3.3 Nested Loops
A loop can be placed inside another loop. Each time the outer loop runs once, the inner loop runs through all of its iterations.
Basic execution order
Start with a small example:
for i in range(2):
for j in range(3):
print("i = %d, j = %d" % (i, j))Output:
i = 0, j = 0
i = 0, j = 1
i = 0, j = 2
i = 1, j = 0
i = 1, j = 1
i = 1, j = 2Read it like this: when outer variable i is 0, the inner variable j goes through 0, 1, 2; then i becomes 1, and the inner loop runs through 0, 1, 2 again.
Multiplication table
Nested loops are useful for generating two-dimensional structures. For a multiplication table, the outer loop controls rows and the inner loop controls columns.
for i in range(1, 10):
for j in range(1, 10):
print("%d*%d=%d\t" % (i, j, i * j), end='')
print()Output:
1*1=1 1*2=2 1*3=3 1*4=4 1*5=5 1*6=6 1*7=7 1*8=8 1*9=9
2*1=2 2*2=4 2*3=6 2*4=8 2*5=10 2*6=12 2*7=14 2*8=16 2*9=18
3*1=3 3*2=6 3*3=9 3*4=12 3*5=15 3*6=18 3*7=21 3*8=24 3*9=27
4*1=4 4*2=8 4*3=12 4*4=16 4*5=20 4*6=24 4*7=28 4*8=32 4*9=36
5*1=5 5*2=10 5*3=15 5*4=20 5*5=25 5*6=30 5*7=35 5*8=40 5*9=45
6*1=6 6*2=12 6*3=18 6*4=24 6*5=30 6*6=36 6*7=42 6*8=48 6*9=54
7*1=7 7*2=14 7*3=21 7*4=28 7*5=35 7*6=42 7*7=49 7*8=56 7*9=63
8*1=8 8*2=16 8*3=24 8*4=32 8*5=40 8*6=48 8*7=56 8*8=64 8*9=72
9*1=9 9*2=18 9*3=27 9*4=36 9*5=45 9*6=54 9*7=63 9*8=72 9*9=81Each row fixes one value of i, then lets j go from 1 to 9.
For the common lower-triangular multiplication table, let j stop at the current value of i:
for i in range(1, 10):
for j in range(1, i + 1):
print("%d*%d=%d\t" % (j, i, i * j), end='')
print()Printing patterns
Nested loops can also control text patterns. The program below prints a triangle made of stars.
for i in range(1, 6):
for j in range(1, i + 1):
print("*", end='')
print()Output:
*
**
***
****
*****The outer loop decides how many rows to print. The inner loop decides how many * characters appear on each row.