Question

Why does this code:

n = int(raw_input("How many rows? "))
for row in xrange(n):
    for col in xrange(row+1):
        print "*",
    print

Display this result (when n = 5)?

*
* *
* * *
* * * *
* * * * *

EDIT – I don’t understand how step by step it gets this form. Does col mean the number of columns? Because it doesn’t make sense as the number of columns is equal to the number of rows even though the input for columns is (rows+1).

Answer 1

Well, let’s go line-by-line and think like we’re the computer.

• n = int(raw_input("How many rows? ")) We get what the users inputs, convert it to an integer, and store the result in n.
• for row in xrange(n): We’re looping over xrange(n), which is more or less the same as [0, 1, 2, 3, 4] (although xrange doesn’t store the full list at once, and so is more memory efficient). So now we know that whenever we see row in the future, we have to replace it with whichever one of those numbers we’re on.
• for col in xrange(row+1): This line is similar to the last one; now col is one of the values in a list counting from 0 to the value stored in row. So if row is 2 on this iteration of the outer loop, the inner loop will run 3 times. The first time col will equal 0, the second 1, and the third time it will equal 2. (For each of those col values, row will stay 2, since row only changes when the outer loop goes through an iteration.)
• print "*", This is where the *s actually get printed. The tricky bit here is the comma at the end of the line. A print statement without a comma just prints something and then moves to the next line. When you add the comma, it basically prints the line but adds a space after the line instead of a new line (\n). So, since this line is called for each col value, it will print a * followed by a space on one line for each col value.
• print Notice that this is not inside the col loop. This statement just prints a newline. The idea here is to print a new line after each row has been printed so that the *s of the next rows are added to a new line, rather than to the previous line. If this print weren’t here, all of the stars would be printed on a single line.

So, for each row value, this program prints row+1 number of *s on one row.

In response to question edit:

I could see how col could be a confusing name. col is just the current column number of the * being printed. To see this more clearly, try replacing print "*", with print col,. You should get the following:

0
0 1
0 1 2
0 1 2 3
0 1 2 3 4

(Remember that range(3) produces [0,1,2], while range(3+1) produces [0,1,2,3].)