Nested Loops and Pattern Programs

Nested Loops and Pattern Programs

In a nested loop the inner loop runs completely for each pass of the outer loop. Pattern programs use the outer loop for rows and the inner loop for the items in a row.

n = 4
for i in range(n, 0, -1):
    for j in range(1, i + 1):
        print(j, end="")
    print()

Output:

1234
123
12
1

To work out the inner loop of any pattern, write down for each row number i what is printed, then find the rule. A pyramid also needs leading spaces: row i of an n-row pyramid starts with n - i spaces.

for i in range(1, 4):
    print(" " * (3 - i) + "*" * (2 * i - 1))

n = int(input())
for i in range(1, n + 1):
    for j in range(1, n + 1):  # how far should j go in row i?
        print(j, end="")
    print()