Practical Programming, 3rd Edition

Nesting Loops in Loops

The block of statements inside a loop can contain another loop. In this code, the inner loop is executed once for each item of list outer:

>>> outer = ['Li', 'Na', 'K']
>>> inner = ['F', 'Cl', 'Br']
>>> for metal in outer:
...     for halogen in inner:
...         print(metal + halogen)
...
...

LiF
LiCl
LiBr
NaF
NaCl
NaBr
KF
KCl
KBr

The number of times that function print is called is len(outer) * len(inner). In Table 13 we show that for each iteration of the outer loop (that is, for each item in outer), the inner loop executes three times (once per item in inner).

Table 13. Nested Loops Over Inner and ...