Conditions

All previous programs have been executed sequentially, line by line. No line could be skipped.

Consider the following problem: for a given real number, determine its absolute value. If number > 0, the program should print its value, otherwise it should print –number . This behavior cannot be achieved by using a sequential program. The program should conditionally select the next step. The following command used for this purpose is called if:

number = float(input("Enter a number:"))

if number > 0:
    absolute_value = number
else:
    absolute_value = -number

print(absolute_value)

This program uses a conditional statement if . After that, we put a condition number > 0 after a colon. We then insert a block of statements that will only be executed if the condition is true (i.e. the value of the expression number > 0 is equal to True). This block can (but doesn’t have to) be followed by a word else, a colon, and another block of instructions that will only be executed if the condition is false (i.e., has the value False).

Blocks in Python

In Python, blocks are marked with indentation i.e. spaces at the beginning of the line. The number of these spaces is arbitrary, but it is important that there are enough of them to make the block visually stand out. Each block must be identified by a colon (:) at the end of the preceding line. Of course, only some commands can start a new block. In the example above, these are the commands if and else. The blocks assigned to them will be executed only if the condition is met (if) or not met (else). We finish the block by removing the indentation of the first non-block line.

Blocks can be nested: next levels are marked by increasing the number of spaces at the beginning (e.g. 4, 8, 12 etc.), and each of them is finished by undoing the indentation accordingly. (8, 4, 0). It is possible to remove indentations corresponding to several blocks at once — then we finish them all at once.

The way of marking blocks with indentation is specific to Python. Other programming languages use special characters (usually braces) to mark the beginning and end of a block.

In summary, a conditional statement in Python has the following syntax:

if condition:
    any number of commands
    executed if the condition is True
else:
    any number of commands
    executed if the condition is False

the keyword else along with its corresponding block can be omitted if nothing should be done if the condition is false. For example, we can replace the variable with number with its absolute value as follows:

number = float(input("Enter a number:"))

if number < 0:
    number = -number

print(number)

In this example, the variable number is assigned the value -number only if number < 0. On the other hand, the command print(number) is executed every time because it is not indented, so it does not belong to the block that is executed only when the tested condition is true.

Nesting conditions

Each Python statement can be placed in any of the blocks, including a different conditional statement. Thus, we obtain nested conditions. Internal condition blocks are indented with more spaces (e.g., 8 spaces). Let’s see an example. Given the coordinates of a point on the plane, print a quarter of it:

x = float(input("Enter x coordinate:"))
y = float(input("Enter y coordinate:"))

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 than or equal to 0
        print("Quadrant IV")
else:
    if y > 0:
        # x is less than or equal to 0, y is greater than 0
        print("Quadrant II")
    else:
        # x is less than or equal to 0, y is less than or equal to 0
        print("Quadrant III")

If we have more than two options to distinguish using the conditional operator, we can use the statement if ... elif ... else (elif is short for else if):

if condition1:
    any number of commands
    executed if condition1 is True
elif condition2:
    any number of commands
    executed if the condition1 if False and condition2 is True
elif condition3:
    any number of commands
    executed if the condition1 and condition2 are False and condition3 is True
else:
    any number of commands
    executed if all conditions are False

Let’s show how it works by rewriting the example above:

x = float(input("Enter x coordinate:"))
y = float(input("Enter y coordinate:"))

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")

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