THEORY AND PRACTICE

Input and Output Data

Tasks

Conditions

Tasks

For Loop

Tasks

Strings

Tasks

While Loop

Tasks

Lists

Tasks

Two-Dimensional Arrays

Tasks

Dictionaries

Tasks

Sets

Tasks

Functions and Recursion

Tasks

Занятие 4. Lines

Difficulty level:

Task«Cryptocurrency»

Hello, crypto enthusiast! You were carried away by the analysis of the names of cryptocurrencies and you assume that in their names there are hidden patterns. To check your theory, you need a tool that will quickly show which symbol is in a certain position in the name of the token.

Task: & nbsp; write a program that makes the following:

first asks the user a name for the user cryptocurrency token.

then asks how many characters (positions) the user wants to check.

after that, in the cycle, the required number of times the user's position number

for each entered The program should check whether it is permissible for the introduced token name.

if the position is correct, the program displays the symbol located in this position.

if the position is incorrect (for example, more than the length of the name, or negative), the program must derive the message about the message about the message about error.

Input format

the name of the token (line)

the number of checks (an integer)

further, depending on the number of checks, the position number (an integer number)

is introduced several times)

Output format

symbol at the indicated position (line) or error message (line)

Example

Input

bitcoin
3
1
5
9

Output

The symbol in position 1: b
symbol in position 5: o
error! Position 9 is not in the word Bitcoin. The length of the word: 7.

Hint

All about strings in Python: from the basics to useful tricks

A string in Python is an ordered sequence of characters. To create a string, it must be enclosed in single ('...'), double ("...") or even triple quotes ("""...""" or "'..."'), which are especially useful for multi-line texts.

The key feature of strings in Python is their immutability. This means that after creating a string, you cannot change its individual characters. Any operation that "modifies" a row actually creates a new one.

string1 = "Hello, World!"
string2 = 'Python is fun'
# Example of a multiline string
multiline_string = """This is a string
that takes
up several lines of code."""

Indexing of rows

Each character in the string has its own unique number, or index. Indexing allows us to access any single character.

It is important to remember: Indexing in Python always starts with 0, not 1. An attempt to access a non-existent index will cause an error IndexError.

w = "Python"
print(w[0]) # Output: P
print(w[1]) # Output: y
print(w[2]) #Output: t
print(w[3]) # Output: h
print(w[4]) # Output: o (not 0, as in the original example)

There is also negative indexing, which is very convenient for accessing characters from the end of a string, where -1 is the last character, -2 is the penultimate, and so on.

print(w[-1]) # Output: n (last character)
print(w[-2]) # Output: o (penultimate character)

Slicing lines

Slices are a powerful tool for extracting a part of a string (substring). They allow you to get not a single character, but a whole range.

General syntax: string[start : end : step]

start : the index from which the slice begins (this character is included in the result).
end : the index at which the slice ends (this character is not included in the result).
step : determines the interval at which characters should be extracted (the default is 1).

Examples of cross-sections

Substring extraction

string = "Hello, World!"
substring = string[0:5] # Characters 0 through 4 inclusive
print(substring) # Output: Hello

Missing starting or ending index If you omit the initial index, the slice will start from the very beginning of the row. If you omit the final one, it will continue to the very end.

string = "Hello, World!"
substring1 = string[:5] # From the beginning to index 5 (not inclusive)
substring2 = string[7:]   # From index 7 to the end of the line
print(substring1) # Output: Hello
print(substring2) # Output: World!

Using a step The step allows you to "jump over" the symbols. For example, step 2 takes every second character.

string = "Hello, World!"
substring = string[::2] # Every second character from the beginning to the end
print(substring) # Output: Hlo ol!

Negative indexes in slices They can be combined to get the parts of the string from the end.

string = "Hello, World!"
substring = string[-6:-1] # We start with the 6th character from the end and go to the 1st from the end.
print(substring) # Output: World

Useful trick: inverting the string The easiest and most popular way to flip a string is to use a slice with a step -1.

string = "Hello, World!"
reversed_string = string[::-1]
print(reversed_string) # Output: !dlroW ,olleH

Adding and multiplying strings

Addition (concatenation) is performed using the + operator. It "glues" the lines into one new one.

str1 = "Hello"
str2 = "World"
result = str1 + ", " + str2 + "!"
print(result) # Output: Hello, World!

Important advice: You cannot add a string with a number directly. To do this, the number must first be converted to a string using the str() function..

# print("Age: " + 25) # Will cause TypeError
print("Age:" + str(25)) # Correct! Output: Age: 25

Multiplication is performed using the * operator, which repeats the string a set number of times. This is useful for creating separators or formatting.

string = "Go"
result = string * 3
print(result) # Output: GoGoGo
print("-" * 20) # Output: --------------------

Useful string operations

String length The built-in len() function returns the number of characters in a string.

my_string = "Hello"
print(len(my_string))  # Will output: 5

Checking for substring occurrence The in and not in operators allow you to check whether one string is contained inside another. The result will be True or False.

Remember: this check is case-sensitive! 'd' and 'D' are different characters.

name = "Daniil"
password = "Qwerty2000!"
print('D' in name) # Outputs: True
print('d' in name) # Outputs: False
print('3' not in password) # Outputs: True

Iterating through characters in a string

The string can be iterated through in the for loop. There are two main ways, and the choice depends on the task.

Method 1: A simple search (more "pythonic") This method is used when you only need the characters of the string themselves. It's simpler and more readable.

a = "1516 the best"
for char in a: # The char variable sequentially takes the value of each character
    if char in "1234567890":
        print(char)

Method 2: Index search This method is needed when not only the symbol itself is important to you, but also its position (index).

a = "1516 the best"
for i in range(len(a)): #i will take values from 0 to len(a)-1
    if a[i] in "1234567890":
print(f"The character '{a[i]}' was found at position {i}")

Conclusion and what's next?

We have reviewed the basic but very important operations.:

Indexing to access individual characters.
Slices are an incredibly flexible tool for extracting substrings.
Concatenation (+) and repetition (*) to create new strings.

This is the foundation upon which all work with text in Python is based. To move on, study the string methods. These are built-in functions that allow you to do even more useful things with strings, for example:

.lower() / .upper() reduce the entire string to lowercase or uppercase.
.strip() - remove spaces and line breaks at the beginning and at the end.
.replace('old', 'new') replace all occurrences of one substring with another.
.split() - split a string into a list by separator (for example, by space).
.startswith('prefix') / .endswith('suffix') check whether the string begins or ends with a certain sequence of characters.

main.py

Test 1

Test 2

Test 3

Test 4

Test 5

Test 6

Test 7

Test 8

Test 9

Test 10

Developer’s solution

# Запрашиваем у пользователя название токена и сохраняем его в переменную
token_name = input()

# Запрашиваем количество проверок, которое хочет сделать пользователь
checks_count_str = input()
# Преобразуем полученную строку в целое число
checks_count = int(checks_count_str)

# Запускаем цикл for, который повторится столько раз, сколько указал пользователь
for i in range(checks_count):
    # Внутри цикла каждый раз запрашиваем номер позиции для проверки
    # f-строка помогает красиво вывести подсказку с допустимыми номерами позиций
    position_str = input()
    # Преобразуем полученную строку в целое число
    position = int(position_str)

    # Условие: проверяем, что введенная позиция находится в допустимых границах
    # len(token_name) возвращает длину строки. Индексы идут от 0 до длины-1.
    if 0 <= position <= len(token_name):
        # Если позиция корректна, получаем символ по этому индексу
        symbol = token_name[position-1]
        # Выводим результат
        print(f"Символ на позиции {position}: {symbol}")
    else:
        # Если позиция некорректна, выводим сообщение об ошибке
        print(f"Ошибка! Позиции {position} нет в слове {token_name}. Длина слова: {len(token_name)}.")

🎉 Congratulations! 🎉

You did an excellent job with the task! It was a challenging problem, but you found the correct solution. You are one step closer to mastering programming! Keep up the good work, because every stage you pass makes you even stronger.