How does the regular expression work in the function? - javascript

I've been dealing with an algorithm which takes a sequence of letters in alphabetic order, and if there is a character missing, it returns that character.
For example: fearNotLetter("abcdfg") would return "e".
My questions are:
What is the logic behind this solution?
Why and how regex is used here?
How does the condition in the for loop work?
function fearNotLetter(str) {
var allChars = '';
var notChars = new RegExp('[^'+str+']','g');
for (var i = 0; allChars[allChars.length-1] !== str[str.length-1] ; i++)
allChars += String.fromCharCode(str[0].charCodeAt(0) + i);
return allChars.match(notChars) ? allChars.match(notChars).join('') : undefined;
}

function fearNotLetter(str) {
var allChars = '';
var notChars = new RegExp('[^'+str+']','g'); //1
for (var i = 0; allChars[allChars.length-1] !== str[str.length-1] ; i++)//2
allChars += String.fromCharCode(str[0].charCodeAt(0) + i);
return allChars.match(notChars) ? allChars.match(notChars).join('') : undefined; //3
}
As the variable name says, it creates a negate of str which means
like !abcdfg
Loop, build the full correct string until the last
character of str, means full letters of the alphabet untill character g => abcdefg
Then it compares with match method of string the full text with the provided text:
dummy way you can think
abcdefg - abcdfg then you get e, if there are multiple characters are missing it concatenates with join method.

Related

Function to capitalize first and last letter of each word not working

I created a function that given any string will return the string with the first and last letter of each word capitalized. So far it works in some words, not on others, can someone help me figure out why?
function Capitalize(str) {
var spl = str.split(" ");
var words = [];
for (let i = 0; i < spl.length; i++) {
//For every word
for (let j = 0; j < spl[i].length; j++) {
//For every letter in each word
var word = spl[i];
var size = spl[i].length;
var firstLetterCapital = word.replace(word[0], word[0].toUpperCase()); //Creates new array
var LastLetterCapital = firstLetterCapital.replace(
word[size - 1],
word[size - 1].toUpperCase()
);
}
words.push(LastLetterCapital);
}
console.log(words.join(" "));
}
Capitalize("hello there");
It works when I type : Capitalize("my name is john smith"), but not with Capitalize("hello there")
I know it's a complete mess and probably a very bad way to do it, but I started programming a month ago so give me a break :)
#symlink has already explained why it is "HellO ThEre" instead of "Hello TherE". He also has given a solution to explicitly target first and last character of the string. I have accomplished not much different than already posted by members, except for .. "may be" a little more explanation.
You can break the entire problem in these four steps.
Get all the words into an array.
Create a function, that takes each word and targets first and last character, changes it and returns the changed word.
Apply a mapping step using the function created above (in step 2) to the entire array of words (obtained in step 1).
Join the transformed array, obtained in step 3, using a blank space as a separator.
I have written two functions that accomplish this task. I am sorry for long name of functions. It helps me keep track of things in a complex program (especially when I am in a hurry!).
Step 2 function
function Capitalize_FirstAndLast_One_Word(word){
// Split the string in array for easy access/manipulation by indexing
Split_String = word.split("")
// Target the first word
Split_String[0] = Split_String[0].toUpperCase();
// Target the last word
Split_String[Split_String.length - 1] = Split_String[Split_String.length - 1].toUpperCase();
// Join the array into a single word
Joined_Back = Split_String.join("")
return Joined_Back;
}
Step 1, 3 and 4 function
function Capitalize_Entire_String(str){
Regular_Exp = new RegExp(/\w+/g);
//Below is step 1
MatchedArray = str.match(Regular_Exp);
//Below is step 3
ConvertedArray = MatchedArray.map(Capitalize_FirstAndLast_One_Word);
// Below is step 4
ReturnedString = ConvertedArray.join(" ");
console.log(ReturnedString);
return ReturnedString;
}
Now you have everything. You can use the function like below.
Capitalize_Entire_String("hello there");
Capitalize_Entire_String("hello there this is a test");
Hope this helps. I am sorry if this turned out to be a redundant answer for you.
Reason your code don't work is the use of replace(). replace() will always replace the first character found.
There is absolutely no reason to run a nested loop. You can achieve this using a single loop.
function cap(str){
let spl = str.split(' ');
for(let i = 0; i < spl.length; i++){
let temp = spl[i];
temp = temp[0].toUpperCase() + temp.slice(1)
temp = temp.slice(0,-1) + temp[temp.length - 1].toUpperCase();
spl[i] = temp;
}
return spl.join(' ');
}
console.log(cap("a quick brown fox"))
An easier way is to use map() and template strings.
const cap = str => str
.split(' ')
.map(x => (
x.length === 1 ?
x.toUpperCase() :
`${x[0].toUpperCase()}${x.slice(1,-1)}${x[x.length -1].toUpperCase()}`)
)
.join(' ')
console.log(cap("a quick brown fox"))
To simplify the function, you could split the string into an array, map each word to the desired format, and join it together into a string again.
function Capitalize(str){
return str.split(" ").map((word) => word.charAt(0).toUpperCase() +
(word.length > 2 ? word.substring(1, word.length - 1) : "") +
(word.length > 1 ? word.charAt(word.length - 1).toUpperCase() : "")).join(" ");
}
console.log(Capitalize("i want to capitalize first and last letters"));
Congrats on starting out programming...
You can use this to achieve what you want to do
function capitalizeFirstAndLastLetters (str) {
const words = str.split(" "); // Split the string into words
const modified = [];
for (const word of words) {
if (word.length <= 2) {
modified.push(word.toUpperCase()); // If the word less than 3 characters, the whole word is capitalized
continue;
}
var firstCapital = word[0].toUpperCase(); // word[0] gets the first index of the string (I.e. the first letter of the word)
var lastCapital = word.slice(-1).toUpperCase(); // The slice function slices a portion of the word. slice(-1) gets the last letter
var middlePart = word.slice(1, -1); // slice(1, -1) means start slicing from the second index (I.e. 1) and ignore the last index
modified.push(firstCapital + middlePart + lastCapital);
}
return modified.join(" "); // Join each element in the modified array with a space to get the final string with each words first and last letters capitalized
}
capitalizeFirstAndLastLetters("hello there I am a boy"); // "HellO TherE I AM A BoY"
Try this, it worked for hello world because I guess you want the outcome to be HellO TherE right?:
function capitalize(str) {
var spl = str.split(" ");
var words = [];
for (let i = 0; i < spl.length; i++) {
//For every word
let changedWord = "";
for (let j = 0; j < spl[i].length; j++) {
//For every letter in each word
if(j == 0 || j == spl[i].length - 1) {
changedWord += spl[i][j].toUpperCase();
} else {
changedWord += spl[i][j].toLowerCase();
}
}
words.push(changedWord);
console.log(words);
}
console.log(words.join(" "));
}
capitalize("hello there");
ALSO: Make your functions name start with lowercase letter. Thats just how it is. Starting with uppercase letters usually are Classes. Just a quick tip
Maybe this does what you want, don't want to change much from your code:
function Capitalize(str) {
var spl = str.split(" ");
var words = [];
for (let i = 0; i < spl.length; i++) {
var word = spl[i];
var firstCapital = word[0].toUpperCase(); // get first character after capitalizing
var lastCapital = word.slice(-1).toUpperCase(); // get last character after capitalizing
var midOriginal = word.slice(1, -1);
words.push(firstCapital + midOriginal + lastCapital) // concat 3 parts
}
console.log(words.join(" "));
}
Capitalize("hello there");
This expression:
var LastLetterCapital = firstLetterCapital.replace(
word[size - 1],
word[size - 1].toUpperCase()
);
Is replacing the first occurrence of the character "e" in "There" with an uppercase "E".
Explanation
The replace() function first translates the first param: word[size - 1] to the literal character "e", then replaces the first occurrence of that character with the uppercase "E", resulting in the string "ThEre".
Solution
Use a regular expression as your first parameter instead, to ensure that the last character is targeted, regardless of whether or not that same character shows up anywhere else in the word:
var LastLetterCapital = firstLetterCapital.replace(/.$/, word[size - 1].toUpperCase());
function Capitalize(str) {
var spl = str.split(" ");
var words = [];
for (let i = 0; i < spl.length; i++) {
//For every word
var word = spl[i];
var size = spl[i].length;
for (let j = 0; j < size; j++) {
//For every letter in each word
var firstLetterCapital = word.replace(word[0], word[0].toUpperCase()); //Creates new array
var LastLetterCapital = firstLetterCapital.replace(/.$/, word[size - 1].toUpperCase());
}
words.push(LastLetterCapital);
}
console.log(words.join(" "));
}
Capitalize("hello there");
This should do the trick:
function Capitalize(str) {
return str.replace(/(\b\w|\w\b)/g, l => l.toUpperCase())
}
console.log(Capitalize('i want to be capitalized in a rather strange way'))
Explanation:
In the regular expression /(\b\w|\w\b)/g, \b means "word boundary" and \w means "word character", so (\b\w|\w\b) matches a word boundary followed by a word character OR a word character followed by a word boundary (i.e. the first and last character of words).
The matches of this expression are then passed to the inline function l => l.toUpperCase() (which itself is the second argument to replace) that capitalizes the passed letter.
the string type is immutable, so why don't you try to convert the string to an array like y = word.split('') and do y[0] = word.charAt(0).toUpperCase() and then convert back to string with y.join('')

How make a more concise code using the For statement?

I'm new in StackOverflow and JavaScript, I'm trying to get the first letter that repeats from a string considering both uppercase and lowercase letters and counting and obtaining results using the for statement. The problem is that the form I used is too long Analyzing the situation reaches such a point that maybe you can only use a "For" statement for this exercise, which I get to iterate, but not with a cleaner and reduced code has me completely blocked, this is the reason why I request help to understand and continue with the understanding and use of this sentence. In this case, the result was tested in a JavaScript script inside a function and 3 "For" sentences obtaining quite positive results, but I can not create it in 1 only For (Sorry for my bad english google translate)
I making in HTML with JavasScript
var letter = "SYAHSVCXCyXSssssssyBxAVMZsXhZV";
var contendor = [];
var calc = [];
var mycalc = 0;
letter = letter.toUpperCase()
console.log(letter)
function repeats(){
for (var i = 0; i < letter.length; i++) {
if (contendor.includes(letter[i])) {
}else{
contendor.push(letter[i])
calc.push(0)
}
}
for (var p = 0; p < letter.length; p++) {
for (var l = 0; l < contendor.length; l++) {
if (letter[p] == contendor[l]) {
calc [l]= calc [l]+1
}
}
}
for (var f = 0; f < calc.length; f++) {
if ( calc[f] > calc[mycalc]) {
mycalc = f
}
}
}
repeats()
console.log("The most repeated letter its: " + contendor[mycalc]);
I Expected: A result with concise code
It would probably be a lot more concise to use a regular expression: match a character, then lookahead for more characters until you can match that first character again:
var letter = "SYAHSVCXCyXSssssssyBxAVMZsXhZV";
const firstRepeatedRegex = /(.)(?=.*\1)/;
console.log(letter.match(firstRepeatedRegex)[1]);
Of course, if you aren't sure whether a given string contains a repeated character, check that the match isn't null before trying to extract the character:
const input = 'abcde';
const firstRepeatedRegex = /(.)(?=.*\1)/;
const match = input.match(firstRepeatedRegex);
if (match) {
console.log(match[0]);
} else {
console.log('No repeated characters');
}
You could also turn the input into an array and use .find to find the first character whose lastIndexOf is not the same as the index of the character being iterated over:
const getFirstRepeatedCharacter = (str) => {
const chars = [...str];
const char = chars.find((char, i) => chars.lastIndexOf(char) !== i);
return char || 'No repeated characters';
};
console.log(getFirstRepeatedCharacter('abcde'));
console.log(getFirstRepeatedCharacter('SYAHSVCXCyXSssssssyBxAVMZsXhZV'));
If what you're actually looking for is the character that occurs most often, case-insensitive, use reduce to transform the string into an object indexed by character, whose values are the number of occurrences of that character, then identify the largest value:
const getMostRepeatedCharacter = (str) => {
const charsByCount = [...str.toUpperCase()].reduce((a, char) => {
a[char] = (a[char] || 0) + 1;
return a;
}, {});
const mostRepeatedEntry = Object.entries(charsByCount).reduce((a, b) => a[1] >= b[1] ? a : b);
return mostRepeatedEntry[0];
};
console.log(getMostRepeatedCharacter('abcde'));
console.log(getMostRepeatedCharacter('SYAHSVCXCyXSssssssyBxAVMZsXhZV'));
If the first repeated character is what you want, you can push it into an array and check if the character already exists
function getFirstRepeating( str ){
chars = []
for ( var i = 0; i < str.length; i++){
var char = str.charAt(i);
if ( chars.includes( char ) ){
return char;
} else {
chars.push( char );
}
}
return -1;
}
This will return the first repeating character if it exists, or will return -1.
Working
function getFirstRepeating( str ){
chars = []
for ( var i = 0; i < str.length; i++){
var char = str.charAt(i);
if ( chars.includes( char ) ){
return char;
} else {
chars.push( char );
}
}
return -1;
}
console.log(getFirstRepeating("SYAHSVCXCyXSssssssyBxAVMZsXhZV"))
Have you worked with JavaScript objects yet?
You should look into it.
When you loop through your string
let characters = "hemdhdksksbbd";
let charCount = {};
let max = { count: 0, ch: ""}; // will contain max
// rep letter
//Turn string into an array of letters and for
// each letter create a key in the charcount
// object , set it to 1 (meaning that's the first of
// that letter you've found) and any other time
// you see the letter, increment by 1.
characters.split("").forEach(function(character)
{
if(!charCount[character])
charCount[character] = 1;
else
charCount[character]++;
}
//charCount should now contain letters and
// their counts.
//Get the letters from charCount and find the
// max count
Object.keys(charCount). forEach (function(ch){
if(max.count < charCount[ch])
max = { count: charCount[ch], ch: ch};
}
console.log("most reps is: " , max.ch);
This is a pretty terrible solution. It takes 2 loops (reduce) and doesn't handle ties, but it's short and complicated.
Basically keep turning the results into arrays and use array methods split and reduce to find the answer. The first reduce is wrapped in Object.entries() to turn the object back into an array.
let letter = Object.entries(
"SYAHSVCXCyXSssssssyBxAVMZsXhZV".
toUpperCase().
split('').
reduce((p, c) => {
p[c] = isNaN(++p[c]) ? 1 : p[c];
return p;
}, {})
).
reduce((p, c) => p = c[1] > p[1] ? c : p);
console.log(`The most repeated letter is ${letter[0]}, ${letter[1]} times.`);

How to insert symbol when encountering duplicates in a string

Lets assume that I have this string: aabbc and between every character which is not equal to the prior character I want to insert a symbol.
Which would result in the following string: aa$bb$c
How can this be achived?
You can do that in following steps:
You can convert string to array using Spread Operator
Then use map() on it
Inside map() check if the element on next to current is not same as current letter then add $ at its end.
Use join() to make array a string.
Remove the last $ using slice() which is extra.
let str = 'aabbc';
let res = [...str].map((x,i,arr) => arr[i+1] !== arr[i] ? x + '$' : x).join('').slice(0,-1)
console.log(res)
You can also use RegExp and match()
let str = 'aabbc';
let res = str.match(/(.)(\1*)/g).join('$')
console.log(res)
Can be done with a single replace:
input.replace(/(.)(?!\1)(?=.)/g, "$1$$")
Explanation of the pattern:
. - matches any single character (except newline); let's call this character X.
(.) - capturing subpattern; captures X so that it can be referenced in the replacement string (explained below).
\1 - backreference; matches another character identical to X.
(?!\1) - negative look-ahead; matches only if the X matched so far, is not followed by another X.
(?=.) - positive look-ahead; matches only if the X matched so far, is followed by any other character; in other words, asserts that the line does not end here. Can be omitted if you don't mind having a trailing $ appended.
Explanation of the replacement string:
$1 - the X captured by the first capturing subpattern (see above).
$$ - a single $.
Example:
var input = "aabbc";
var out = input.replace(/(.)(?!\1)(?=.)/g, "$1$$");
console.log(out);
Maybe try to do:
var charArray = myString.split(''); //this should split the string in an array of the characters. ["a", "a", "b", "b", "c"]
var newString = "";
for (int i = 0; i < charArray.length -1; i++) { //I do length -1 so to not cause an IndexOutOfBoundException at charArrayi+1]
if (charArray[i] === charArray[i+1]) {
newString += charArray[i] + charArray[i+1] + "$";
}
}
Something like that maybe?
I haven't actually tested this, I just typed it in here.
But in my head this feels right. :)
const a = 'aabbc';
let last = '';
let str = '';
for (let i = 0; i < a.length; i += 1) {
str += a[i];
if (last === a[i]) {
str += '$';
}
last = a[i];
}
console.log(str);
You can do this by using simple for loop as mentioned below:
var string = "aabbc";
var checkChar = "";
for (let i = 0; i < string.length; i++) {
checkChar += string.charAt(i);
if (string.charAt(i - 1) === string.charAt(i)){
checkChar += "$";
}
}
console.log(checkChar)

Replacing multiple characters in a string with a string rewrites characters

I have a variable (in this example var str = "I!%1$s-I!%2$s TTL!%3$s";), in which I want to replace the % with elements from an array (var regex = ['aaa', 'bbb', 'ccc'];).
I google around a bit and found this solution, but I'm having trouble implementing it. My problem is that I want to replace a single character with multiple characters, and then continue the string, but this just overwrites the characters. I actually have no idea why.
Any help is appreciated, my code below
String.prototype.replaceAt = function(index, character) {
return this.substr(0, index) + character + this.substr(index + character.length);
}
var str = "I!%1$s-I!%2$s TTL!%3$s";
var regex = ['replace', 'replace', 'replace'];
//find position of %
var find = /%/gi,
result, pos = [];
while ((result = find.exec(str))) {
pos.push(result.index);
}
//replace % with regex elements
for (x = 0; x < pos.length; x++) {
str = str.replaceAt(pos[x], regex[x]);
}
document.write(str);
Use replacement function, like this
var str = "I!%1$s-I!%2$s TTL!%3$s";
var regex = ['[123]', '[456]', '[789]'];
console.log(str.replace(/%(\d+)/g, function(match, group1) {
return regex[parseInt(group1) - 1] + group1;
}));
// I![123]1$s-I![456]2$s TTL![789]3$s
The RegEx /%(\d+)/g matches anything of the pattern % followed by one or more digits. And it captures the digits as a group. Then the exact match and the group is passed to the function to get the actual replacement. In the function, you convert the group to a number with parseInt and return the respective value from the regex array.

Count how many times a single letter appears in a word

I am very new to JavaScript, and I'm trying to figure out how to count how many times a single letter appears in a word. For example, how many times does 'p' appear in 'apple'
Here is what I have written so far but am having trouble figuring out where am I going wrong.
var letterInWord = function (letter, word) {
var letter = 0;
var word = 0;
for (var i = 0; i < letter.charAt; i+= 1) {
if (letter.charAt(i) === " " = true) {
letter++;
console.log('The letter `letter` occurs in `word` 1 time.');
}
}
return letter;
};
You've got a number of problems:
You're reusing parameter names as local variable names. Use different identifiers to track each bit of information in your function.
letter.charAt is undefined, if letter is a number, and is a function if letter is a string. Either way, i < letter.charAt makes no sense.
If you're searching for letter in word why do you want to look at letter.charAt(i)? You probably want word.charAt(i).
" " = true makes no sense at all.
Perhaps you meant something like this?
var letterInWord = function (letter, word) {
var count = 0;
for (var i = 0; i < word.length; i++) {
if (word.charAt(i) === letter) {
count++;
}
}
return count;
};
'apple'.match(/p/g).length // outputs 2
in other words:
var letterInWord = function (letter, word) {
return (word.match( new RegExp(letter, 'g') ) || []).length;
};
FIDDLE
Here's a smaller function that also works with characters like $ or * (and since it's calling length on a string, there's no need to use || [])
'apple'.replace(/[^p]/g,'').length // outputs 2
function charcount(c, str) {
return str.replace(new RegExp('[^'+c+']','g'),'').length
}
console.log = function(x) { document.write(x + "<br />"); };
console.log( "'*' in '4*5*6' = " + charcount('*', '4*5*6') ) // outputs 2
console.log( "'p' in 'pineapples' = " + charcount('p', 'pineapples') )// outputs 3

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