Programming languages and Python: variables, input and output

A computer understands only commands made of 0s and 1s, yet you type print(7 * 6) and it instantly answers 42. Who translates between these two languages? In this lesson you will find out, and you will learn Python’s first essential tools: variables, reading from the keyboard, printing to the screen and arithmetic. In the informatics part of Azerbaijan’s university entrance exam (DİM, group I), 9 of the 30 tasks are Python programs — and all of them are built from the bricks of this lesson.

Programs, programming languages and translators

Languages are divided into levels by how close they are to the machine. Machine language is the only language the processor executes directly: commands are binary codes and depend on the type of processor. Low-level languages (for example, assembly language) write machine commands with short letter codes such as MOV and ADD; they are also tied to a particular processor. High-level languages (Python, C++, Java, Pascal) are close to human language and mathematical notation, do not depend on the processor, and one of their commands turns into many machine commands.

Python is an interpreted language. If the program fails at its 5th command while running (for example, division by zero), the output of the first four commands is already on the screen. Strictly speaking, Python first turns the whole text into intermediate bytecode — that is why a program with a typing (syntax) error does not start at all — but at school and in the exam Python is classed as an interpreted language.

Constants, variables and assignment

The assignment operator = works like this: first the expression on the right is calculated, then the result is stored in the variable on the left, and the old value is lost. That is why x = x + 1 is not an equation but the command “increase x by 1”. Official exam explanations describe it as “the value … is assigned to the variable”. Python also allows several assignments in one line: a, b = 3, 8.

• a name starts with a letter or _, followed by letters, digits and _: a1, sum_2, _k are valid, while 2a, my-sum and a b are not;
• upper and lower case differ: a and A are two different variables;
• keywords (if, else, for, while, and, or, not, True) cannot be variable names.

a = 7
b = 2.5
s = '7'
t = a > 5
print(type(a), type(b), type(s), type(t))
print(a * 2, b * 2, s * 2, t)

<class 'int'> <class 'float'> <class 'str'> <class 'bool'>
14 5.0 77 True

In Python the type of a variable is set by the value assigned to it and can change while the program runs: after x = 5, x is a whole number; after x = 'five' it is a string. So in every task ask yourself: does the variable hold a number or text at this moment? The same + sign adds numbers but glues strings together.

Input and output: input() and print()

The print() function shows the values in its brackets on the screen. Several values separated by commas are printed on one line with one space between them, and every new print() starts a new line. The sep parameter changes the separator and end changes the line ending: print(1, 2, 3, sep='-') → 1-2-3, while print('a', end='') keeps the next output on the same line.

input() reads the line the user types on the keyboard and **always returns it as a string (str)**: even if you type 5, you get '5'. To calculate with it, convert the string: int(input()) for a whole number, float(input()) for a real number. That is exactly how the exam programs begin: n = int(input()). Several values typed on one line with spaces are read with input().split() — it cuts the line at the spaces and returns a list of strings; we convert each part with int().

name = input()
age = int(input())
a, b = input().split()
a = int(a)
b = int(b)
print(name, age, a + b)
print(name, 'is', age, sep='_')

Read the task statement carefully: if the numbers are typed each on its own line, repeat int(input()) as many times as needed (a = int(input()), b = int(input())); if they are typed on one line, separated by spaces, use input().split(). Both forms appear in the written exam tasks: for example, the elements of a list may come as one line separated by single spaces, while their count n comes on a line of its own.

name = 'Leyla'
age = int('14')
line = '3 8'
a, b = line.split()
a = int(a)
b = int(b)
print(name, age, a + b)
print(name, 'is', age, sep='_')

Leyla 14 11
Leyla_is_14

Arithmetic: //, % and the order of operations

a = 7
b = 45
c = b // a % 4 * 3 + b % a ** 2
print(c)

51

Digits of a number and type conversions

n = 739
a = n // 100
b = n // 10 % 10
c = n % 10
print(c * 100 + b * 10 + a, a + b + c, abs(a - c))

937 19 2

Type conversions. str() turns a number into a string, and int() turns a string or a real number into a whole number: str(45) + '1' → 451, int('45') + 1 → 46, int(7.9) → 7 (the fraction is simply cut off, not rounded). len(str(n)) gives the number of digits: len(str(2026)) → 4. abs(x) returns the absolute value: abs(-12) → 12.

Exam tasks: what does the program print?

In the exam a program is usually 5–15 lines long and the question is always the same: “determine the result of the program”. A closed task has 5 answer options; in a coded task you write the answer yourself. In both cases the winning method is the same — “run” the program line by line, like a computer.

1. 1
   Open a table of variables

   Draw a column for each variable and write the starting values. If there is input(), write the given input with the right type: int(input()) → a number, input() → a string.

2. 2
   Go line by line

   After each assignment update only that variable’s column and cross out the old value. The right-hand side is always computed with the old values.

3. 3
   Follow the priority

   In a long expression do ** first, then * / // % from left to right, then + -. Write down the intermediate results.

4. 4
   Watch the types

   / always gives a float (8 / 2 → 4.0); for strings + joins and * repeats.

5. 5
   Check the output format

   print(a, b) separates values with one space, and each print is on a new line. The answer options often differ exactly in spaces and lines.

t = 7384
# hours, minutes, seconds

2 3 4

line = '38 7'
# split, convert, print quotient, remainder, check

5 3 38