Python 2, 26 bytes

from textwrap import*

fill

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Meh... built-ins are boring... instead, have a nice 87-byte solution here:

s,n=input()

x=''

for i in s.split():c=n<len(x+i);exec'print x'*c;x=x*-~-c+i+' '

print x

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Outputs trailing spaces.

Perl 6, 46 29 bytes

{;*.comb(/.**{1..$_}[s|$]/)}

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Regex based solution that takes input curried, like f(n)(s) and returns a list of lines. Each line except the last has a trailing whitespace

Explanation:

{;*                         }   # Anonymous code block that returns a Whatever lambda

   .comb(/                /)    # Split the string by

          .**{1..$_}            # Up to n characters

                    [s|$]      # Terminated by a whitespace char or the end of the string

PHP, 8 bytes

Admittedly not the most original solution, but PHP has a native function that matches your requirements perfectly!

wordwrap:

string wordwrap ( string $str [, int $width = 75 [, string $break = "n" [, bool $cut = FALSE ]]] )

Wraps a string to a given number of characters using a string break character.

Use like so:

$str = "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed eget erat lectus. Morbi mi mi, fringilla sed suscipit ullamcorper, tristique at mauris. Morbi non commodo nibh. Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Sed at iaculis mauris. Praesent a sem augue. Nulla lectus sapien, auctor nec pharetra eu, tincidunt ac diam. Sed ligula arcu, aliquam quis velit aliquam, dictum varius erat.";

echo wordwrap($str, 50);

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JavaScript (ES6),  75 73  72 bytes

Takes input as (string)(n).

s=>n=>s.split` `.map(w=>r=(u=r?r+' '+w:w)[n]?(o+=r+`

`,w):u,o=r='')&&o+r

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Variables

The formatted output is stored in $o$ (in green below).

The updated line $u$ is defined as the concatenation of:

• the current line $r$ (in black below)


• a space if $r$ is not empty, or nothing otherwise (in orange below)


• the new word $w$ (in blue below)

We need to insert a line break whenever the $n$-th character of $u$ is set (0-indexed, in red below).

Example

$n=16$ and $s$ = "LOREM IPSUM DOLOR"

Adding "LOREM":
$$smallbegin{array}{|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|}
hline
00&01&02&03&04&05&06&07&08&09&10&11&12&13&14&15&color{red}{16}\ hline
color{blue}L&color{blue}O&color{blue}R&color{blue}E&color{blue}M&&&&&&&&&&&&\ hlineend{array}
$$

Adding "IPSUM":
$$smallbegin{array}{|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|}
hline
00&01&02&03&04&05&06&07&08&09&10&11&12&13&14&15&color{red}{16}\ hline
L&O&R&E&M&color{orange}bullet&color{blue}I&color{blue}P&color{blue}S&color{blue}U&color{blue}M&&&&&&\ hlineend{array}
$$

Adding "DOLOR":
$$smallbegin{array}{|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|}
hline
00&01&02&03&04&05&06&07&08&09&10&11&12&13&14&15&color{red}{16}\ hline
L&O&R&E&M&bullet&I&P&S&U&M&color{orange}bullet&color{blue}D&color{blue}O&color{blue}L&color{blue}O&color{blue}R\ hlineend{array}
$$

$$smallbegin{array}{|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|}
hline
00&01&02&03&04&05&06&07&08&09&10&11&12&13&14&15&color{red}{16}\ hline
color{green}L&color{green}O&color{green}R&color{green}E&color{green}M&color{green}bullet&color{green}I&color{green}P&color{green}S&color{green}U&color{green}M&color{green}hookleftarrow&&&&&\ hline
D&O&L&O&R&&&&&&&&&&&&\ hlineend{array}
$$

Vim, 15 bytes/keystrokes

DJ:se tw=<C-r>"

gq_

A text formatting question? I know just the tool for the job! And it even has my name in the first two keystrokes :D

<C-r> means ctrl-r.

This could ever so slightly shorter in V, but I prefer answering in vanilla vim for answers that really show off how concise vim can be for the right challenge. And the difference is so small anyway.

This could also be the following for 15 bytes as well:

:se tw=<C-r><C-w>

ddgq_

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Haskell, 70 bytes

s!n|length s<=n=[s]|(t,_:d)<-splitAt(until((<'!').(s!!))pred n)s=t:d!n

Python 2, 74 bytes

s,n=input()

while s:i=n;exec"i-=' '<(s+' '*n)[i];"*n;print s[:i];s=s[i+1:]

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R, 36 27 bytes

R has this as a built-in (strwrap), we return a vector of split lines.

function(s,n)strwrap(s,n+1)

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Retina 0.8.2, 37 bytes

.+$

$*

!`(?=S.*¶(1)+)(?<-1>.)+(?=s)

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.+$

$*

Convert n to unary.

(?=S.*¶(1)+)(?<-1>.)+(?=s)

Match non-whitespace, then look ahead to n and count it as $#1. Then go back and use a balancing group to match up to n characters followed by whitespace.

!`

Output the matches as a list of lines.

Charcoal, 19 bytes

Ｎθ←Ｆ⪪Ｓ «¿‹⁺ＬιⅈθＭ→⸿ι

Try it online! Link is to verbose version of code. Takes input of n and s on separate lines. Explanation:

Ｎθ

Input n.

←

Move the cursor left one square to balance the right movement from the first iteration of the loop.

Ｆ⪪Ｓ «

Split the string on spaces and loop over the words.

¿‹⁺Ｌιⅈθ

Calculate whether the next word will reach the right edge.

Ｍ→

If it will not then move one square right.

⸿

If it will then start a new line.

ι

Output the word.

Red, 125, 117, 114 112 bytes

func[s n][d: 0 parse s[any[to" "p:" "opt[[to" "| to end]q:(if(-1 - d + index? q)> n[p/1: #"^/"d: index? p])]]]s]

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JavaScript, 39 bytes

s=>n=>eval(`s.match(/.{1,${n}} |.+/g)`)

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Powershell, 40 83 bytes

Test case with n=80 added.

param($s,$n)$s-split' '|%{if(($o+$_|% le*)-lt$n){$o+=' '*!!$o+$_}else{$o;$o=$_}}

$o

Test script:

$f = {



param($s,$n)$s-split' '|%{if(($o+$_|% le*)-lt$n){$o+=' '*!!$o+$_}else{$o;$o=$_}}

$o



}



@(

,(50, "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed eget erat lectus. Morbi mi mi, fringilla sed suscipit ullamcorper, tristique at mauris. Morbi non commodo nibh. Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Sed at iaculis mauris. Praesent a sem augue. Nulla lectus sapien, auctor nec pharetra eu, tincidunt ac diam. Sed ligula arcu, aliquam quis velit aliquam, dictum varius erat.",

"Lorem ipsum dolor sit amet, consectetur adipiscing",

"elit. Sed eget erat lectus. Morbi mi mi, fringilla",

"sed suscipit ullamcorper, tristique at mauris.",

"Morbi non commodo nibh. Pellentesque habitant",

"morbi tristique senectus et netus et malesuada",

"fames ac turpis egestas. Sed at iaculis mauris.",

"Praesent a sem augue. Nulla lectus sapien, auctor",

"nec pharetra eu, tincidunt ac diam. Sed ligula",

"arcu, aliquam quis velit aliquam, dictum varius",

"erat.")

,(80, "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed eget erat lectus. Morbi mi mi, fringilla sed suscipit ullamcorper, tristique at mauris. Morbi non commodo nibh. Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Sed at iaculis mauris. Praesent a sem augue. Nulla lectus sapien, auctor nec pharetra eu, tincidunt ac diam. Sed ligula arcu, aliquam quis velit aliquam, dictum varius erat.",

"Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed eget erat lectus.",

"Morbi mi mi, fringilla sed suscipit ullamcorper, tristique at mauris. Morbi non",

"commodo nibh. Pellentesque habitant morbi tristique senectus et netus et",

"malesuada fames ac turpis egestas. Sed at iaculis mauris. Praesent a sem augue.",

"Nulla lectus sapien, auctor nec pharetra eu, tincidunt ac diam. Sed ligula arcu,",

"aliquam quis velit aliquam, dictum varius erat.")

) | %{

    $n,$s,$expected = $_

    $result = &$f $s $n

    "$result"-eq"$expected"

    # $result   # uncomment this line to dispaly a result

}

Output:

True

True

Japt, 24 bytes

¸r@[XY]q(X·Ì+S+Y Ê>V?R:S

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Thanks to Bubbler for removing some duplication.

Explanation:

¸                           :Split into words

 r@                         :Reduce back into a string by:

         X                  : Start with the already reduced portion

          ·Ì                : Get the last line

            +S+Y            : Add a space and the next word

                 Ê>V?       : If the length of that line is greater than n:

                     R      :  Choose newline

                      :     : Otherwise:

                       S    :  Choose space

   [XY]q(                   : Join the next word to the result using the chosen character

Bash/Coreutils, 8 bytes

fold -sw <n>

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The fold utility, that's bundled along with coreutils, does exactly what the question asks. The -s flag makes it so that it only breaks on whitespaces(ie. when the word ends), the value of n is passed using the argument -w. Input is accepted via stdin

Thanks to @Erik the Outgolfer, a golfed version :

Python 3, 94 bytes

def f(t,n):

 while t:i=n+(t[min(len(t)-1,n)]==" "or-t[n-1::-1].find(' '));print(t[:i]);t=t[i:]

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# Python 3, 130 bytes

def f(t,n):

 l=

 while len(t):

  i=(n-t[:n][::-1].find(' '),n+1)[t[min(len(t)-1,n)]==" "]

  l.append(t[:i])

  t=t[i::]

 return l

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Not so golfed version...

JavaScript + HTML + CSS, 117 64 bytes

-53 bytes courtesy of @Neil

n=50

s="Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed eget erat lectus. Morbi mi mi, fringilla sed suscipit ullamcorper, tristique at mauris. Morbi non commodo nibh. Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Sed at iaculis mauris. Praesent a sem augue. Nulla lectus sapien, auctor nec pharetra eu, tincidunt ac diam. Sed ligula arcu, aliquam quis velit aliquam, dictum varius erat."

f=(n,s)=>document.body.innerHTML+=`<tt><p style=width:${n}ch>${s}`

f(n,s)

C# (Visual C# Interactive Compiler), 91 bytes

s=>n=>s.Split(' ').Aggregate((a,w)=>a+(a.Length-a.LastIndexOf('n')+w.Length>n?'n':' ')+w)

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Mathematica, 16 bytes

InsertLinebreaks

Built-in function. Takes a string and an integer as input and returns a string as output.

InsertLinebreaks["string", n]

 inserts newline characters to make no line longer than n characters.

Jelly, 12 bytes

ḲŒṖK€€ḣ€ƑƇṪY

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Unfortunately, this is too slow to work for the provided test case in under a minute over TIO.

C# (.NET Core), 162 bytes

stringt(string n,int a){var g="";for(int i=0;i++<Math.Floor((double)n.Length/a);)g+=$"^.{{{i*a-1}}}|";return Regex.Split(n,$@"(?n)(?<=({g.Trim('|')})S*)s");}}

This function uses a regex which matches the closest whitespace that is near the nth or multiple of nth character and splits the string based on it.

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The TIO link is a full program, and the function has a static keyword so the function can be called from main.

Test Regex

05AB1E, 18 bytes

õs#vDy«g²›i,}yðJ}?

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Explanation:

õ                   # Push an empty string "" to the stack

 s                  # Swap to take the (implicit) string input

  #                 # Split it by spaces

   v            }   # For-each `y` over the words:

    D               #  Duplicate the top of the stack

                    #  (which is the empty string in the very first iteration)

     y«             #  Append the current word `y`

       g            #  Get its length

        ²›i }       #  If its lengthy is larger than the second input:

           ,        #   Pop and output the current duplicated value with trailing newline

             yð     #  Push the word `y` and a space " "

               J    #  Join the entire stack together

                 ?  # After the loop, output the last part as well (without newline)

Java 8, 135 bytes

n->s->{String r="",S=s.split(" "),t=r;for(int i=0;i<S.length;)if((t+S[i]).length()>n){r+=t+"n";t="";}else t+=S[i++]+" ";return r+t;}

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Explanation:

n->s->{                      // Method with integer & String parameters and String return

  String r="",               //  Result-String, starting empty

         S=s.split(" "),   //  Input-String split by spaces

         t=r;                //  Temp-String, starting empty as well

  for(int i=0;i<S.length;)   //  Loop `i` in the range [0, amount_of_words):

    if((t+S[i]).length()>n){ //   If `t` and the word are larger than the integer input:

      r+=t+"n";             //    Add `t` and a newline to the result

      t="";}                 //    And reset `t` to an empty String

     else                    //   Else:

       t+=S[i++]+" ";        //    Append the word and a space to `t`

                             //    (and then increase `i` by 1 with `i++` for the next word

                             //     of the next iteration)

  return r+t;}               //  Return the result-String appended with `t` as result