Lesson 3
Conditions: if, then, else
1. Syntax
All the programs in the first lesson were executed sequentially, line after line. No line could be skipped.
Let's consider the following problem: for the given integer X determine its absolute value. If X>0 then the program should print the value X, otherwise it should print -X. This behavior can't be reached using the sequential program. The program should conditionally select the next step. That's where the conditions help:
x = int(input())
if x > 0:
print(x)
else:
print(-x)
This program uses a conditional statement if. After the if
we put a condition (x > 0) following by a colon.
After that we put a block of instructions which will be executed only if the condition
is true (i.e. evaluates to True). This block may be followed by the word
else, colon and another block of instructions which will be executed only if the condition is false
(i.e. evaluates to False). Is the case above, the condition is false, so the 'else' block
is executed. Each block should be indented using spaces.
To sum up, the conditional statement in Python has the following syntax:
if condition:
true-block
several instructions that are executed
if the condition evaluates to True
else:
false-block
several instructions that are executed
if the condition evaluates to False
The else keyword with the 'false' block may be omitted in case nothing should be
done if the condition is false. For example, we can replace the variable x
with its absolute value like this:
x = int(input())
if x < 0:
x = -x
print(x)
In this example the variable x is assigned to -x only if x < 0.
In contrast, the instruction print(x) is executed every time, because it's not indented,
so it doesn't belong to the 'true' block.
Indentation is a general way in Python to separate blocks of code. All instructions within the same block should be indented in the same way, i.e. they should have the same number of spaces at the beginning of the line. It's recommended to use 4 spaces for indentation.
The indentation is what makes Python different from the most of other language, in which
the curly braces { and } are used to form the blocks.
By the way, there's a builtin-function for absolute value in Python:
x = int(input()) print(abs(x))
2. Nested conditions
Any Python instruction may be put into 'true' blocks and 'false' block, including another conditional statement. This way we get nested conditions. The blocks of inner conditions are indented using twice more spaces (eg. 8 spaces). Let's see an example. Given the coordinates of the point on the plane, print its quadrant.
x = int(input())
y = int(input())
if x > 0:
if y > 0:
# x is greater than 0, y is greater than 0
print("Quadrant I")
else:
# x is greater than 0, y is less or equal than 0
print("Quadrant IV")
else:
if y > 0:
# x is less or equal than 0, y is greater than 0
print("Quadrant II")
else:
# x is less or equal than 0, y is less or equal than 0
print("Quadrant III")
In this example we use the comments: the explanatory text that has no effect on program execution.
This text starts with the hash # and lasts till the end of the line.
3. Comparison operators
Usually the condition after if has one or more of the following operators:
<- less — the condition is true if left side is less than right side.
>- greater — the condition is true if left side is greater than right side.
<=- less or equal.
>=- greater or equal.
==- equal.
!=- not equal.
For example, the condition x * x < 1000
means “the value of the expression x * x is less than 1000”,
and the condition 2 * x != y means “the doubled value
of the variable x is not equal to the value of the variable y”.
The comparison operators in Python may be grouped together like this: a == b == cx <= y >= 10
4. Bool objects and logical operators
When we sum two integer objects using the + operator, like 2 + 5, we get a new object:
7. In the same way, when we compare two integers using the < operator, like
2 < 5, we get a new object: True.
print(2 < 5) print(2 > 5)
True and False objects have a special type called bool.
As every type name can be used to cast objects into that type, let's see what this cast gives
for numbers:print(bool(-10)) # True
print(bool(0)) # False - zero is the only false number
print(bool(10)) # True
print(bool('')) # False - empty string is the only false string
print(bool('abc')) # True
Sometimes you need to check several conditions at once.
For example, you can check if a number n is divisible by 2 using the condition
n % 2 == 0
(n gives a remainder 0 when dividing by 2).
If you need to check that two numbers n and m are both divisble by 2,
you should check both n % 2 == 0
and m % 2 == 0.
To do that, you join them using an operator and
(logical AND): n % 2 == 0 and m % 2 == 0.
Python has logical AND, logical OR and negation.
Operator and is a binary operator which evaluates to True
if and only if both its left-hand side and right-hand side are True.
Operator or is a binary operator which evaluates to True
if at least one of its sides is True.
Operator not is a unary negation, it's followed by some value.
It's evaluated to True if that value is False and vice versa.
Let's check that at least one of the two numbers ends with 0:
a = int(input())
b = int(input())
if a % 10 == 0 or b % 10 == 0:
print('YES')
else:
print('NO')
Let's check that the number a is positive and
the number b is non-negative:
if a > 0 and not (b < 0):
Instead of not (b < 0) we can write (b >= 0).
5. 'elif' word
If you have more than two options to tell apart using the conditional operator, you can use
if... elif... else statement.
Let's show how it works by rewriting the example with point (x,y) on the plane and quadrants from above:
x = int(input())
y = int(input())
if x > 0 and y > 0:
print("Quadrant I")
elif x > 0 and y < 0:
print("Quadrant IV")
elif y > 0:
print("Quadrant II")
else:
print("Quadrant III")
In this case the conditions in if and elif are checked one after another until
the first true condition is found. Then only the true-block for that condition is being executed. If all the
conditions are false, the 'else' block is being executed, if it's present.