How to print a text from inside a command without affecting its width?

Question

I am trying to print within a command, but after printing I see the text (command's argument) to be printed with changed length...

I wonder if there is a reason of that and how can I avoid it.

I will try to answer my question... but probably I will not give the best answer... So, If you want, you can try to answer...

MWE:

\documentclass[a4paper,10pt]{article}
\usepackage[utf8]{inputenc}
\setlength\parskip{10pt}
\setlength\parindent{0pt}

\newcommand{\myprint}[1]{
#1
}


\begin{document}



\section{In a section}

This is a text that will occupy  one full line and some more, I hope I can show it



\myprint{This is a text that will occupy  one full line and some more, I hope I can show it}

\end{document}

Output:

Answer 1

Edit: Long answer to understand what happens

(For shorter answer look at @Henri Menke's answer)

Many LaTeX users are also programmers, and being a LaTeX user you become somehow a kind of programmer. Coding has some rules on styling our structure and so writing a C program we may choose between the following styles:

1.

#include <stdio.h>

int main(void)
{
  int i=0;
  for (i=0;i<10;i++)
  {
     printf("Loop Count i=%d",i);
  }
     return 0;
}

2.

#include <stdio.h>

int main(void){
   int i=0;
   for (i=0;i<10;i++){
      printf("Loop Count i=%d",i);
   }
   return 0;
}

or 3.

#include <stdio.h>

int main(void){
   int i=0;
   for (i=0;i<10;i++){printf("Loop Count i=%d",i);}
   return 0;
}

In C programming the result is exactly the same but something similar in LaTeX coding will not have the same results.

Empty lines

One reason is that in LaTeX coding, an empty line is equivalent to a \par command and inserts a paragraph. So the next example will generate the following output:

\documentclass[a4paper,10pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{parskip}

\setlength{\parindent}{8pt}

\setlength{\parskip}{15pt}
\newcommand{\myprint}[1]{

#1

}


\begin{document}



\section{In a section}

First line in first section

\def\mymaintext{This is my  text that will occupy a big part of remaining line.}

Test before.\mymaintext
Test line after.

Test before.\myprint{\mymaintext}
Test line after.
\end{document}

Spaces

By removing the empty lines we remove the \par commands from our print command but we don't remove the spaces:

\newcommand{\myprint}[1]{
#1
}

The above command will generate an extra space before our argument (we will have one space there) and an extra space after (two spaces after).

The following changes are equivalent:

\newcommand{\myprint}[1]{#1}

or

\newcommand{\myprint}[1]{%
#1%
}

and we have removed the extra empty spaces.

For more look at the topic of space tokens that @TH. gave me in the comments of the question.

---

The following answer was my first answer before understanding exactly what happens and I'm leaving it here for someone who will read the comments

I don't really know why it happens, but I found that a solution is to print in a minipage of width=\linewidth.

\documentclass[a4paper,10pt]{article}
\usepackage[utf8]{inputenc}
\setlength\parskip{10pt}
\setlength\parindent{0pt}

\newcommand{\myprint}[1]{
\begin{minipage}{\linewidth}
#1
\end{minipage}
}


\begin{document}



\section{In a section}

This is a text that will occupy  one full line and some more, I hope I can show it



\myprint{This is a text that will occupy  one full line and some more, I hope I can show it}

\end{document}

Output:

Answer 2

Remove the spurious spaces (marked with <--). I also removed your bogus parameters for \parskip and \parindent. If you don't want to indent a single paragraph, use \noindent. If you do not want to indent every paragraph, use the parskip package but this will make typographers' eyes bleed.

\documentclass{article}
\usepackage[utf8]{inputenc}

\newcommand{\myprint}[1]{% <--
#1% <--
}

\begin{document}

\noindent This is a text that will occupy  one full line and some more, I hope I can show it

\noindent\myprint{This is a text that will occupy  one full line and some more, I hope I can show it}

\end{document}