String prepend

From Rosetta Code
Task
String prepend
You are encouraged to solve this task according to the task description, using any language you may know.

Basic Data Operation
This is a basic data operation. It represents a fundamental action on a basic data type.

You may see other such operations in the Basic Data Operations category, or:

Integer Operations
Arithmetic | Comparison

Boolean Operations
Bitwise | Logical

String Operations
Concatenation | Interpolation | Comparison | Matching

Memory Operations
Pointers & references | Addresses

Task

Create a string variable equal to any text value.

Prepend the string variable with another string literal.

If your language supports any idiomatic ways to do this without referring to the variable twice in one expression, include such solutions.


To illustrate the operation, show the content of the variable.

Ada[edit]

In Ada, a variable of type String cannot change its length. So the variable S which we will change, need to be of the type Unbounded_String. Thus the need for conversions from String literal to Unbounded_String for initialization, and from Unbounded_String to String for printing.

with Ada.Text_IO; with Ada.Strings.Unbounded; use Ada.Strings.Unbounded;
 
procedure Prepend_String is
S: Unbounded_String := To_Unbounded_String("World!");
begin
S := "Hello " & S;-- this is the operation to prepend "Hello " to S.
Ada.Text_IO.Put_Line(To_String(S));
end Prepend_String;
Output:
Hello World!

ALGOL 68[edit]

Works with: ALGOL 68 version Revision 1.
Works with: ALGOL 68G version Any - tested with release algol68g-2.7.
Works with: ELLA ALGOL 68 version Any (with appropriate job cards).
File: String_prepend.a68
#!/usr/bin/a68g --script #
# -*- coding: utf-8 -*- #
 
STRING str := "12345678";
"0" +=: str;
print(str)
Output:
012345678

AutoHotkey[edit]

s := "foo"
s := s "bar"
Msgbox % s
Output:
foobar

AWK[edit]

 
# syntax: GAWK -f STRING_PREPEND.AWK
BEGIN {
s = "bar"
s = "foo" s
print(s)
exit(0)
}
 
Output:
foobar

BASIC[edit]

S$ = " World!"
S$ = "Hello" + S$
PRINT S$
 
Output:
Hello World!

Applesoft BASIC[edit]

Works with: Applesoft BASIC

BBC BASIC[edit]

Works with: BBC BASIC

Bracmat[edit]

  World!:?string
& str$("Hello " !string):?string
& out$!string
Hello World!

C[edit]

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
 
int main()
{
char str[100]="my String";
char *cstr="Changed ";
char *dup;
sprintf(str,"%s%s",cstr,(dup=strdup(str)));
free(dup);
printf("%s\n",str);
return 0;
}
Output:
Changed my String

C++[edit]

include <vector>
#include <algorithm>
#include <string>
#include <iostream>
 
int main( ) {
std::vector<std::string> myStrings { "prepended to" , "my string" } ;
std::string prepended = std::accumulate( myStrings.begin( ) ,
myStrings.end( ) , std::string( "" ) , []( std::string a ,
std::string b ) { return a + b ; } ) ;
std::cout << prepended << std::endl ;
return 0 ;
}
Output:
prepended tomy string

C#[edit]

using System;
 
namespace PrependString
{
class Program
{
static void Main(string[] args)
{
string str = "World";
str = "Hello " + str;
Console.WriteLine(str);
Console.ReadKey();
}
}
}
Hello World

Clojure[edit]

(def s (ref "World"))
(dosync (alter s #(str "Hello " %)))
 
user=> @s
"Hello World"

COBOL[edit]

       identification division.
program-id. prepend.
data division.
working-storage section.
1 str pic x(30) value "World!".
1 binary.
2 len pic 9(4) value 0.
2 scratch pic 9(4) value 0.
procedure division.
begin.
perform rev-sub-str
move function reverse ("Hello ") to str (len + 1:)
perform rev-sub-str
display str
stop run
.
 
rev-sub-str.
move 0 to len scratch
inspect function reverse (str)
tallying scratch for leading spaces
len for characters after space
move function reverse (str (1:len)) to str
.
end program prepend.
Hello World!
Works with: GNU Cobol version 2.0
       >>SOURCE FREE
PROGRAM-ID. prepend.
 
DATA DIVISION.
WORKING-STORAGE SECTION.
01 str PIC X(30) VALUE "world!".
 
PROCEDURE DIVISION.
MOVE FUNCTION CONCATENATE("Hello, ", str) TO str
DISPLAY str
.
END PROGRAM prepend.

ColdFusion[edit]

Classic tag based CFML[edit]

 
<cfoutput>
<cfset who = "World!">
#"Hello " & who#
</cfoutput>
 
Output:
Hello World! 

Script Based CFML[edit]

<cfscript>
who = "World!";
greeting = "Hello " & who;
writeOutput( greeting );
</cfscript>
Output:
Hello World! 

Common Lisp[edit]

A macro is necessary in order to prepend a string in-place:

(defmacro prependf (s &rest strs)
"Prepend the given string variable with additional strings. The string variable is modified in-place."
`(setf ,s (concatenate 'string ,@strs ,s)))
 
(defvar *str* "foo")
(prependf *str* "bar")
(format T "~a~%" *str*)
Output:
barfoo

D[edit]

import std.stdio;
 
void main() {
string s = "world!";
s = "Hello " ~ s;
writeln(s);
}
Output:
Hello world!

Déjà Vu[edit]

local :s "world!"
set :s concat( "Hello " s)
!print s
Output:
Hello world!

EchoLisp[edit]

 
define-syntax-rule
(set!-string-prepend a before)
(set! a (string-append before a)))
→ #syntax:set!-string-prepend
 
(define name "Presley")
→ name
(set!-string-prepend name "Elvis ")
name
"Elvis Presley"
 

Elena[edit]

import extensions.
 
program =
[
var s := "World".
s := "Hello " + s.
console writeLine:s.
 
// Alternative way
var s2 := String new:"World".
s2 insert:"Hello " at:0.
console writeLine:s2.
console readChar.
].

Elixir[edit]

 
str1 = "World!"
str = "Hello, " <> str1
 
Output:

"Hello, World!"

Emacs Lisp[edit]

version 1[edit]

 
(defun glue (str1 str2)
(concat str1 str2) )
 

version 2[edit]

 
(defun glue (str1 str2)
(format "%s%s" str1 str2) )
 

Eval:

 
(setq str "World!")
(setq str (glue "Hello, " str) )
(insert str)
 

Output:

Hello, World!

Erlang[edit]

Output:
1> S = "world".
"world"
2> "Hello " ++ S.
"Hello world"

ERRE[edit]

 
......
S$=" World!"
S$="Hello"+S$
PRINT(S$)
......
 
Output:
Hello World!

F#[edit]

let mutable s = "world!"
s <- "Hello, " + s
printfn "%s" s

Factor[edit]

 
"world"
"Hello " prepend
 

Forth[edit]

Forth has no string prepend word, but adding it is not difficult. This demonstration starts from the low level operations that Forth gives us and quickly builds a simple set of "WORDS" (sub-routines) that let us move strings from address to address. Strings are just an address on the data stack so we can reference them as many times as we need to. Our prepend word makes use of the Return stack as a temporary storage for the address of the string we want to prepend. Standard Forth also provides a named general purpose buffer called PAD, so we make use of that too. With this PREPEND becomes part of the language.

\ the following functions are commonly native to a Forth system. Shown for completeness
 
: C+! ( n addr -- ) dup c@ rot + swap c! ; \ primitive: increment a byte at addr by n
 
: +PLACE ( addr1 length addr2 -- ) \ Append addr1 length to addr2
2dup 2>r count + swap move 2r> c+! ;
 
: PLACE ( addr1 len addr2 -- ) \ addr1 and length, placed at addr2 as counted string
2dup 2>r 1+ swap move 2r> c! ;
 
\ Example begins here
: PREPEND ( addr len addr2 -- addr2)
>R \ push addr2 to return stack
PAD PLACE \ place the 1st string in PAD
R@ count PAD +PLACE \ append PAD with addr2 string
PAD count R@ PLACE \ move the whole thing back into addr2
R> ; \ leave a copy of addr2 on the data stack
 
: writeln ( addr -- ) cr count type ; \ syntax sugar for testing

Test our language extensions interactively at the console

256 buffer: string1 ok                                     
s" needs no introduction!" string1 place  ok                
string1 writeln 
needs no introduction! ok

s" This string "  string1 prepend writeln
This string needs no introduction! ok

Fortran[edit]

Early inability[edit]

Early Fortran had almost no ability to manipulate text except via overwriting text literals in a FORMAT statement used in a READ, that would then appear when the same FORMAT statement was used in a WRITE (!) perhaps as a heading.

Initial difficulty[edit]

With Fortran IV came the ability to use arrays of integers and the A1 format code in READ and WRITE statements for them. With sixteen-bit integers, one might use A2 and so forth, but the numerical values of the integers would not be convenient especially if the sign bit was involved. This would be even more difficult with floating-point variables. Still, the desire for good headings and annotations and flexible layout flogged one onwards. Following the Pascal "Hello world!" example, one might proceed somewhat as follows:
      INTEGER*4 I,TEXT(66)
DATA TEXT(1),TEXT(2),TEXT(3)/"Wo","rl","d!"/
 
WRITE (6,1) (TEXT(I), I = 1,3)
1 FORMAT ("Hello ",66A2)
 
DO 2 I = 1,3
2 TEXT(I + 3) = TEXT(I)
TEXT(1) = "He"
TEXT(2) = "ll"
TEXT(3) = "o "
 
WRITE (6,3) (TEXT(I), I = 1,6)
3 FORMAT (66A2)
END

This old-style source is acceptable to the F90 compiler as it stands. By chance, two characters per integer fits nicely but in many cases having one character per variable is easier for manipulation. So, as usual with Fortran, it's all done with arrays. The DATA statement demonstrates that a quoted string is acceptable as a value for an integer; it is just a matter of bit patterns, and this type miscegnation will work with floating-point variables also though resulting in even stranger numerical values. Looking more closely, note that an INTEGER*4 variable can hold four eight-bit characters but only two-character text literals have been specified. Unlike integer constants, which might be considered to have leading zero digits, text literals are deemed to have trailing spaces as needed: "Wo" is deemed to be "Wo " to make up to the recipient's capacity for four characters, and when format code A2 is specified, the leftmost two characters in the variable are taken. The strange ideas of "little-endianism" have never flourished on mainframes! Thus, if the format code were to be A4, then "Wo " would appear, not " Wo".

The first output (to standard output: unit 6) thus prepends the text "Hello " via the workings of the nominated FORMAT statement without a further mention of variable TEXT, itself not being modified in this action. Thus, this is an example of a single-mention possibility.

Some versions of Fortran offered the ability to write to a variable such as an array rather than to a nominated output unit, via a statement like WRITE (TEXT,1) (TEXT(I),I = 1,3), which array could then itself be written to the actual output via normal statements. This would involve a working variable within the routines for formatted I/O to hold the output, and thus provides one of the reasons that Fortran I/O implementations seldom enable re-entrancy - as with a WRITE statement whose output expression list includes a function evaluation, which function itself attempts to WRITE something, say to a log file, with both WRITE statements employing formatting statements. More modern compilers now require the recipient for this sort of WRITE statement to be of type CHARACTER, so the older style is blocked - and re-entrancy is still a problem.

Still another variant involved writing to unit number zero, which did not actually send anything to an output recipient. Instead, the scratchpad used by the formatted I/O system would retain whatever was produced, which could then be read back via unit number zero. Indeed, reading from unit zero would reveal whatever had been the last line of the previous I/O statement. This would be of interest if a format error had been reported on a READ during some mass data acquisition, so that the error message could show the problematic input that had been obtained rather than just complain. But this facility was not common, and did not become a part of the F90 standard. Perhaps a BACKSPACE and re-read to a text variable will work instead...

Retreating from FORMAT usage to the case of manipulating a "string" variable so as to prepend a given text to the working variable, first the existing content must be moved right to make room (again, an even number of characters is involved) which is achieved via the DO-loop, using certain constants. If on the other hand, text were to be removed from the front, then a loop would be needed to shift the surviving content leftwards. In doing this, one must pay attention to any overlaps and the direction of the loop! By chance, this exercise starts the placement after the end of the existing text but if instead the shift were to be two units, then the first-placed unit would land atop the tail end of the existing text. Thus, for rightwards shifts, one should start with the end of the surviving text and work back to its start.

Having made space, the next statements merely assign some bit patterns to elements of TEXT, and then the result is revealed, again using known constants instead of the associated variables of the more general approach. The result from the two WRITE statements is of course
Hello world!
Hello world!

Character facility[edit]

With F77 came the CHARACTER type...
      CHARACTER*66 TEXT
TEXT = "World!"
TEXT = "Hello "//TEXT
WRITE (6,*) TEXT
END

This means that variable TEXT has space for 66 characters, addressed as TEXT(first:last) starting with one. There is no associated string length facility, so the first assignment places the six characters of the supplied literal, followed by spaces all the way to the end of TEXT. Alternatively, TEXT(1:6) = "World!" would place only six characters, leaving the rest of TEXT to hold whatever it may. This would probably be unsuitable for the next statement, which prepends "Hello " to the content of TEXT (including positions past six) and assigns the result to TEXT, overwriting its previous content - with the aid of a temporary working area. Although in principle there could be cunning schemes that update the recipient "in place" with a minimum of character copying to and fro, this doesn't happen. Only characters up to the capacity of the recipient will be transferred from the expression's result, and if the result is shorter than the capacity of the recipient, trailing spaces will be added. All of this is extra effort! And when TEXT is written out, all 66 characters will be sent forth. It is useful to have a function that locates the last non-blank character!

Modern[edit]

With F90, and standardised in F2003, came extensions that enable a variable to be "cut-to-fit" on each usage. The first assignment would discard any storage associated with TEXT and re-assign space matching the size of the expression's result, so TEXT would have six characters. In the next statement, the expression would be evaluated and produce twelve characters (six from "Hello ", and the six of the current size of TEXT), then the current storage for text would be discarded and TEXT re-allocated to be of size twelve. At some cost in overhead. On the other hand, rather than copy the result of an expression from the scratchpad to the recipient, with re-allocation, the recipient variable could be repointed to the result area: no copying needed.

FreeBASIC[edit]

' FB 1.05.0 Win64
 
Var s = "prepend"
s = "String " + s
Print s
Sleep
Output:
String prepend

Gambas[edit]

You can run this code. Copy the code, click this link, paste it in and press 'Run !'

Public Sub Main()
Dim sString1 As String = "world!"
Dim sString2 As String = "Hello "
 
sString1 = sString2 & sString1
 
Print sString1
 
End

Output:

Hello world!

Go[edit]

s := "world!"
s = "Hello, " + s

Haskell[edit]

 
Prelude> let f = (++" World!")
Prelude> f "Hello"
 
Output:
"Hello world!"

Icon and Unicon[edit]

s := "world!"
s := "Hello, " || s

J[edit]

   s=: 'value'
s
value
s=: 'new ',s
s
new value

Java[edit]

public class Prepend {
 
public static void main(String[] args) {
String s = "world!";
System.out.println("Hello " + s);
}
}
Output:
Hello world!

JavaScript[edit]

// No built-in prepend
var s=", World"
s = "Hello" + s
print(s);

jq[edit]

"world!" as $s
| "Hello " + $s

Julia[edit]

s = "world!"
s = "Hello " * s

Kotlin[edit]

// version 1.0.6
 
fun main(args: Array<String>) {
var s = "Obama"
s = "Barack " + s
println(s)
 
// It's also possible to use this standard library function
// though this is not what it's really intended for
var t = "Trump"
t = t.prependIndent("Donald ")
println(t)
}
Output:
Barack Obama
Donald Trump

Lasso[edit]

local(x = ', World!')
#x->merge(1,'Hello')
#x // Hello, World!

LFE[edit]

Using the concatenation operator:

 
> (set s "world")
"world"
> (++ "hello " s)
"hello world"
 

Using the concatenation function:

 
> (set s "world")
"world"
> (string:concat "hello " s)
"hello world"
 

Lingo[edit]

str = "world!"
put "Hello " before str
put str
-- "Hello world!"

LiveCode[edit]

The idiomatic way is to use "before"
put "world" into x
put "hello" before x
put x // hello world

Lua[edit]

By concatenation:

 
s = "12345678"
s = "0" .. s
print(s)

By string formatting:

 
s = "12345678"
s = string.format("%s%s", "0", s)
print(s)

By list joining:

 
s = "12345678"
s = table.concat({"0", s})
print(s)
Output:
of each solution:
    012345678

Maple[edit]

l := " World";
m := cat("Hello", l);
n := "Hello"||l;
o := `||`("Hello", l);
Output:
                            " World"
                         "Hello World"
                         "Hello World"
                         "Hello World"

Mathematica[edit]

a = "any text value";
a = "another string literal" <> a (* using concatenation (no built-in prepend) *)
Output:
"another string literalany text value"

Mercury[edit]

:- module string_prepend.
:- interface.
:- import_module io.
:- pred main(io::di, io::uo) is det.
:- implementation.
:- import_module string.
main(!IO) :-
S = "World!\n",
io.write_string("Hello " ++ S, !IO).
Output:
Hello World!

NetRexx[edit]

s_ = 'world!'
s_ = 'Hello, 's_
say s_
Output:
Hello, world!

NewLISP[edit]

(setq str "bar")
(push "foo" str)
(println str)

Nim[edit]

var str = "12345678"
str = "0" & str
echo str

Objeck[edit]

class Prepend  {
function : Main(args : String[]) ~ Nil {
s := "world!";
"Hello {$s}"->PrintLine();
}
}
Hello World!

Oforth[edit]

" World" "Hello" swap + println
Output:
Hello World

PARI/GP[edit]

Not supported in GP.

s = "world!";
s = Str("Hello, ", s)
Output:
%1 = "Hello, world!"

Pascal[edit]

Works with: Free Pascal version 2.6.2
program StringPrepend;
{$mode objfpc}{$H+}
 
uses
{$IFDEF UNIX}{$IFDEF UseCThreads}
cthreads,
{$ENDIF}{$ENDIF}
Classes
{ you can add units after this };
 
var
s: String = ' World !';
begin
s := 'Hello' + s;
WriteLn(S);
ReadLn;
end.
Output:
Hello  World !

Perl[edit]

my $str = 'bar';
substr $str, 0, 0, 'Foo';
print $str;
Output:
Foobar

Perl 6[edit]

# explicit concatentation
$_ = 'byte';
$_ = 'kilo' ~ $_;
.say;
 
# interpolation as concatenation
$_ = 'buck';
$_ = "mega$_";
.say;
 
# lvalue substr
$_ = 'bit';
substr-rw($_,0,0) = 'nano';
.say;
 
# regex substitution
$_ = 'fortnight';
s[^] = 'micro';
.say;
 
# reversed append assignment
$_ = 'cooper';
$_ [R~]= 'mini';
.say;
Output:
kilobyte
megabuck
nanobit
microfortnight
minicooper

Phix[edit]

string s = "World"
s = "Hello "&s

NB: s = prepend(s,"Hello ") gives typecheck: s is {"Hello ",'W','o','r','l','d'}, rather than the "Hello World" you probably wanted.

PicoLisp[edit]

(setq Str1 "12345678!")
(setq Str1 (pack "0" Str1))
(println Str1)
Output:
"012345678!"

PL/I[edit]

 
Pre_Cat: procedure options (main); /* 2 November 2013 */
declare s character (100) varying;
s = ' bowl';
s = 'dust' || s;
put (s);
end Pre_Cat;
 
dust bowl

Plain TeX[edit]

\def\prepend#1#2{% #1=string  #2=macro containing a string
\def\tempstring{#1}%
\expandafter\expandafter\expandafter
\def\expandafter\expandafter\expandafter
#2\expandafter\expandafter\expandafter
{\expandafter\tempstring#2}%
}
\def\mystring{world!}
\prepend{Hello }\mystring
Result : \mystring
\bye

Here is an equivalent code with eTeX capabilities:

\def\prepend#1#2{% #1=string  #2=macro containing a string
\edef#2{\unexpanded{#1}\unexpanded\expandafter{#2}}%
}
\def\mystring{world!}
\prepend{Hello }\mystring
Result : \mystring
\bye

PowerShell[edit]

 
$str = "World!"
$str = "Hello, " + $str
$str
 
Hello, World!

Prolog[edit]

Works with: SWI-Prolog

In its admirable wisdom, Prolog is generally unfriendly to state mutations and destructive assignment. However, it is also very flexible. Using the traditional representation of strings as lists of character codes, and the non-logical predicate `setarg/3`, we can destructively set the head and tail of the list to achieve a mutation of the variable holding the string. I define an operator for the purpose:

 
:- op(200, xfx, user:(=+)).
 
%% +Prepend =+ +Chars
%
% Will destructively update Chars
% So that Chars = Prepend prefixed to Chars.
% eazar001 in ##prolog helped refine this approach.
 
[X|Xs] =+ Chars :-
append(Xs, Chars, Rest),
nb_setarg(2, Chars, Rest),
nb_setarg(1, Chars, X).
 

Example of this abomination in action:

 
?- Str = `World!`, `Hello, ` =+ Str.
Str = "Hello, World!".

Note: I can't imagine when I would want to do this in Prolog.

PureBasic[edit]

S$ = " World!"
S$ = "Hello" + S$
If OpenConsole()
PrintN(S$)
 
Print(#CRLF$ + #CRLF$ + "Press ENTER to exit"): Input()
CloseConsole()
EndIf
Output:
Hello World!

Python[edit]

File: string_prepend.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-
 
s = "12345678"
s = "0" + s # by concatenation
print(s)
Output:
012345678

Racket[edit]

;there is no built-in way to set! prepend in racket
(define str "foo")
(set! str (string-append "bar " str))
(displayln str)
 
;but you can create a quick macro to solve that problem
(define-syntax-rule (set-prepend! str value)
(set! str (string-append value str)))
 
(define macrostr " bar")
(set-prepend! macrostr "foo")
(displayln macrostr)
 
Output:
bar foo
foo bar

REXX[edit]

zz= 'llo world!'          /*─────────────── using literal abuttal.────────────*/
zz= 'he'zz /*This won't work if the variable name is X or B */
say zz
 
 
gg = "llo world!" /*─────────────── using literal concatenation.──────*/
gg = 'he' || gg
say gg
 
 
aString= 'llo world!' /*─────────────── using variable concatenation.─────*/
bString= "he"
aString= bString || aString
say aString

output

hello world!
hello world!
hello world!

Ring[edit]

 
aString = "World!"
bString = "Hello, " + aString
see bString + nl
 

Ruby[edit]

There is a method for prepending a string, aptly named "prepend".

str = "llo world"
str.prepend("He")
p str #=> "Hello world"

Scala[edit]

Evaluation in Scala worksheet

  val s = "World" // Immutables are recommended   //> s  : String = World
val f2 = () => ", " //Function assigned to variable
//> f2  : () => String = <function0>
val s1 = "Hello" + f2() + s //> s1  : String = Hello, World
println(s1); //> Hello, World

Seed7[edit]

$ include "seed7_05.s7i";
 
const proc: main is func
local
var string: s is "world!";
begin
s := "Hello " & s;
writeln(s);
end func;
Output:
Hello world!

Sidef[edit]

var str = 'llo!';
str.sub!(/^/, 'He');
say str;

or

var str = 'llo!';
str.prepend!('He');
say str;
Output:
Hello!

SNOBOL4[edit]

    s = ', World!'
OUTPUT = s = 'Hello' s
END
Output:
Hello, World!

Swift[edit]

var str = ", World"
str = "Hello \(str)"
println(str)
Output:
Hello, World!

Tcl[edit]

Concatenation is a fundamental feature of Tcl's basic language syntax.

set s "llo world"
set s "he$s"
puts $s
Output:
hello world

Ursa[edit]

decl string s
 
# set s to "world"
set s "world"
 
# prepend "hello "
set s (+ "hello " s)
 
# outputs "hello world"
out s endl console

VBA[edit]

Function StringPrepend()
Dim s As String
s = "bar"
s = "foo" & s
Debug.Print s
End Function

VBScript[edit]

s = "bar"
s = "foo" & s
WScript.Echo s
Output:
foobar

Wart[edit]

s <- "12345678"
s <- ("0" + s)

zkl[edit]

s:="bar"; s="foo" + s;           s.println();
s:="bar"; s=String("foo",s); s.println();
s:="bar"; s="%s%s".fmt("foo",s); s.println();
// a Data is a byte buffer/editor:
s:=Data(Void,"bar").insert(0,"foo").text; s.println();
Output:
foobar
foobar
foobar
foobar