ruby - Difference between += for Integers/Strings and << For Arrays? -

i'm confused different results i'm getting when performing simple addition/concatenation on integers, strings , arrays in ruby. under impression when assigning variable b (see below), , changing value of a, b remain same. , in first 2 examples. when modify array in 3rd example, both , b modified.

a = 100 b = a+= 5 puts puts b 

---

a = 'abcd' b = a += 'e' puts puts b 

---

a = [1,2,3,4] b = a << 5 puts a.inspect puts b.inspect 

the following returned in terminal above code:

ricks-macbook-pro:programs rickthomas$ ruby variablework.rb 105 100 abcde abcd [1, 2, 3, 4, 5] [1, 2, 3, 4, 5] ricks-macbook-pro:programs rickthomas$  

i given following explanation programming instructor:

assigning new variable giving additional label, doesn't make copy.

it looks += method, <<, , you'd expect behave similarly. in reality, it's "syntactic sugar", added language make things easier on developers.

when run += 1, ruby converts = + 1.

in case, we're not modifying fixnum in a. instead, we're re-assigning on top of it, blowing away previous value of a.

on other hand, when run b << "c", you're modifying underlying array appending string "c" it.

my questions these:

1) mentions syntactic sugar, isn't << is, i.e. syntactic sugar .push method?

2) why matter if += syntactic sugar or more formal method? if there difference between two, doesn't mean previously-understood of syntactic sugar ("syntax within programming language designed make things easier read or express") incomplete, since isn't purpose?

3) if assigning b doesn't make copy of a, why doesn't wiping away a's old value mean b's old value wiped away 3 cases (integer, string , array)?

as can see, i'm pretty turned around on thought understood until now. appreciated!

you see, names (variable names, a , b) don't hold values themselves. point value. when make assignment

a = 5 

then a points value 5, regardless of pointed previously. important.

a = 'abcd' b = 

here both a , b point same string. but, when this

a += 'e' 

it's translated to

a = + 'e' # = 'abcd' + 'e' 

so, name a bound new value, while b keeps pointing "abcd".

a = [1,2,3,4] b = a << 5 

there's no assignment here, method << modifies existing array without replacing it. because there's no replacement, both a , b still point same array , 1 can see changes made another.