I need to reverse a string using a for loop but I just can't do it. What I'm doing wrong?
class Program
{
static void Main(string[] args)
{
string s;
string sReverse;
Console.WriteLine("Say any word please: ");
s = Console.ReadLine();
for (int i = 0; i < s.Length; i++)
{
s[s.Length - i] = sReversa[i];
}
}
}
Strings are immutable. You can't change them character by character.
If you want random access to the characters in a string, convert it to a char array first (using ToCharArray), then convert it back when you're done (String has a constructor that accepts a char array).
string a = "1234";
char[] b = a.ToCharArray();
b[1] = 'X';
a = new string(b);
Console.WriteLine("{0}", a);
Output:
1X34
There are much easier ways to reverse a string (e.g. LINQ would let you do var output = new string(input.Reverse().ToArray());), but if you want to use a for loop and your current approach, this is probably the piece of information you are missing.
These will be the minimal changes required in your code:
public static void Main()
{
string s;
string sReverse = string.Empty; // Initialise (always recommended)
Console.WriteLine("Say any word please: ");
s = Console.ReadLine();
for (int i = s.Length-1; i >=0 ; i--) // Chnage the order of indexing
{
sReverse += s[i]; // makes a new string everytime since strings are immutable.
}
Console.WriteLine(sReverse); // print your new string
}
As others said this may not be the best approach for very large number of string manipulation / formation. Instead, use StringBuilder.
StringBuilder is suited in situations where you need to perform repeated modifications to a string, the overhead associated with
creating a new String object can be costly.
Below is the snippet on how to use StringBuilder:
using System;
using System.Text; // Add this for StringBuilder
public class Program
{
public static void Main()
{
string s;
StringBuilder sReverse = new StringBuilder(); // Instantiate the object.
Console.WriteLine("Say any word please: ");
s = Console.ReadLine();
for (int i = s.Length-1; i >=0 ; i--)
{
sReverse.Append(s[i]); // Keep Appending to original string.
}
Console.WriteLine(sReverse.ToString()); // finally convert to printable string.
}
}
string reverse = string.Join("", "some word".Reverse());
You can not use s[s.Length - i] = sReversa[i];
Because Strings are immutable.
Instead, you can use sReverse = sReverse + s[Length];
The following code will give you reverse of a string:
static void Main(string[] args)
{
string s;
string sReverse = "";
int Length;
Console.WriteLine("Say any word please: ");
s = Console.ReadLine();
Length = s.Length - 1;
for (int i = Length; i >= 0; i--)
{
sReverse = sReverse + s[Length];
Length--;
}
Console.WriteLine("{0}", sReverse);
Console.ReadLine();
}
Note: For a large number of iteration, this might be a performance issue.
The simplest way to make your code work with minimal changes is to use a StringBuilder. Unlike string a StringBuilder is mutable.
Here's the code:
Console.WriteLine("Say any word please: ");
string s = Console.ReadLine();
StringBuilder sb = new StringBuilder(s);
for (int i = 0; i < s.Length; i++)
{
sb[s.Length - i - 1] = s[i];
}
string r = sb.ToString();
If you must use a for loop the way you want, you will have to allocate a new char array, initialize it with the string's characters from back to front, then take advantage of the string constructor that takes a char array as input. The code looks as follows:
public static string ReverseWithFor(string s)
{
if (string.IsNullOrEmpty(s)) return s;
char[] a = new char[s.Length];
for (int i = 0; i < s.Length; i++)
a[s.Length - 1 - i] = s[i];
return new string(a);
}
Related
I have tried this:
using System;
using System.Collections;
using System.Collections.Generic;
public class HelloWorld
{
public static string reverseWords(string str){
ArrayList strArr = new ArrayList();
int start = 0;
string revStr = "";
for(int i = 0; i < str.Length; i++){
if(str[i] == ' '){ // if there's a space,
while(start <= str[i - 1]){ // loop thru the iterated values before space
strArr.Add(str[start]); // add them to the ArrayList
start++; // increment `start` until all iterated values are-
} // stored and also for the next word to loop thru
}
}
for(int j = strArr.Count - 1; j >= 0; j--){
revStr += strArr[j] + " "; // keep appending ArrayList values to the-
} // string from the last to the first value
return revStr;
}
public static void Main(string[] args)
{
Console.WriteLine(reverseWords("Our favorite color is Pink"));
//Expected output : Pink is color favorite Our
}
}
And it's giving this error:
System.IndexOutOfRangeException: Index was outside the bounds of the array.
Please help me understand why this is not working. And also, if there's better way to do this ReverseWord function manually(not using any built-in functions at all).
I'm sorry if this is such a noob question. Any constructive criticism is appreciated. Thanks!
Here is a little improved version of your code that actually works for what you are willing to do.
using System;
using System.Collections;
public class HelloWorld
{
public static string reverseWords(string str){
ArrayList strArr = new ArrayList();
string currentWordString = string.Empty;
string revStr = string.Empty;
for(int i = 0; i < str.Length; i++){
if(str[i] == ' '){ // if there's a space,
strArr.Add(currentWordString); // add the accumulated word to the array
currentWordString = string.Empty; // reset accumulator to be used in next iteration
}else {
currentWordString += str[i]; // accumulate the word
}
}
strArr.Add(currentWordString); // add last word to the array
for(int j = strArr.Count - 1; j >= 0; j--){
revStr += strArr[j] + " "; // keep appending ArrayList values to the-
} // string from the last to the first value
return revStr;
}
public static void Main(string[] args)
{
Console.WriteLine(reverseWords("Our favorite color is Pink"));
//Expected output : Pink is color favorite Our
}
}
I'll let you do the remaining. Like removing the trainling space at the end of the sentence. add seperators other than space (e.g comma, semicolons...)
Try this
"Our favorite color is Pink".Split('\u0020').Reverse().ToList().ForEach(x =>
{
Console.WriteLine(x);
});
This will help
public static string ReverseCharacters(string str)
{
if(str == null)
{
throw new ArgumentNullException(nameof(str));
}
int lastIndex = str.Length - 1;
char[] chars = new char[str.Length];
char temp;
for(int i = 0; i < str.Length/2+1; i++)
{
// Swap. You could refactor this to its own method if needed
temp = str[i];
chars[i] = str[lastIndex - i];
chars[lastIndex - i] = temp;
}
return new string(chars);
}
public static string ReverseWords(string str)
{
if (str == null)
{
throw new ArgumentNullException(nameof(str));
}
if (string.IsNullOrWhiteSpace(str))
{
return str;
}
string space = " ";
StringBuilder reversed = new StringBuilder();
// reverse every characters
var reversedCharacters = ReverseCharacters(str);
// split words (space being word separator here)
var reversedWords = reversedCharacters.Split(space);
// for every revered word characters, reverse it back one more time and append.
foreach(var reversedWord in reversedWords)
{
reversed.Append(ReverseCharacters(reversedWord)).Append(space);
}
// remove last extra space
reversed = reversed.Remove(reversed.Length - 1, 1);
return reversed.ToString();
}
Here is the test result:
I'm doing an exercise that says I have to enter a phrase and put it in array, then I have to delete all the repeated characters and show the new phrase.
I did it like this but I don't know how to get char from string array and put it into another string array.
PS: I must only use the basics of C# and arrays :(
namespace Exercice3
{
class Program
{
static void Main(string[] args)
{
Console.WriteLine("Entrez une phrase SVP!!");
string phrase = Console.ReadLine();
string[] phraseArray = new string[]{ phrase };
string[] newPhrase = new string[phrase.Length];
for (int i = 0; i <= phrase.Length - 1; i++)
{
for (int j = 1; j <= phrase.Length - 1; j++)
{
if (phraseArray[i] != phraseArray[j])
newPhrase = phraseArray[i]; //Probleme here
}
}
Console.WriteLine(newPhrase);
}
}
}
something like this would do it. You don't have to do it this way...
using System;
using System.Collections.Generic;
public class Program
{
public static void Main()
{
var newColl = new List<char>();
foreach(char c in "aaabbbccc".ToCharArray())
{
if (!newColl.Contains(c))
newColl.Add(c);
}
Console.WriteLine(new string(newColl.ToArray()));
}
}
Output:
abc
Array-only method
using System;
using System.Collections.Generic;
public class Program
{
public static void Main()
{
const string orig = "aaabbbcccddd";
int origLen = orig.Length;
char[] newArr = new char[origLen]; // get max possible spots in arr
int newCount = 0;
for(int i = 0; i < origLen; i++)
{
bool yes = false;
for(int j = 0; j < newCount + 1; j++)
{
if (newArr[j] == orig[i])
{
yes = true;
break;
}
}
if (!yes)
{
newArr[newCount] = orig[i];
newCount++;
}
}
Console.WriteLine(new string(newArr));
}
}
Output:
abcd
The main problem here is that you are using arrays of strings. That is inappropriate because you are trying to iterate the characters of the string.
You need to construct your array of characters like this:
char[] phraseArray = phrase.ToCharArray();
This should allow you the ability to iterate the set of characters, check for duplicates, and form the new character array.
The reason why you're getting an IndexOutOfRangeException is because if you look at the two arrays:
string[] phraseArray = new string[]{ phrase };
And
string[] newPhrase = new string[phrase.Length];
The length of both arrays are completely different, phraseArray has a length of 1 and newPhrase will be set to the length of your phrase, so if you insert a world of more than 1 character both arrays will not match and then either:
if (phraseArray[i] != phraseArray[j])
Or
newPhrase = phraseArray[i];
Will fail, specially newPhrase = phraseArray[i];, because here you're trying to replace newPhrase an array, for a character phrase[i]. Basically your solution won't work. So what you can do is do what #Travis suggested, basically change your arrays from String[]to char[], then:
char[] phraseArray = phrase.ToCharArray();
Or Instead of using arrays you can just use another string to filter your phrase. The first string being your phrase and the second would be the string you'd add your non repeated characters to. basically this:
Console.WriteLine("Entrez une phrase SVP!!");
string phrase = Console.ReadLine();
string newPhrase = "";
// loop through each character in phrase
for (int i = 0; i < phrase.Length; i++) {
// We check if the character of phrease is within our newPhrase, if it isn't we add it, otherwise do nothing
if (newPhrase.IndexOf(phrase[i]) == -1)
newPhrase += phrase[i]; // here we add it to newPhrase
}
Console.WriteLine(newPhrase);
Don't forget to read the comments in the code.
Edit:
Based on the comments and suggestions given I implemented a similar solution:
Console.WriteLine("Entrez une phrase SVP!!");
char[] phrase = Console.ReadLine().ToCharArray();
char[] newPhrase = new char[phrase.Length];
int index = 0;
// loop through each character in phrase
for (int i = 0; i < phrase.Length; i++) {
// We check if the character of phrease is within our newPhrase, if it isn't we add it, otherwise do nothing
if (Array.IndexOf(newPhrase, phrase[i]) == -1)
newPhrase[index++] = phrase[i]; // here we add it to newPhrase
}
Console.WriteLine(newPhrase);
I am doing this in this way but its remove string previous characters, its out put is (Magic,Agic,Gic,Ic,C) but I want the whole string to be concate before and after.
public string[] Transform(string st)
{
string[] arr = new string[st.Length];
string[] arr1 = new string[st.Length];
for (int x = 0; x < st.Length; x++)
{
arr1[x] = char.ToLower(st[x]) + "".ToString();
}
for (int i = 0; i < st.Length; i++)
{
string st1 = "";
{
st1 = char.ToUpper(st[i]) + st.Substring(i + 1);
}
arr[i] = st1;
}
return arr;
}
You can do this with a single loop:
public static string[] Transform(string str)
{
var strs = new List<string>();
var sb = new StringBuilder();
for (int i = 0; i < str.Length; i++)
{
sb.Clear();
sb.Append(str);
sb[i] = char.ToUpper(str[i]);
strs.Add(sb.ToString());
}
return strs.ToArray();
}
What this does is adds the str to a StringBuilder and then modifies the indexed character with the upper case version of that character. For example, the input abcde will give:
Abcde
aBcde
abCde
abcDe
abcdE
Try it out on DotNetFiddle
If you wanted to get really fancy I'm sure there is some convoluted LINQ that can do the same, but this gives you a basic framework for how it can work.
You forgot to add left part of the string. Try to do like this:
st1 = st.ToLower().Substring + char.ToUpper(st[i]) + st.Substring(i + 1);
Here. This is twice as fast as the method that uses a string builder and a List
public static string[] Transform(string str)
{
var strs = new string [str.Length];
var sb = str.ToCharArray();
char oldCh;
for (int i = 0; i < str.Length; i++)
{
oldCh = sb[i];
sb[i] = char.ToUpper(sb[i]);
strs[i] = new string (sb);
sb[i] = oldCh;
}
return strs;
}
There's no need to clear and keep reading the string to the string builder. We also know the size of the array so that can be allocated at the start.
I wrote an answer for your questions (it's second code snippet), you can modify it for your needs, like changing the return type to string[], or use ToArray() extension method if you wanna stick with it. I think it's more readable this way.
I decided to put a the end little profiler to check CPU usage and memory compared to #Ron Beyer answer.
Here is my first attempt:
public static void Main()
{
var result = Transform("abcde");
result.ToList().ForEach(WriteLine);
}
public static IEnumerable<string> Transform(string str)
{
foreach (var w in str)
{
var split = str.Split(w);
yield return split[0] + char.ToUpper(w) + split[1];
}
}
Result:
Abcde
aBcde
abCde
abcDe
abcdE
Code fiddle https://dotnetfiddle.net/gnsAGX
There is one huge drawback of that code above, it works only if the passed word has unique letters. Therefore "aaaaa" won't produce proper result.
Here is my second successful attempt that seems works with any string input. I used one instance of StringBuilder to decrease the number of objects that would need to be created and manage on one instance, instead of so much copying objects so it's more optimized.
public static void Main()
{
var result = Transform("aaaaa");
result.ToList().ForEach(WriteLine);
}
public static IEnumerable<string> Transform(string str)
{
var result = new StringBuilder(str.ToLower());
for( int i = 0; i < str.Length; i++)
{
result[i] = char.ToUpper(str[i]);
yield return result.ToString();
result[i] = char.ToLower(str[i]);
}
}
Result:
Aaaaa
aAaaa
aaAaa
aaaAa
aaaaA
Code fiddle: https://dotnetfiddle.net/tzhXtP
Measuring execute time and memory uses.
I will use dotnetfiddle.net status panel, to make it easier.
Fiddle has few limitations like time execution of code 10 sec and used memory
besides differences are very significant.
I tested programs with 14 000 repetitions, my code additionally changes the output to array[].
My answer (https://dotnetfiddle.net/1fLVw9)
Last Run: 12:23:09 pm
Compile: 0.046s
Execute: 7.563s
Memory: 16.22Gb
CPU: 7.609s
Compared answer (https://dotnetfiddle.net/Zc88F2)
Compile: 0.031s
Execute: 9.953s
Memory: 16.22Gb
CPU: 9.938s
It slightly reduces the execution time.
Hope this helps!
public static string[] Transform(string str)
{
var strs = new string [str.Length];
var sb = str.ToCharArray();
char oldCh;
for (int i = 0; i < str.Length; i++)
{
oldCh = sb[i];
sb[i] = char.ToUpper(sb[i]);
strs[i] = new string (sb);
sb[i] = oldCh;
}
return strs;
}
I'm facing a problem while developing an application.
Basically,
I have a fixed string, let's say "IHaveADream"
I now want to user to insert another string, for my purpose of a fixed length, and then concatenate every character of the fixed string with every character of the string inserted by the user.
e.g.
The user inserts "ByeBye"
then the output would be:
"IBHyaevBeyAeDream".
How to accomplish this?
I have tried with String.Concat and String.Join, inside a for statement, with no luck.
One memory-efficient option is to use a string builder, since both the original string and the user input could potentially be rather large. As mentioned by Kris, you can initialize your StringBuilder capacity to the combined length of both strings.
void Main()
{
var start = "IHaveADream";
var input = "ByeBye";
var sb = new StringBuilder(start.Length + input.Length);
for (int i = 0; i < start.Length; i++)
{
sb.Append(start[i]);
if (input.Length >= i + 1)
sb.Append(input[i]);
}
sb.ToString().Dump();
}
This only safely accounts for the input string being shorter or equal in length to the starting string. If you had a longer input string, you'd want to take the longer length as the end point for your for loop iteration and check that each array index is not out of bounds.
void Main()
{
var start = "IHaveADream";
var input = "ByeByeByeByeBye";
var sb = new StringBuilder(start.Length + input.Length);
var length = start.Length >= input.Length ? start.Length : input.Length;
for (int i = 0; i < length; i++)
{
if (start.Length >= i + 1)
sb.Append(start[i]);
if (input.Length >= i + 1)
sb.Append(input[i]);
}
sb.ToString().Dump();
}
You can create an array of characters and then re-combine them in the order you want.
char[] chars1 = "IHaveADream".ToCharArray();
char[] chars2 = "ByeBye".ToCharArray();
// you can create a custom algorithm to handle
// different size strings.
char[] c = new char[17];
c[0] = chars1[0];
c[1] = chars2[0];
...
c[13] = chars1[10];
string s = new string(c);
var firstString = "Ihaveadream";
var secondString = "ByeBye";
var stringBuilder = new StringBuilder();
for (int i = 0; i< firstString.Length; i++) {
stringBuilder .Append(str[i]);
if (i < secondString.Length) {
stringBuilder .Append(secondStr[i]);
}
}
var result = stringBuilder.ToString();
If you don't care much about memory usage or perfomance you can just use:
public static string concatStrings(string value, string value2)
{
string result = "";
int i = 0;
for (i = 0; i < Math.Max(value.Length, value2.Length) ; i++)
{
if (i < value.Length) result += value[i].ToString();
if (i < value2.Length) result += value2[i].ToString();
}
return result;
}
Usage:
string conststr = "IHaveADream";
string input = "ByeBye";
var result = ConcatStrings(conststr, input);
Console.WriteLine(result);
Output: IBHyaevBeyAeDream
P.S.
Just checked perfomance of both methods (with strBuilder and simple cancatenation) and it appears to be that both of this methods take same time to execute (if you have just one operation). The main reason for it is that string builder take considerable time to initialize while with use of concatenation we don't need that.
But in case if you have to process something like 1500 strings then it's different story and string builder is more of an option.
For 100 000 method executions it showed 85 (str buld) vs 22 (concat) ms respectively.
My Code
Ok, so a friend of mine asked me to help him out with a string reverse method that can be reused without using String.Reverse (it's a homework assignment for him). Now, I did, below is the code. It works. Splendidly actually. Obviously by looking at it you can see the larger the string the longer the time it takes to work. However, my question is WHY does it work? Programming is a lot of trial and error, and I was more pseudocoding than actual coding and it worked lol.
Can someone explain to me how exactly reverse = ch + reverse; is working? I don't understand what is making it go into reverse :/
class Program
{
static void Reverse(string x)
{
string text = x;
string reverse = string.Empty;
foreach (char ch in text)
{
reverse = ch + reverse;
// this shows the building of the new string.
// Console.WriteLine(reverse);
}
Console.WriteLine(reverse);
}
static void Main(string[] args)
{
string comingin;
Console.WriteLine("Write something");
comingin = Console.ReadLine();
Reverse(comingin);
// pause
Console.ReadLine();
}
}
If the string passed through is "hello", the loop will be doing this:
reverse = 'h' + string.Empty
reverse = 'e' + 'h'
reverse = 'l' + 'eh'
until it's equal to
olleh
If your string is My String, then:
Pass 1, reverse = 'M'
Pass 2, reverse = 'yM'
Pass 3, reverse = ' yM'
You're taking each char and saying "that character and tack on what I had before after it".
I think your question has been answered. My reply goes beyond the immediate question and more to the spirit of the exercise. I remember having this task many decades ago in college, when memory and mainframe (yikes!) processing time was at a premium. Our task was to reverse an array or string, which is an array of characters, without creating a 2nd array or string. The spirit of the exercise was to teach one to be mindful of available resources.
In .NET, a string is an immutable object, so I must use a 2nd string. I wrote up 3 more examples to demonstrate different techniques that may be faster than your method, but which shouldn't be used to replace the built-in .NET Replace method. I'm partial to the last one.
// StringBuilder inserting at 0 index
public static string Reverse2(string inputString)
{
var result = new StringBuilder();
foreach (char ch in inputString)
{
result.Insert(0, ch);
}
return result.ToString();
}
// Process inputString backwards and append with StringBuilder
public static string Reverse3(string inputString)
{
var result = new StringBuilder();
for (int i = inputString.Length - 1; i >= 0; i--)
{
result.Append(inputString[i]);
}
return result.ToString();
}
// Convert string to array and swap pertinent items
public static string Reverse4(string inputString)
{
var chars = inputString.ToCharArray();
for (int i = 0; i < (chars.Length/2); i++)
{
var temp = chars[i];
chars[i] = chars[chars.Length - 1 - i];
chars[chars.Length - 1 - i] = temp;
}
return new string(chars);
}
Please imagine that you entrance string is "abc". After that you can see that letters are taken one by one and add to the start of the new string:
reverse = "", ch='a' ==> reverse (ch+reverse) = "a"
reverse= "a", ch='b' ==> reverse (ch+reverse) = b+a = "ba"
reverse= "ba", ch='c' ==> reverse (ch+reverse) = c+ba = "cba"
To test the suggestion by Romoku of using StringBuilder I have produced the following code.
public static void Reverse(string x)
{
string text = x;
string reverse = string.Empty;
foreach (char ch in text)
{
reverse = ch + reverse;
}
Console.WriteLine(reverse);
}
public static void ReverseFast(string x)
{
string text = x;
StringBuilder reverse = new StringBuilder();
for (int i = text.Length - 1; i >= 0; i--)
{
reverse.Append(text[i]);
}
Console.WriteLine(reverse);
}
public static void Main(string[] args)
{
int abcx = 100; // amount of abc's
string abc = "";
for (int i = 0; i < abcx; i++)
abc += "abcdefghijklmnopqrstuvwxyz";
var x = new System.Diagnostics.Stopwatch();
x.Start();
Reverse(abc);
x.Stop();
string ReverseMethod = "Reverse Method: " + x.ElapsedMilliseconds.ToString();
x.Restart();
ReverseFast(abc);
x.Stop();
Console.Clear();
Console.WriteLine("Method | Milliseconds");
Console.WriteLine(ReverseMethod);
Console.WriteLine("ReverseFast Method: " + x.ElapsedMilliseconds.ToString());
System.Console.Read();
}
On my computer these are the speeds I get per amount of alphabet(s).
100 ABC(s)
Reverse ~5-10ms
FastReverse ~5-15ms
1000 ABC(s)
Reverse ~120ms
FastReverse ~20ms
10000 ABC(s)
Reverse ~16,852ms!!!
FastReverse ~262ms
These time results will vary greatly depending on the computer but one thing is for certain if you are processing more than 100k characters you are insane for not using StringBuilder! On the other hand if you are processing less than 2000 characters the overhead from the StringBuilder definitely seems to catch up with its performance boost.