Math & Science ⇒ pgfplots | Conditions on functions

[LaTeX Beginner]

Hi guys!

I have a function defined in LaTeX, and I want to use this function inside other one, in this way:

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\def\A{...}
\ded\B{A<0}

Mi doubt is the next: How can I express in LaTeX the condition A<... or A>...? I mean, I want to use the "negative side" (A<0) of this function.

Thank you so much!

[Stefan Kottwitz]

The etoolbox package provides such tests, have a look at section 3.6.4 Arithmetic Tests in the etoolbox documentation.

What do you plan to do? You said negative side of a function, would you like to plot functions, or what is the plan?

Stefan

[LaTeX Beginner]

The etoolbox package provides such tests, have a look at section 3.6.4 Arithmetic Tests in the etoolbox documentation.

What do you plan to do? You said negative side of a function, would you like to plot functions, or what is the plan?

Stefan

Hi Stefan, thanks for the reply.

I've plotted a function, and this function has a negative part and a positive part. So I want to define a new function (B) that only takes the negative values of A, in order to operate with it. Do you understand me?

[Stefan Kottwitz]

Sure, but it's quite theoretical. Please post a minimal working example. So we have something to test and to implement, and we can see if you work with gnuplot, PSTricks, TikZ, pgfplots, or something else.

Stefan

[LaTeX Beginner]

Yeah, sure, I post here an example:

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\documentclass[11pt,a4paper,spanish]{article}
\usepackage[T1]{fontenc}
\usepackage{selinput}
\usepackage{babel}
\usepackage{geometry}
\usepackage{tikz}
\usepackage{textcomp}
\usepackage{pgfplots}

\usepgfplotslibrary{polar}
\usepgfplotslibrary[polar]
\usetikzlibrary{pgfplots.polar}
\usetikzlibrary[pgfplots.polar]

\begin{document}
\begin{center}
\pgfplotsset{width=15cm,compat=1.5.1}
\begin{tikzpicture}

%%%%%%%%%%% CARTESIAN COORDINATES

%\begin{axis}

%\addplot+[red,domain=0:360,samples=360] {sin(\x)};

%\end{axis}

%%%%%%%%%%%

\begin{polaraxis}[title=\large\bfseries{Polar coordinates}]
				
\addplot+[red,domain=0:360,samples=360] {sin(\x)}; 						

\end{polaraxis}
\end{tikzpicture}
\end{center}
\end{document} 

This is an example, a sin function representation. If you run it, you'll get a sin(x) function in polar coordinates (the same example in cartesian coordinates is commented). The sin function oscillates between -1 and 1. So, I want to define a new function (B, for example), that only takes the negative values of sin(x), these values for which sin(x)<0

I hope I explained it well

[Stefan Kottwitz]

Ok, that's something clearer. So I changed the topic title from "Use of "<" or ">" in LaTeX" to "pgfplots | Conditions on functions", to make clear it's not about printing those < and > symbols or the like, and in the hope that this is noticed by a pgfplots expert.

Stefan

[LaTeX Beginner]

Thanks Stefan. I hope someone can help me

[cgnieder]

You can exploit the fact that the mathematical expression in

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\addplot {<expr>} ;

can be any valid pgfmath expression (pgfplots also refers to the pgfmanual).

One of the expression available is the “ifthenelse” expression

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 <if> ? <then> : <else>

So this is a way to go:

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\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{pgfplots}

\begin{document}

\begin{tikzpicture}
 \begin{axis}[mark=none]
   \addplot[red,samples=100]   { x^2 } ;
   \addplot[green,samples=300] { x^2 > 4 ? -x^2+8 : x^2 } ;
 \end{axis}
\end{tikzpicture}

\end{document}

ifthenelse.png (7.47 KiB) Viewed 10157 times

Regards

[LaTeX Beginner]

Thank you so much, Clemens

Tomorrow I will try it, and I tell you.

Regards.

[LaTeX Beginner]

I tried it, and it works very well! I don't knew how to use the if?then:else structure, and I think that it could be very useful to me

But I've realised that I didn't explain my doubt well... Taking the sin(x) example, you can see that it has a negative part (-1:0) and a positive part (0:1). If I want to operate in this way:

A=(sin(\x)*(sin(\x)>0))

How can I do that? I want to operate with the sin(\x) function and with his positive part(sin(\x)>0)

Thank you again!