: don’t trust indentation!

– Use explicit block statements (through curly braces)

You must use a block if you have more than a single statement under your condition-

al/loop

I do this all the time, no matter. I personally belie ve it oers a cleaner, more consistent

code style that has better defined semantics

Dangling else’s

• Technical semantics (the actual logic here):

– if and else will associate with the next statement

– That statement can be a single program statement, or a compound statement

By making the next statement a block statement, we avoid the problem completely,

even if we only execute one statement in that block statement.

Removes ambiguity in all cases while adding minimal amount of lines to your

program (not that ever print source code anyway, so the length of your program

doesn’t really matter)

Errors

•

When debugging, we need to evaluate what is causing the problem in our program and how we

can fix it

• Three main types of errors:

–

Syntax errors - invalid code. The compiler/interpreter cannot succesfully execute the pro-

gram. Will be detected by the computer. Basically means you violated the rules of the

Mark Edmonds 9

Javascript: Control Statements

language. Example: forgetting to put a closing curly brace at the end of an if...else.

– Semantic errors - valid code, but does not produce the results you expect. Example: using

the wrong variable or operator somewhere in your code

–

Design errors - valid code and produces the results you expect. However, your understanding

of the problem is wrong. Example: using the wrong forumla for something.

Errors - Semantic & Design

• Semantic and design errors are very similar but have dierent implications for debugging

–

A semantic error means we understand the problem and need to adjust our code to reflect

that understanding

–

A design error means we don’t understand the problem and will never be able to produce a

working program

• A design error is a more significant problem than semantic error!

Fixing Errors

• Ways to fix errors:

–

Syntax error: computer will usually give a hint as to what’s causing the problem. Go inspect

your code to see what might be wrong (generally easy, but can be incredibly frustrating)

–

Semantic error: assuming you have correct pseudocode, something in your program doesn’t

match the pseudocode. Compare the two and make sure the operations match up. This is

when you can’t believe you’ve wasted an hour verifying your pseudocode matches your

code but you mistyped a + for a *.

–

Design error: Your pseudocode is wrong. Fix the pseudocode first. There’s no specific

advice to give since this error is always problem-specific. This is when you email your

professor/supervisor/(maybe) customer for guidance.

Reptition (loops)

• Repeat an action a number of times while a condition is true

• Example shopping pseudocode:

1 While there are more items on my shopping list

2 Purchase next item and cross it off my list

• When the condition becomes false, the loop exits

• Critical part: the loop must do something that will eventually cause the condition to be false

Mark Edmonds 10

Javascript: Control Statements

– Otherwise, we are stuck in an infinite loop!

Reptition (loops)

• Example for shopping:

1 // shopping list

2 var shopping_list = ["pants", "grocercies", "car"];

3 var i = 0;

4 // purchase all items in the list

5 while(i < shopping_list.length)

6 {

7 purchase(shopping_list[i]); // purchase the item

8 i = i + 1; // move to the next item

9 }

10 function purchase(item){

11 window.alert("Purchased " + item);

12 }

Reptition (loops)

• If you think visually, consider the following code flowchart

1 var product = 2;

2 while ( product <= 1000 )

3 {

4 product = 2 * product;

5 }

Reptition (loops)

• If you think visually, consider the following code + flowchart

Mark Edmonds 11

Javascript: Control Statements

• We can think about a loop as a repeated cycle in a flowchart until a condition becomes false

Exercise

Class Average

• Write pseudocode and javascript to average a class’s scores on an exam.

•

The program should prompt the user for the number of students in the class, then ask for each

score.

• The scores should be printed nicely into a table with the average at the bottom.

Exercise

Class Average

• Pseudocode:

1 Set total to zero

2 Set grade counter to zero

3 Input number of students

4 Print table header

5 While grade counter is less than number of students

6 Input the next grade

7 Add the grade into the total

8 Add one to the grade counter

9 Print grade to table

10 Set the class average to the total divided by number of students

11 Print the class average to table

Mark Edmonds 12

Javascript: Control Statements

Exercise: class_average.html

1 <!DOCTYPE html>

2 <html>

4 <head>

5 <meta charset="utf-8">

6 <title>Class Average Problem</title>

7 <style>

8 table,

9 th,

10 td {

11 border: 1px solid black;

12 }

14 table {

15 border-collapse: collapse;

16 }

18 th,

19 td {

20 padding: 15px;

21 text-align: left;

22 }

23 </style>

24 <script>

25 var total; // sum of grades

26 var gradeCounter; // number of grades entered

27 var grade; // grade typed by user

28 var average; // average of all grades

29 // initialization phase

30 total = 0; // clear total

31 gradeCounter = 0;

32 var numStudents = window.prompt("Enter the number of students: ", "

0");

33 numStudents = parseInt(numStudents);

35 if (numStudents == 0)

36 {

37 document.writeln("The number of students is 0");

38 }

39 else

40 {

Mark Edmonds 13

Javascript: Control Statements

41 //setup table

42 document.writeln("<table>");

43 document.writeln("<caption><strong>Exam scores</strong></caption>

");

44 document.writeln("<thead>\n<tr>\n<th>Student Number</th>\n<th>

Grade</th>\n</thead>");

45 document.writeln("<tbody>");

47 // loop, processing phase

48 while (gradeCounter < numStudents) {

49 // prompt for input and read grade from user

50 grade = window.prompt("Enter integer grade:", "0");

51 // convert grade from a string to an integer

52 grade = parseInt(grade);

53 // add gradeValue to total

54 total = total + grade;

55 // add 1 to gradeCounter

56 gradeCounter = gradeCounter + 1;

57 // add data to table

58 document.writeln("<tr>\n<td>" + gradeCounter + "</td>\n<td>" +

grade + "</td>\n</tr>");

59 } // end while

61 // calculate the average

62 average = total / numStudents;

63 // display average of exam grades

64 document.writeln("</tbody>");

65 document.writeln("<tfoot>\n<tr>\n<th>Average</th>\n<th>" +

average + "</th>\n</tr>\n</tfoot>");

66 document.writeln("</table>");

67 }

68 </script>

69 </head>

71 <body>

72 </body>

74 </html>

Exercise: class_average_functions.html

• Next, let’s consider breaking our program into multiple parts.

Mark Edmonds 14

Javascript: Control Statements

• We’ll have one function to collect the grades

• Another function to display the grades

• And another function to calculate the average

Exercise: class_average_functions.html

1 <!DOCTYPE html>

2 <html>

4 <head>

5 <meta charset="utf-8">

6 <title>Class Average Problem</title>

7 <style>

8 table,

9 th,

10 td {

11 border: 1px solid black;

12 }

14 table {

15 border-collapse: collapse;

16 }

18 th,

19 td {

20 padding: 15px;

21 text-align: left;

22 }

23 </style>

24 <script>

25 function collectGrades() {

26 var total; // sum of grades

27 var gradeCounter; // number of grades entered

28 var grade; // grade typed by user

29 var average; // average of all grades

30 var grades = []; // array to store grades

31 // initialization phase

32 total = 0; // clear total

33 gradeCounter = 0;

34 var numStudents = window.prompt("Enter the number of students: ",

"0");

35 numStudents = parseInt(numStudents);

Mark Edmonds 15

Javascript: Control Statements

37 // loop, processing phase

38 while (gradeCounter < numStudents) {

39 // prompt for input and read grade from user

40 grade = window.prompt("Enter integer grade:", "0");

41 // convert grade from a string to an integer

42 grade = parseInt(grade);

43 // add gradeValue to total

44 total = total + grade;

45 // add 1 to gradeCounter

46 gradeCounter = gradeCounter + 1;

47 // add grade to our grades array

48 grades.push(grade);

49 } // end while

50 return grades;

51 }

53 function displayGrades(grades, average){

54 if (grades.length == 0)

55 {

56 document.writeln("The number of students is 0");

57 return;

58 }

59 //setup table

60 document.writeln("<table>");

61 document.writeln("<caption><strong>Exam scores</strong></caption>

");

62 document.writeln("<thead>\n<tr>\n<th>Student Number</th>\n<th>

Grade</th>\n</thead>");

63 document.writeln("<tbody>");

65 // loop, processing phase

66 for(var i = 0; i < grades.length; i++) {

67 // add data to table

68 document.writeln("<tr>\n<td>" + i + "</td>\n<td>" + grades[i] +

"</td>\n</tr>");

69 } // end while

71 // display average of exam grades

72 document.writeln("</tbody>");

73 document.writeln("<tfoot>\n<tr>\n<th>Average</th>\n<th>" +

average + "</th>\n</tr>\n</tfoot>");

74 document.writeln("</table>");

Mark Edmonds 16

Javascript: Control Statements

75 }

77 function calculateAverage(array){

78 // loop over grades to calculate average

79 var average = 0;

80 if (array.length > 0){

81 for(var i = 0; i < array.length; i++){

82 average += array[i];

83 }

84 average /= array.length;

85 }

86 return average;

87 }

89 var grades = collectGrades();

91 var average = calculateAverage(grades);

93 displayGrades(grades, average);

94 </script>

95 </head>

97 <body>

98 </body>

100 </html>

Exercise

Real Estate License

•

A college oers a course that prepares students for the state licensing exam for real estate brokers.

Last year, 10 of the students who completed this course took the licensing exam. Naturally, the

college wants to know how well its students performed.

•

You’ve been asked to write a script to summarize the results. You’ve been given a list of these 10

students. Next to each name is written a 1 if the student passed the exam and a 2 if the student

failed.

Exercise

Real Estate License

Mark Edmonds 17

Javascript: Control Statements

• Your script should analyze the results of the exam as follows:

Input each test result (i.e., a 1 or a 2). Display the message “Enter result” on the screen each

time the script requests another test result.

2. Count the number of test results of each type.

Display a summary of the test results indicating the number of students who passed and

the number of students who failed.

4. If more than eight students passed the exam, print the message “Bonus to instructor!”

Exercise

Real Estate License

• Pseudocode:

1 Initialize passes to zero

2 Initialize failures to zero

3 Initialize student to zero

4 While student counter is less than ten

5 Input the next exam result

6 If the student passed

7 Add one to passes

8 Else

9 Add one to failures

10 Add one to student counter

11 Print the number of passes

12 Print the number of failures

13 If more than eight students passed

14 Print "Bonus to Instructor!";

Exercise: bonus.html

1 <!DOCTYPE html>

2

3

4 <html>

6 <head>

7 <meta charset="utf-8">

8 <title>Analysis of Examination Results</title>

9 <script>

10 // initializing variables in declarations

Mark Edmonds 18

Javascript: Control Statements

11 var passes = 0; // number of passes

12 var failures = 0; // number of failures

13 var student = 0; // student counter

14 var result; // an exam result

15 // process 10 students; counter-controlled loop

16 while (student < 10) {

17 result = window.prompt("Enter result (1=pass,2=fail)", "0");

18 if (result == "1")

19 {

20 passes = passes + 1;

21 }

22 else

23 {

24 failures = failures + 1;

25 }

26 student = student + 1;

27 } // end while

28 document.writeln("<h1>Examination Results</h1>");

29 document.writeln("<p>Passed: " + passes +"; Failed: " + failures +

"</p>");

31 if (passes > 8)

32 {

33 document.writeln("<p>Bonus to instructor!</p>");

34 }

35 </script>

36 </head>

38 <body></body>

40 </html>

Assignment Operators

•

Modifying a variable (changing its value) is extremely common. We have a shorthand way doing

this:

1 c = c + 3;

2 c += 3;

• More generally, any statement of the form:

Mark Edmonds 19

Javascript: Control Statements

1 variable = variable operator expression;

• Can always be written as:

1 variable operator= expression; // operator could be +, -, *, /, %

2 // for example

3 c *= 4; // multiply by 4

Assignment Operators

Figure 2: Assignment operators

Mark Edmonds 20

Javascript: Control Statements

Increment and Decrement

Figure 3: Increment and decrement operators

Increment and Decrement

• Key dierence:

– pre changes the value, then returns the new value

– post returns the current value, then change the value

– These operators break PEMDAS; they have a higher precedence than *, /, %

Increment and Decrement

• Consider the following. Carefully consider the value of counter1 and counter2:

1 var a = 10;

2 var counter1 = 0;

3 var counter2 = 0;

4 var i = 0;

5 while(counter1++ < a)

6 {

7 //loop 1

8 console.log("Loop 1, i: ", i);

9 i++;

10 }

11 i=0;

12 while(++counter2 < a)

13 {

14 //loop 2

Mark Edmonds 21

Javascript: Control Statements

15 console.log("Loop 2, i: ", i);

16 i++;

17 }

18 console.log(counter1);

19 console.log(counter2);

Increment and Decrement

• What will be the final value of counter1 and counter2?

Increment and Decrement

• What will be the final value of counter1 and counter2?

– counter1

will be 11 (loop 1 runs 10 times, but counter1 is incremented an extra time (postin-

crement))

– counter2 will be 10 (loop 2 runs 9 times)

Additional Repetition Structures

• for

– Functionally equivalent to while

1 for(initialization_statement; loop_condition; loop_end_statement)

2 {

3 // loop body

4 }

5 // in practice

6 for (var i = 0; i < 10; i++)

7 {

8 // loop body

9 }

10 // which is the same as

11 var i = 0;

12 while (i < 10){

13 i++;

14 }

Additional Repetition Structures

• do...while

Mark Edmonds 22

Javascript: Control Statements

– Like a while loop, but guarantees that the loop will execute at least once

– Condition is checked at the end of the loop

1 var i = 0;

2 do {

3 // loop body

4 i++;

5 }

6 while(i < 10);

Example: sortedList.html

1 <!doctype html>

3

4

5 <html>

6 <head>

7 <meta charset="utf-8" />

8 <title>Sorted List</title>

9 </head>

10 <body>

11 <h1>People</h1>

12 <ol id="ol"></ol>

13 <script>

14 var name; // name entry

15 var people = []; // array of people

17 // get names

18 while (true) {

19 name = prompt("Name: ", "Done");

20 if (name === "Done") {

21 break; // stop getting names

22 }

23 people.push(name); // add name to end of array

24 }

26 // sort the array

27 people.sort();

29 // output the list

Mark Edmonds 23

Javascript: Control Statements

30 document.getElementById("ol").innerHTML = "<li>" + people.join("</li><

li>") + "</li>";

31 </script>

32 </body>

33 </html>

Mark Edmonds 24