In the current chapter, we are going to examine nested conditional statements in the Python language, by which our program can execute conditions, that contain other nested conditional statements. We call them "nested" because we put the if condition into another if condition. We are going to examine the more complex logical conditions through proper examples.
Pretty often the program logic requires the use of if or if-else statements, which are contained one inside another. They are called nested if or if-else statements. As implied by the title "nested", these are if or if-else statements that are placed inside other if or else statements.
if condition1:
if condition2:
# body
else:
# bodyNesting of more than three conditional statements inside each other is not considered a good practice and has to be avoided, mostly through optimization of the structure/the algorithm of the code and/or by using another type of conditional statement, which we are going to examine below in this chapter.
Depending on age (decimal number and gender (m / f), print a personal title:
- “Mr.” – a man (gender “m”) – 16 or more years old.
- “Master” – a boy (gender “m”) under 16 years.
- “Ms.” – a woman (gender “f”) – 16 or more years old.
- “Miss” – a girl (gender “f”) under 16 years.
| Input | Output | Input | Output |
|---|---|---|---|
| 12 f |
Miss | 17 m |
Mr. |
| Input | Output | Input | Output |
|---|---|---|---|
| 25 f |
Ms. | 13.5 m |
Master |
We should notice that the output of the program depends on a few things. First, we have to check what is the entered gender and then check the age. Respectively, we are going to use a few if-else blocks. These blocks will be nested, meaning from the result of the first, we are going to define which one of the others to execute.
After reading the input data from the console, the following program logic should be executed:
Test your solution here: https://judge.softuni.org/Contests/Practice/Index/1051#0.
A Bulgarian entrepreneur opens small shops in a few cities with different prices for the following products:
| product / city | Sofia | Plovdiv | Varna |
|---|---|---|---|
| coffee water beer sweets peanuts |
0.50 0.80 1.20 1.45 1.60 |
0.40 0.70 1.15 1.30 1.50 |
0.45 0.70 1.10 1.35 1.55 |
Calculate the price by the given city (string), product (string) and quantity (float).
| Input | Output | Input | Output |
|---|---|---|---|
| coffee Varna 2 |
0.90 | peanuts Plovdiv 1 |
1.50 |
| Input | Output | Input | Output |
|---|---|---|---|
| beer Sofia 6 |
7.20 | water Plovdiv 3 |
2.10 |
We convert all of the letters into lower register using the function .lower(), to compare products and cities no matter what the letters are – small or capital ones.
Test your solution here: https://judge.softuni.org/Contests/Practice/Index/1051#1.
Let's take a look at how we can create more complex logical conditions. We can use the logical "AND" (and), logical "OR" (or), logical negation (not) and brackets (()).
As we saw, in some tasks we have to make many checks at once. But what happens when to execute some code more conditions have to be executed and we don't want to make a negation (else) for each one of them? The option with nested if blocks is valid, but the code would look very unordered and for sure – hard to read and maintain.
The logical "AND" (operator and) means a few conditions have to be fulfilled simultaneously. The following table of truthfulness is applicable:
| a | b | a and b |
|---|---|---|
| True True False False |
True False True False |
True False False False |
The and operator accepts a couple of Boolean (conditional) statements, which have a True or False value, and returns one bool statement as a result. Using it instead of a couple of nested if blocks, makes the code more readable, ordered and easy to maintain. But how does it work, when we put a few conditions one after another? As we saw above, the logical "AND" returns True, only when it accepts as arguments statements with the value True. Respectively, when we have a sequence of arguments, the logical "AND" checks either until one of the arguments is over, or until it meets an argument with value False.
Example:
a = True
b = True
c = False
d = True
result = a and b and c and d
# False (as d is not being checked)The program will run in the following way: It starts the check form a, reads it and accepts that it has a True value, after which it checks b. After it has accepted that a and b return True, it checks the next argument. It gets to c and sees that the variable has a False value. After the program accepts that argument c has a False value, it calculates the expression before c, independent of what the value of d is. That is why the evaluation of d is being skipped and the whole expression is calculated as False.
Checks whether point {x, y} is placed inside the rectangle {x1, y1} – {x2, y2}. The input data is read from the console and consists of 6 lines: the decimal numbers x1, y1, x2, y2, x and y (as it is guaranteed that x1 < x2 and y1 < y2).
| Input | Output | Visualization |
|---|---|---|
| 2 -3 12 3 8 -1 |
Inside |  |
A point is internal for a given polygon if the following four conditions are applied at the same time:
- The point is placed to the right from the left side of the rectangle.
- The point is placed to the left from the right side of the rectangle.
- The point is placed downwards from the upper side of the rectangle.
- The point is placed upwards from the down side of the rectangle.
Test your solution here: https://judge.softuni.org/Contests/Practice/Index/1051#2.
The logical "OR" (operator or) means that at least one among a few conditions is fulfilled. Similar to the operator and, the logical "OR" accepts a few arguments of bool (conditional) type and returns True or False. We can easily guess that we obtain a value True whenever at least one of the arguments has a True value. This is shown at the truth table below:
In school, the teacher said: "Ivan or Peter should clean the board". For completing this condition (the board to be clean), it's possible only Ivan to clean it, only Peter to clean it or both of them to do it.
| a | b | a or b |
|---|---|---|
| True True False False |
True False True False |
True True True False |
We have already learned what the logical "OR" represents. But how is it being achieved? Just like with the logical "AND", the program checks from left to right the arguments that are given. To obtain True from the expression, just one argument must have a True value. Respectively, the checking continues until an argument with such value is met or until the arguments are over.
Here is one example of the or operator in action:
a = False
b = True
c = False
d = True
result = a or b or c or d
# True (as c and d are not being checked)The program checks a, accepts that it has a value False and continues. Reaching b, it understands that it has a True value and the whole expression is calculated as True, without having to check c or d, because their values wouldn't change the result of the expression.
Let's check whether a given product is a fruit or a vegetable. The "fruits" are banana, apple, kiwi, cherry, lemon and grapes. The "vegetables" are tomato, cucumber, pepper and carrot. Everything else is "unknown".
| Input | Output |
|---|---|
| banana tomato java |
fruit vegetable unknown |
We have to use a few conditional statements with logical "OR" (or):
Test your solution here: https://judge.softuni.org/Contests/Practice/Index/1051#3.
Logical negation (operator not) means that a given condition is not fulfilled.
| a | not a |
|---|---|
| True | False |
The operator not accepts as an argument a bool variable and returns its value.
(the truth becomes a lie and the lie becomes a truth).
A given number is valid if it is in the range [100 … 200] or it is 0. Validate an invalid number.
| Input | Output |
|---|---|
| 75 | invalid |
| 150 | (no output) |
| 220 | invalid |
Test your solution here: https://judge.softuni.org/Contests/Practice/Index/1051#4.
Like the rest of the operators in programming, the operators and and or have a priority, as in the case and is with higher priority than or. The operator () serves for changing the priority of operators and is being calculated first, just like in mathematics. Using parentheses also gives the code better readability and is considered a good practice.
Sometimes the conditions may be very complex, so they can require a long bool expression or a sequence of conditions. Let's take a look at a few examples.
Write a program that checks whether a point {x, y} is placed onto any of the sides of a rectangle {x1, y1} – {x2, y2}. The input data is read from the console and consists of 6 lines: the decimal numbers x1, y1, x2, y2, x and y (as it is guaranteed that x1 < x2 and y1 < y2). Print "Border" (if the point lies on any of the sides) or "Inside / Outside" (in the opposite case).
| Input | Output | Input | Output |
|---|---|---|---|
| 2 -3 12 3 12 -1 |
Border | 2 -3 12 3 8 -1 |
Inside / Outside |
The point lies on any of the sides of the rectangle if:
- x coincides with x1 or x2 and at the same time y is between y1 and y2 or
- y coincides with y1 or y2 and at the same time x is between x1 and x2.
The previous conditional statement can be simplified by this way:
The second way with an additional boolean variable is longer but it's also more readable than the first, right? We advise you when writing boolean conditions to make them easier for reading than understanding and not short. If you are forced to, use additional variables with similar names. Names of the boolean variables should be with reasonable names. They should hint at what value will be stored in them.
All that's left is to write the code, that prints "Inside / Outside" if the point is not on one of the sides of the rectangle.
Test your solution here: https://judge.softuni.org/Contests/Practice/Index/1051#5.
A fruit shop during weekdays sells at the following prices:
| Fruit | Price |
|---|---|
| banana apple orange grapefruit kiwi pineapple grapes |
2.50 1.20 0.85 1.45 2.70 5.50 3.85 |
During the weekend days the prices are higher:
| Fruit | Price |
|---|---|
| banana apple orange grapefruit kiwi pineapple grapes |
2.70 1.25 0.90 1.60 3.00 5.60 4.20 |
Write a program that reads from the console a fruit (banana / apple / …), a day of the week (Monday / Tuesday / …) and a quantity (a decimal number) and calculates the price according to the prices from the tables above. The result has to be printed rounded up to 2 digits after the decimal point. Print “error” if it is an invalid day of the week or an invalid name of a fruit.
| Input | Output | Input | Output |
|---|---|---|---|
| orange Sunday 3 |
2.70 | kiwi Monday 2.5 |
6.75 |
| Input | Output | Input | Output |
|---|---|---|---|
| grapes Saturday 0.5 |
2.10 | tomato Monday 0.5 |
error |
Test your solution here: https://judge.softuni.org/Contests/Practice/Index/1051#6.
A company is giving the following commissions to its traders according to the city, in which they are working and the volume of sales s:
| City | 0 <= s <= 500 | 500 < s <= 1000 | 1000 < s <= 10000 | s > 10000 |
|---|---|---|---|---|
| Sofia Varna Plovdiv |
5% 4.5% 5.5% |
7% 7.5% 8% |
8% 10% 12% |
12% 13% 14.5% |
Write a program that reads the name of a city (string) and the volume of sales (float) and calculates the rate of the commission fee. The result has to be shown rounded up to 2 digits after the decimal point. When there is an invalid city or volume of sales (a negative number), print "error".
| Input | Output | Input | Output | Input | Output |
|---|---|---|---|---|---|
| Sofia 1500 |
120.00 | Plovdiv 499.99 |
27.50 | Kaspichan -50 |
error |
When reading the input, we could convert the city into small letters (with the function .lower()). Initially, we set the commission fee to -1. It will be changed if the city and the price range are found in the table of commissions.
To calculate the commission according to the city and volume of sales, we need a few nested if statements, as in the sample code below:
Test your solution here: https://judge.softuni.org/Contests/Practice/Index/1051#7.
Let's write a program that prints the day of the week depending on the given number (1 … 7) or "Error!" if invalid input is given.
| Input | Output |
|---|---|
| 1 7 -1 |
Monday Sunday Error |
 |
It is a good practice to put at the first place those case statements that process the most common situations and leave the case constructions processing the rarer situations at the end, before the default construction. Another good practice is to arrange the case labels in ascending order, regardless of whether they are integral or symbolic. |
Test your solution here: https://judge.softuni.org/Contests/Practice/Index/1051#8.
Write a program that prints the type of the animal depending on its name:
- dog -> mammal
- crocodile, tortoise, snake -> reptile
- others -> unknown
| Input | Output | Input | Output | Input | Output |
|---|---|---|---|---|---|
| tortoise | reptile | dog | mammal | elephant | unknown |
We can solve the example with a few if-elif conditional statements by doing so:
Test your solution here: https://judge.softuni.org/Contests/Practice/Index/1051#9.
Let's review the new constructions and program techniques we have met in this chapter:
if condition1:
if condition2:
# body
else:
# bodyif (x == left or x == right) and (y >= top or y <= bottom):
print(...) Let's practice using more complex conditions. We will solve a few practical exercises.
In a cinema hall, the chairs are ordered in a rectangle shape in r rows and c columns. There are three types of screenings with tickets of different prices:
- Premiere – a premiere screening, with a price of 12.00 BGN.
- Normal – a standard screening, with a price of 7.50 BGN.
- Discount – a screening for children and students at a reduced price – 5.00 BGN.
Write a program that enters a type of screening (string), a number for rows and a number for columns in the hall (integer numbers) and calculates the total income from tickets from a full hall. The result has to be printed in the same format as in the examples below – rounded up to 2 digits after the decimal point.
| Input | Output | Input | Output |
|---|---|---|---|
| Premiere 10 12 |
1440.00 leva | Normal 21 13 |
2047.50 leva |
While reading the input, we could convert the screening type into small letters (with the function .lower()). We create and initialize a variable that will store the calculated income. In another variable, we calculate the full capacity of the hall. We use an if-elif conditional statement to calculate the income according to the type of the projection and print the result on the console in the given format (look for the needed Python functionality on the internet).
Sample code (parts of the code are blurred with the purpose to stimulate your thinking and solving skills):
Test your solution here: https://judge.softuni.org/Contests/Practice/Index/1051#10.
Vladimir is a student, lives in Sofia and goes to his hometown from time to time. He is very keen on volleyball, but is busy during weekdays and plays volleyball only during weekends and on holidays. Vladimir plays in Sofia every Saturday when he is not working, and he is not traveling to his hometown and also during 2/3 of the holidays. He travels to his hometown h times a year, where he plays volleyball with his old friends on Sunday. Vladimir is not working 3/4 of the weekends, during which he is in Sofia. Furthermore, during leap years Vladimir plays 15% more volleyball than usual. We accept that the year has exactly 48 weekends, suitable for volleyball. Write a program that calculates how many times Vladimir has played volleyball throughout the year. Round the result down to the nearest whole number (e.g. 2.15 -> 2; 9.95 -> 9).
The input data is read from the console:
- The first line contains the word “leap” (leap year) or “normal” (a normal year with 365 days).
- The second line contains the integer p – the count of holidays in the year (which are not Saturday or Sunday).
- The third line contains the integer h – the count of weekends, in which Vladimir travels to his hometown.
| Input | Output | Input | Output |
|---|---|---|---|
| leap 5 2 |
45 | normal 3 2 |
38 |
| Input | Output | Input | Output |
|---|---|---|---|
| normal 11 6 |
44 | leap 0 1 |
41 |
As usual, we read the input data from the console and, to avoid making mistakes, we convert the text into small letters with the function .lower(). Consequently, we calculate the weekends spent in Sofia, the time for playing in Sofia and the common playtime. At last, we check whether the year is a leap, we make additional calculations when necessary and we print the result on the console rounded down to the nearest integer (look for a Python class with such functionality).
A sample code (parts of the code are blurred on purpose to stimulate independent thinking and solving skills):
Test your solution here: https://judge.softuni.org/Contests/Practice/Index/1051#11.
The figure consists of 6 blocks with size h * h, placed as in the figure below. The lower left angle of the building is on position {0, 0}. The upper right angle of the figure is on position {2*h, 4*h}. The coordinates given in the figure are for h = 2:
Write a program that enters an integer h and the coordinates of a given point {x, y} (integers) and prints whether the point is inside the figure (inside), outside of the figure (outside) or on any of the borders of the figure (border).
| Input | Output | Input | Output |
|---|---|---|---|
| 2 3 10 |
outside | 2 3 1 |
inside |
| Input | Output | Input | Output |
|---|---|---|---|
| 2 2 2 |
border | 2 6 0 |
border |
| Input | Output | Input | Output |
|---|---|---|---|
| 2 0 6 |
outside | 15 13 55 |
outside |
| Input | Output | Input | Output |
|---|---|---|---|
| 15 29 37 |
inside | 15 37 18 |
outside |
| Input | Output | Input | Output |
|---|---|---|---|
| 15 -4 7 |
outside | 15 30 0 |
border |
A possible logic for solving the task (not the only correct one):
- We might split the figure into two rectangles with a common side:
- A point is outer (outside) for the figure when it is outside both of the rectangles.
- A point is inner (inside) for the figure if it is inside one of the rectangles (excluding their borders) or lies on their common side.
- In every other case, the point lies on the border of the rectangle (border).
Sample code (parts of the code are blurred to stimulate logical thinking and solving skills):
Test your solution here: https://judge.softuni.org/Contests/Practice/Index/1051#12.