Dec  = Decimal Value
Char = Character

'D' has the int value 68
if we write ('A' <= c && c <= 'Z'), this will be true when c is an uppercase letter
if we write 'D'-'A' it evaluates to 68-65, or the int 3
if we write: char c = 'B'+32; then c holds 'b'


Dec  Char                           Dec  Char     Dec  Char     Dec  Char
---------                           ---------     ---------     ----------
  0  NUL (null)                      32  SPACE     64  @         96  `
  1  ---                             33  !         65  A         97  a
  2  ---                             34  "         66  B         98  b
  3  ---                             35  #         67  C         99  c
  4  ---                             36  $         68  D        100  d
  5  ---                             37  %         69  E        101  e
  6  ---                             38  &         70  F        102  f
  7  BEL (bell)                      39  '         71  G        103  g
  8  BS  (backspace)                 40  (         72  H        104  h
  9  TAB (horizontal tab)            41  )         73  I        105  i
 10  LF  (NL line feed, new line)    42  *         74  J        106  j
 11  ---                             43  +         75  K        107  k
 12  ---                             44  ,         76  L        108  l
 13  CR  (carriage return)           45  -         77  M        109  m
 14  ---                             46  .         78  N        110  n
 15  ---                             47  /         79  O        111  o
 16  ---                             48  0         80  P        112  p
 17  ---                             49  1         81  Q        113  q
 18  ---                             50  2         82  R        114  r
 19  ---                             51  3         83  S        115  s
 20  ---                             52  4         84  T        116  t
 21  ---                             53  5         85  U        117  u
 22  ---                             54  6         86  V        118  v
 23  ---                             55  7         87  W        119  w
 24  ---                             56  8         88  X        120  x
 25  ---                             57  9         89  Y        121  y
 26  ---                             58  :         90  Z        122  z
 27  ESC (escape)                    59  ;         91  [        123  {
 28  ---                             60  <         92  \        124  |
 29  ---                             61  =         93  ]        125  }
 30  ---                             62  >         94  ^        126  ~
 31  ---                             63  ?         95  _        127  DEL


(adapted from https://www.cs.cmu.edu/~pattis/15-1XX/common/handouts/ascii.html, 2022.03.15)