arrays

docs/arrays.md

@@ -1,4 +1,92 @@
 # Awk arrays
+
+Here is a simple array definition and a way to scan its elements.
+
+```awk
+#!/usr/bin/awk -f
+BEGIN { 
+  D["a"]="A"
+  D["b"]="B"
+  D["c"]="C"
+  
+  for (i in D){        # loop over the  index
+    print i" : "D[i]
+  }
+}
+```
+
+Output:
+```
+a : A
+b : B
+c : C
+```
+> Note: by default, when a for loop traverses an array, the order is undefined, meaning that the awk implementation determines the order in which the array is traversed. This order is usually based on the internal implementation of arrays and will vary from one version of awk to the next.. Here is how to sort them [Predefined Array Scanning order](https://www.gnu.org/software/gawk/manual/html_node/Controlling-Scanning.html)
+
+## Multidimensional arrays - [docs](https://www.gnu.org/software/gawk/manual/html_node/Multidimensional.html)
+A multidimensional array is an array in which an element is identified by a sequence of indices instead of a single index. For example, a two-dimensional array requires two indices. The usual way (in many languages, including awk) to refer to an element of a two-dimensional array named grid is with `grid[x,y]`.
+
+```awk
+#!/usr/bin/awk -f      
+BEGIN {                
+  D["a","A"]="aA"     
+  D["a","B"]="aB"     
+  D["a","C"]="aC"     
+                       
+  D["b","A"]="bA"     
+                       
+  for (i in D){        # loop over the first index
+    print i" : "DD[i]
+    print "--------"   
+  }                    
+}
+```
+Output:
+```
+aA : aA
+--------
+aB : aB
+--------
+aC : aC
+--------
+bA : bA
+--------
+```
+
+> If you look carefully, `i` iterates over an index that is a string concatenated of both indexes. In other words, the combined string is used as a single index into an ordinary, one-dimensional array. This makes it somewhat dificult to iterate ovet the second index... but could be used in some specific solutions like [Manipulating the output from a genome analysis - vcf and gff](./Case_studies/manipulating_vcf.md).
+
+## Array of arrays - [docs](https://www.gnu.org/software/gawk/manual/html_node/Arrays-of-Arrays.html)
+
+The so called "Array of Arrays" implementation is easier for scanning (iterating) than the above  [multidimensional array](https://www.gnu.org/software/gawk/manual/html_node/Multidimensional.html) implementation.
+
+```awk
+#!/usr/bin/awk -f
+BEGIN { 
+  D["a"]["A"]="aA"
+  D["a"]["B"]="aB"
+  D["a"]["C"]="aC"
+  
+  D["b"]["A"]="bA"
+
+  for (i in D){                    # loop over the first  index
+    for (j in D[i])                # loop over the second index
+      print i","j" : "D[i][j]
+    print "--------"
+  }
+}
+```
+Output
+```
+a,A : aA
+a,B : aB
+a,C : aC
+--------
+b,A : bA
+--------
+```
+
+## More
+
 If you want to learn or just check what other "tricks" one could do with arrays in Awk, here a suggested tutorial on the topic - look for "AWK tips and tricks" section on the page.
 
 !!! quote 

mkdocs.yml

@@ -6,6 +6,7 @@ nav:
   - 2.Teasing with grep: 2.Teasing_with_grep.md
   - 3.Shell we awk: 3.Shell_we_awk.md
   - 4.Brief commands: 4.Brief_commands.md
+  - 4.a Awk arrays: arrays.md
   - 5.String manipulation: 5.String_manipulation.md
   - 6.One line programs: 6.One_line_programs.md
   - Python vs. awk: Python_vs_awk.md
@@ -34,7 +35,6 @@ nav:
   - Fixed size fields: Other/Fixed_size_fields.md
   - Backreferences: Other/Backreferences.md
   - Multi-Line records: https://www.gnu.org/software/gawk/manual/html_node/Multiple-Line.html
-  - Awk arrays: arrays.md
   - Awk or Bash: awk_bash.md
   - Localization problems: Other/Localization.md
   - Awk vs. nawk vs. gawk: http://www.thegeekstuff.com/2011/06/awk-nawk-gawk/