if, while, and for statements

if statement

A conditional statement is what we use when we want our code to make decisions. For example, suppose we wanted to divide a number by 2 only if it is even. We can do this in Sage by using an if statement. The general syntax for Python's if-else statement is

if <condition>:
    <statement>
         :
else:
    <statement>
         :

<condition> is an expression that can use relational operators <, >, <=, >=, == (is equal), and != (is not equal), as well as logical operators: and, or, not. Its value is either True or False. If the condition is True, the statements indented under if are executed, otherwise the statements under else are executed. The else clause is optional: you can have if alone. In that case, if the condition is True the program executes the statements indented under if, otherwise the program skips them.

n = 15
if n%3==0:
    print('n is divisible by 3')
else
    print('n is not divisible by 3')

Output:
n is divisible by 3

Often you'll need to string a chain of several if-else statements together. For example

def letterGrade(score):
    if score >= 90:
        return 'A'
    else:
        if score >= 80:
            return 'B'
        else:
            if score >= 70:
                return 'C'
            else:
                if score >= 60:
                    return 'D'
                else:
                    return 'F'

Python lets you simplify the indentation and compress the if-else on one line by using the keyword elif. The above code can be shortened to

def letterGrade(score):
    if score >= 90:
        return 'A'
    elif score >= 80:
        return 'B'
    elif score >= 70:
        return 'C'
    elif score >= 60:
        return 'D'
    else:
        return 'F'

while loop

while loops are one of the most useful techniques in programming. Essentially, a while loop allows us to repeat the same block of statements multiple times (but with different values of variables) while a certain condition holds true. The general syntax for Python's while loop is

while <condition>:
    <statement>
         :

As long as the condition remains true the program repeats the statements in the while block.

The next example uses a while loop to add the integers from 1 to 10.

i = 1
sum1ton = 0
while i <= 10:
    sum1ton += i   # equivalent to sum1ton = sum1ton + i
    i += 1         # increments i by 1
sum1ton

Output:
55

for loop

for loops are traditionally used when you have a block of code which you want to repeat a fixed number of times. In Python, for loops iterate over a fixed list. As an alternative, the while loop could be used, however, while is used when a condition is to be met, or if you want a block of code to theoretically repeat forever, for example repeatedly asking for user input until the format the user provides is correct. The general syntax for Python's for loop is

for  x in <list>:
    <statement>
         :

Here is an example of using the for loop to step through the entries of a list and square each one.

L = [1,2,3,4,5]
for x in L:
    print(x**2)

Output:
1
4
9
16
25

In the next example we use a for loop together with an if statement to print the list of integers from 1 to 20 which are divisible by 3.

for x in range(1,21):
    if x%3 == 0:
        print(x)

Output:
3
6
9
12
15
18

This has been a quick introduction to get up and running with python.