title: Convert categorical variables to numeric in R
body: <p>I have a huge database and I am having many categorical variables.
  You can watch it here:</p>
  
  <pre><code>&gt; M=data.frame(Type_peau,PEAU_CORPS,SENSIBILITE,IMPERFECTIONS,BRILLANCE ,GRAIN_PEAU,RIDES_VISAGE,ALLERGIES,MAINS,
  +              INTERET_ALIM_NATURELLE,INTERET_ORIGINE_GEO,INTERET_VACANCES,INTERET_COMPOSITION,DataQuest1,Priorite2,
  +              Priorite1,DataQuest4,Age,Nbre_gift,w,Nbre_achat)
  &gt; # pour voir s'il y a des données manquantes
  &gt; str(M)
  'data.frame':   836 obs. of  21 variables:
   $ Type_peau             : Factor w/ 5 levels "","Grasse","Mixte",..: 3 4 5 3 4 3 3 3 2 3 ...
   $ PEAU_CORPS            : Factor w/ 4 levels "","Normale","Sèche",..: 2 3 3 2 2 2 3 2 3 2 ...
   $ SENSIBILITE           : Factor w/ 4 levels "","Aucune","Fréquente",..: 4 4 4 2 4 3 4 2 4 4 ...
   $ IMPERFECTIONS         : Factor w/ 4 levels "","Fréquente",..: 3 4 3 4 3 2 3 4 3 3 ...
   $ BRILLANCE             : Factor w/ 4 levels "","Aucune","Partout",..: 4 2 2 4 4 4 4 4 3 4 ...
   $ GRAIN_PEAU            : Factor w/ 4 levels "","Dilaté","Fin",..: 4 4 4 2 4 2 4 4 2 4 ...
   $ RIDES_VISAGE          : Factor w/ 4 levels "","Aucune","Très visibles",..: 2 2 2 4 4 2 4 2 4 2 ...
   $ ALLERGIES             : Factor w/ 4 levels "","Non","Oui",..: 2 2 2 2 2 2 2 2 2 2 ...
   $ MAINS                 : Factor w/ 4 levels "","Moites","Normales",..: 3 4 4 3 3 3 3 4 4 4 ...
   $ INTERET_ALIM_NATURELLE: Factor w/ 4 levels "","Beaucoup",..: 2 4 4 4 2 2 2 4 4 2 ...
   $ INTERET_ORIGINE_GEO   : Factor w/ 5 levels "","Beaucoup",..: 2 4 2 5 2 2 2 2 2 2 ...
   $ INTERET_VACANCES      : Factor w/ 6 levels "","À la mer",..: 3 4 2 2 3 2 3 2 3 2 ...
   $ INTERET_COMPOSITION   : Factor w/ 4 levels "","Beaucoup",..: 2 2 2 4 2 2 2 2 4 2 ...
   $ DataQuest1            : Factor w/ 4 levels "-20","20-30",..: 4 3 4 4 4 3 3 2 3 2 ...
   $ Priorite2             : Factor w/ 7 levels "éclatante","hydratée",..: 3 1 3 4 3 2 7 1 4 6 ...
   $ Priorite1             : Factor w/ 7 levels "éclatante","hydratée",..: 4 6 1 5 1 6 1 2 6 4 ...
   $ DataQuest4            : Factor w/ 2 levels "nature","urbain": 2 2 2 2 2 1 2 2 2 2 ...
   $ Age                   : int  32 37 23 44 33 30 43 43 60 31 ...
   $ Nbre_gift             : int  1 4 1 1 2 1 1 1 1 1 ...
   $ w                     : num  0.25 0.25 0.5 0.25 0.5 0 0 0 0 0.75 ...
   $ Nbre_achat            : int  3 4 7 3 6 9 22 13 7 16 ...
  </code></pre>
  
  <p>I need to convert all categorical variables to numeric automatically. For example for the variable <strong>Type_peau</strong>, it is :</p>
  
  <pre><code> head(Type_peau)
  [1] Mixte   Normale Sèche   Mixte   Normale Mixte  
  Levels:  Grasse Mixte Normale Sèche
  </code></pre>
  
  <p>I want it :</p>
  
  <pre><code>head(Type_peau)
  [1] 2 3 4 2 3 2
  Levels: 1 2 3 4
  </code></pre>
  
  <p>How can I do that automatically for all categorical variables?</p>