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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>
CHANGING CASE ; MEASURING LENGTH AND EXTRACTING PARTS ;FINDING SEGMENTS
;FINDING A CHARACTER AT A LOCATION ; REPLACING CHARACTERS
</title>
<link rel="stylesheet" href="./style.css" />
</head>
<body>
<div class="blue"></div>
<div class="heading">
<div class="space"></div>
<div class="head">
<h1>
Changing case <br />
Measuring length and extracting parts <br />
Strings: Finding segments <br />
Finding a character at a location <br />
Replacing characters
</h1>
</div>
<div class="space"></div>
</div>
<div class="blue-buttom">
<div class="para">
Assinment # 21-25 <br />
JAVASCRIPT
</div>
</div>
<div class="chap">
<h1><br />C h a p t e r s</h1>
<br />
</div>
<div class="moiz">
<h1>-: Changing case :-</h1>
<p class="ali">
You ask the user to enter her city. Then you check her city against a
list of the 5 cleanest <br />
cities. <br />
If the user enters "Cheyenne" or any of the other cleanest cities, your
code displays an <br />
alert telling her that it's one of the cleanest cities. <br />
But what if she enters "cheyenne" instead of "Cheyenne"—as some users
inevitably will? <br />
When that happens, there will be no match. JavaScript is literal-minded.
<br />
A human knows that in this context "cheyenne" means "Cheyenne." But
JavaScript doesn't. <br />
We need some way to get JavaScript to recognize the uncapitalized
version as a match. <br />
One way would be to expand the cleanestCities array to include the
uncapitalized <br />
versions of all the city names: <br />
var cleanestCities = ["Cheyenne", "cheyenne", "Santa Fe", "santa fe",
"Tucson", tucson", "Gr eat Falls",<br />
"great falls", "Honolulu", "honolulu"]; <br />
This works up to a point, but it's a lot of extra coding. Plus, if the
user enters "santa Fe ,"<br />
"Santa fe," or "sAnta Fe," we're back to the original problem. To cover
all these possibilit es<br />
and others, it would take a mile of code. <br />
The solution is to code the array elements in lower-case, and convert
the user's input, <br />
whatever it is, to lower-case, so we always have apples to compare with
apples. <br />
1 var cityToCheck = prompt("Enter your city"); <br />
2 cityToCheck = cityToCheck.toLowerCase(); <br />
3 var cleanestCities = ["cheyenne", "santa fe", "tucson", "great falls",
"honolulu"]; <br />
4 for (var i = 0; i <= 4; i++) { <br />
5 if (cityToCheck === cleanestCities[i]) { <br />
6 alert("It's one of the cleanest cities"); <br />
7 } <br />
8 } <br />
Line 2 is what's new here: <br />
2 cityToCheck = cityToCheck.toLowerCase(); <br />
The converted string is assigned to a variable. In this case, it's the
same variable whose <br />
string is being converted, cityToCheck. Note that the keyword
toLowerCase must be in <br />
camelCase. <br />
Note too that the toLowerCase method converts all the characters of the
string to lowercase, not just the initial letters. For example,
"ChEyEnNe" becomes "cheyenne." <br />
You could go the other way and convert everything to upper-case, then
test against <br />
"CHEYENNE," "SANTA FE, " etc. Most coders prefer the lower-case method.
To convert the <br />
string to upper-case, you'd write: <br />
2 cityToCheck = cityToCheck.toUpperCase(); <br />
</p>
</div>
<div class="moiz">
<h1>-: Strings: Measuring length and extracting parts :-</h1>
<p class="ali">
You've asked the user to give you the name of a city. You want to
convert the name she's <br />
given you to a name with an initial cap. Whether she's input "boston,"
"BOSTON", or <br />
"bosTon," you want to normalize the input to "Boston." The toLowerCase
and toUpperCase <br />
methods you learned in the last chapter won't get the job done on their
own, because they make <br />
the same wholesale change to every character in the string. But if you
break the string up into <br />
two segments, you can use these methods to get the string into the shape
you want. (For now, <br />
I'll ignore the possibility that the city name might be made up of two
or more words, like New <br />
Orleans or Sault Ste. Marie.) <br />
To copy a section of a string, you use the slice method. Suppose the
user has entered a <br />
string, and the string has been assigned to the variable cityToCheck.
The following code <br />
copies the first character of the string and assigns it to the variable
firstChar. The original <br />
value of cityToCheck doesn't change. If cityToCheck is "Boston",
firstChar is "B". <br />
var firstChar = cityToCheck.slice(0, 1); <br />
Things to be aware of: <br />
A string is indexed like an array. Only, instead of each index number
referring to an <br />
element, it refers to a character. <br />
Like array indexing, string indexing begins with 0. <br />
In the slice method, the first number inside the parentheses is the
index of the first <br />
character in the slice. The second number is not, however, the last
character in the slice. <br />
It's the first character after the slice. If you subtract the first
index from the second index, <br />
you'll always get the length of the slice. <br />
Here's another example. <br />
var someChars = cityToCheck.slice(2, 5); <br />
Again let's say that the string is "Boston". The slice begins with the
index-2 (the third) <br />
character, "s". It ends with the character before the index-5 character,
"n". someChars is "sto". <br />
If you omit the second number inside the parentheses, JavaScript
includes all the <br />
characters to the end of the string. <br />
var someChars = cityToCheck.slice(2); <br />
The slice begins with the index-2 (the third) character, "s". Since no
cutoff at the end is <br />
specified, the slice ends with the last character of the string.
someChars is "ston". <br />
Now we have a way to capitalize the first character of a string and
insure that the <br />
remaining letters are lower-case. <br />
1 var firstChar = cityToCheck.slice(0, 1); <br />
2 var otherChars = cityToCheck.slice(1); <br />
3 firstChar = firstChar.toUpperCase(); <br />
4 otherChars = otherChars.toLowerCase(); <br />
5 var cappedCity = firstChar + otherChars; <br />
Here's what happens in the code above, line-by-line: <br />
1. Copies the first character of the string and assigns it to the
variable firstChar. <br />
2. Copies all the characters from the second one to the end and assigns
them to the variable <br />
otherChars. <br />
3. Caps the first character. <br />
4. Lower-cases the other characters. <br />
5. Concatenates both parts to re-form the whole string. <br />
Sometimes it's useful to know how many characters are in a string. For
example, suppose <br />
you want to slice the first three characters from any string than
exceeds three characters in <br />
length, for example, slicing "Nov" from "November". To find the number
of characters in a <br />
string, you use the same language you've already learned to find the
number of elements in an <br />
array. <br />
1 var month = prompt("Enter a month"); <br />
2 var charsInMonth = month.length; <br />
3 if (charsInMonth > 3) { <br />
4 monthAbbrev = month.slice(0, 3); <br />
5 } <br />
Line 2 counts the characters in the string and assigns the number to the
variable <br />
charsInMonth. <br />
Being able to measure the number of characters in a string can come in
handy. For <br />
example, suppose you want to loop through a string, checking to see if
it has any double spaces <br />
in it. You can use the character count as the loop limiter. Here's some
code that checks for <br />
double spaces in a string and displays an alert if they're found. <br />
1 var str = prompt("Enter some text"); <br />
2 var numChars = str.length; <br />
3 for (var i = 0; i < numChars; i++) { <br />
4 if (str.slice(i, i + 2) === " ") { <br />
5 alert("No double spaces!"); <br />
6 break; <br />
7 } <br />
8 } <br />
Line 2 counts the number of characters in the string and assigns the
number to the variable <br />
numChars. In line 3, this number is used as the loop limiter. The loop
continues to run only as <br />
long as the counter, i, is less than the number of characters in the
string. (Remember, the length <br />
is 1-based, and the counter is 0-based, so the loop has to stop 1 short
of the length number.) <br />
Line 4 moves through the string character-by-character, examining each
2-character segment, <br />
looking for double spaces. <br />
</p>
</div>
<div class="moiz">
<h1>-: Strings: Finding segments :-</h1>
<p class="ali">
The New Yorker magazine doesn't allow the phrase "World War II. " They
say it should <br />
be "the Second World War." So let's search the following sentence for
the banned characters <br />
and replace them with the phrase that the New Yorker prefers. <br />
It is startling to think that, even in the darkest depths of World War
II, J. R. R. Tolkien was <br />
writing the trilogy, which contains, with the weird applicability
available only to poetry and <br />
myth, the essential notion that the good gray wizard can understand the
evil magi precisely <br />
because he is just enough like them to grasp their minds and motives in
ways that they cannot <br />
grasp his. <br />
You already know a way to find the banned segment and replace it.
Suppose the <br />
paragraph above has been assigned to the variable text. <br />
1 for (var i = 0; i < text.length; i++) { <br />
2 if (text.slice(i, i + 12) === "World War II") { <br />
3 text = text.slice(0, i) + "the Second World War" + text.slice(i + 12);
<br />
4 } <br />
5 } <br />
The code loops through the string looking for "World War II." Line 2
progresses through <br />
the string character-by-character, examining each 12-character sequence.
If it finds "World <br />
War II," line 3 concatenates three segments: all the characters
preceding "World War II," the <br />
substitute phrase "the Second World War," and then all the characters
following "World War <br />
II." <br />
But JavaScript has a more efficient way to accomplish this, using the
indexOf method. <br />
var firstChar = text.indexOf("World War II"); <br />
If the segment exists, the method finds the index of the first character
of the segment and <br />
assigns it to the variable firstChar. If the segment doesn't exist, the
method assigns -1 to the<br />
variable, so you know it's not there. <br />
Now we can replace the banned phrase with the preferred phrase with less
coding. <br />
1 var firstChar = text.indexOf("World War II"); <br />
2 if (firstChar !== -1) { <br />
3 text = text.slice(0, firstChar) + "the Second World War" +
text.slice(firstChar + 12); <br />
4 { <br />
Line 1 checks for the phrase, assigning the index of the first character
of the phrase to the <br />
variable firstChar—if the phrase is found. If it isn't found, -1 is
assigned to the variable. If <br />
the variable doesn't have the value -1 (line 2)—if the phrase has been
found—the <br />
concatenation in line 3 replaces the offending phrase with the correct
pharse. <br />
The indexOf method finds only the first instance of the segment you're
looking for. In the <br />
example above, you could overcome this limitation by looping. You'd
change the first instanc <br />
of "World War II" to "the Second World War," then in the next loop
iteration, find the next <br />
surviving instance and change that, and so on. <br />
To find the last instance of a segment in a string, use lastIndexOf. The
following code <br />
finds the index of the first character of the last instance of the
segment, the second "be". The <br />
variable segIndex winds up with a value of 16, the index of "b" in the
second "be". <br />
1 var text = "To be or not to be."; <br />
2 var segIndex = text.lastIndexOf("be"); <br />
</p>
</div>
<div class="moiz">
<h1>-: Strings: Finding a character at a location :-</h1>
<p class="ali">
The user has entered his first name. The string has been assigned to the
variable <br />
firstName. You want to extract the first character. You already know one
way to do it. <br />
var firstChar = firstName.slice(0, 1); <br />
Here's an alternate way to do it that's more direct. <br />
var firstChar = firstName.charAt(0) <br />
The code above finds a single character at index-0 (the beginning) of
the string <br />
represented by the variable firstName and assigns it to the variable
firstChar. <br />
The following code finds the last character in the string. <br />
var lastChar = name.charAt(name.length - 1); <br />
The following code cycles through a string looking for an exclamation
point. If the <br />
character is found, an alert displays. <br />
1 for (var i = 0; i < text.length; i++) { <br />
2 if (text.charAt(i) === "!") { <br />
3 alert("Exclamation point found!"); <br />
4 break; <br />
5 } <br />
6 } <br />
Note: The indexOf method can only identify the character at a particular
location. It can't <br />
change the character at a location. <br />
</p>
</div>
<div class="moiz">
<h1>-: Strings: Replacing characters :-</h1>
<p class="ali">
In previous chapters you learned two different ways to replace "World
War II" with "the <br />
Second World War" in a string. First, there was the loop-and-slice
approach. <br />
1 for (var i = 0; i < text.length; i++) { <br />
2 if (text.slice(i, i + 12) === "World War II") { <br />
3 text = text.slice(0, 1) + "the Second World War" + text.slice(i + 12);
<br />
4 } <br />
5 } <br />
You improved on that rather crude approach when you learned the indexOf
method. <br />
1 var firstChar = text.indexOf("World War II"); <br />
2 if (firstChar !== -1) { <br />
3 text = text.slice(0, firstChar) + "the Second World War" +
text.slice(firstChar + 12) <br />; 4 } <br />
But JavaScript provides a more straightforward way still, the replace
method. <br />
var newText = text.replace("World War II", "the Second World War");
<br />
The first string inside the parentheses is the segment to be replaced.
The second string is <br />
the segment to be inserted. In the above code, the segment "World War
II" is replaced by the <br />
segment "the Second World War" in the string represented by the variable
text, and the <br />
revised string is assigned to the new variable newText. <br />
If you assign the revised string to a new variable, as in the example
above, the original <br />
string is preserved. If you want the original string to be replaced by
the revised string, assign<br />
the revised string to the original variable. <br />
text = text.replace("World War II", "the Second World War"); <br />
In the examples above, only the first instance of a string is replaced.
If you want to <br />
replace all instances, you must let JavaScript know that you want a
global replace. <br />
var newText = text.replace(/World War II/g, "the Second World War");
<br />
In a global replace, you enclose the segment to be replaced by slashes
instead of <br />
quotation marks, and follow the closing slash with "g" for "global." The
segment to be inserted<br />
is enclosed by quotation marks, as in a one-time replace. <br />
</p>
</div>
<div class="moiz">
<h1>Assignment</h1>
<p class="ali">
<object
data="./chapters21-25.pdf"
type="application/pdf"
width="100%"
height="500px"
>
<p>
Unable to display PDF file.
<a href="./chapters21-25.pdf">Download</a>
instead.
</p>
</object>
</p>
</div>
<div class="chap">
<h1><br />Perform The Assignment</h1>
<br />
</div>
<script src="./app.js"></script>
</body>
</html>