Python Programs with Output
Complete programs, each explained step by step with its output. Run them here, open them in the editor, or step through them with Visualize.
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Numbers
Even or odd, primes, factorials, Fibonacci, Armstrong numbers and other school programs.
- Armstrong number check Checks whether a number equals the sum of its digits, each raised to the power of how many digits it has.
- Even or odd check Checks whether a number is even or odd with the % operator, which gives the remainder after dividing by 2.
- Factorial of a number Finds the factorial of a number, the product of every whole number from 1 to n, with a for loop.
- Fibonacci sequence Builds the first n terms of the Fibonacci sequence, where each term is the sum of the two before it, as a list.
- GCD and LCM Finds the greatest common divisor (HCF) of two numbers with Euclid's algorithm, then their least common multiple from it.
- Grade calculator Turns marks out of 100 into a letter grade from A to F by checking, from the top down, which range they fall in.
- Largest of three numbers Finds the largest of three numbers by comparing them with if, elif and else.
- Leap year check Checks whether a year is a leap year: divisible by 4, except century years, which must be divisible by 400.
- Multiplication table Prints the multiplication table of a number from 1 to 10, one line per multiple, with a for loop.
- Palindrome number check Checks whether a number reads the same backwards as forwards by reversing its digits and comparing.
- Perfect number check Checks whether a number equals the sum of its proper divisors, the smaller numbers that divide it exactly, as 28 does.
- Prime number check Checks whether a number is prime by trying every divisor from 2 up to its square root.
- Prime numbers in a range Lists every prime number between two numbers, such as 10 and 50, by checking each number in turn.
- Quadratic equation roots Finds the roots of ax² + bx + c = 0 from the discriminant: two real roots, one repeated root, or two complex roots.
- Reverse a number Reverses the digits of a number, so 1234 becomes 4321, by taking them off the end with % 10 and // 10.
- Simple and compound interest Works out the simple and the compound interest on the same principal, rate and number of years, shown to two decimal places.
- Simple calculator A calculator function that adds, subtracts, multiplies or divides two numbers, with a message instead of an error for division by zero.
- Sum of digits Adds up the digits of a number, taking the last digit with % 10 and removing it with // 10 until none are left.
- Swap two numbers Swaps the values of two variables in one line with a, b = b, a, without a temporary variable.
- Temperature conversion Converts a temperature from Celsius to Fahrenheit and from Fahrenheit back to Celsius with the two standard formulas.
Patterns
Star, number and Pascal triangle patterns printed with loops.
- Diamond pattern Prints a diamond of stars: the rows of a pyramid going down to the widest one, then the same rows again in reverse.
- Number triangle pattern Prints a triangle of numbers where row 1 is 1, row 2 is 1 2, row 3 is 1 2 3, and so on for as many rows as you ask.
- Pascal's triangle Prints Pascal's triangle, working out each row from the one above it: every number inside a row is the sum of the two above it.
- Pyramid pattern Prints a centred pyramid of stars: each row has one space less in front and two more stars than the row above.
- Right-angled triangle pattern Prints a right-angled triangle of stars with one more star on each row, using a loop and string repetition.
Strings
Palindromes, vowels, word counts, anagrams and other text programs.
- Anagram check Checks whether two words or phrases are anagrams, made of exactly the same letters, by sorting the letters of each and comparing them.
- Count vowels Counts the vowels (a, e, i, o and u) in a string, capital letters included, by checking each character in turn.
- Palindrome string check Checks whether a word or sentence reads the same backwards as forwards, ignoring capitals, spaces and punctuation.
- Reverse words in a string Reverses the order of the words in a sentence, keeping each word spelt forwards, with split(), list slicing and join().
- Word frequency Counts how many times each word appears in a sentence, with a dictionary that has the words as keys and their counts as values.
Searching and sorting
Linear and binary search, and the classic sorting algorithms step by step.
- Bubble sort Sorts a list by swapping neighbours that are out of order, pass after pass, and stops early once a pass makes no swaps.
- Insertion sort Sorts a list by taking each item in turn and inserting it into its place among the sorted items before it.
- Linear search Finds a value in a list by checking each item in turn, from the first to the last, until one matches.
- Selection sort Sorts a list by finding the smallest item left and swapping it to the front of the unsorted part, one place at a time.
- Binary search Finds a value in a sorted list by checking the middle item and halving the part still to search at every step.
- Merge sort Sorts a list by splitting it in half, sorting each half the same way, then merging the two sorted halves into one.
- Quick sort Sorts a list in place by moving smaller items before a pivot and the rest after it, then sorting each side: O(n log n) time on average.
Data structures
Stacks, queues, linked lists, trees, heaps and techniques built on lists.
- Balanced brackets Checks whether every bracket in a string is closed in the right order, using a list as a stack of the brackets still open.
- Queue A first-in, first-out print queue built on collections.deque: jobs join at the back, leave from the front, and peek shows the next one.
- Binary search tree Builds a binary search tree, where smaller values go left and larger ones right, then searches it and lists its values in sorted order.
- Binary tree traversals Visits every node of a small binary tree in in-order, pre-order and post-order with three short recursive functions.
- Linked list A singly linked list of Node objects that adds values at the end, deletes a value, reverses itself and prints as 1 -> 2 -> 3.
- Prefix sums Stores the running totals of a list in one O(n) pass, then answers each range sum query with one subtraction, in O(1).
- Sliding window Finds the largest sum of k items in a row by sliding a window along the list, adding the item that enters and taking away the one that leaves.
- Top k items with a heap Finds the k largest items in a list with heapq, keeping a min-heap of the k biggest seen so far, so the list never has to be sorted.
Graphs
Breadth-first and depth-first search, shortest paths, ordering and connected groups.
- Breadth-first search Visits the nodes of a graph from a start node, nearest first, keeping the nodes still to visit in a queue.
- Depth-first search Visits the nodes of a graph from a start node, following each path as deep as it goes before backing up to try the next.
- Dijkstra's shortest path Finds the shortest distance from one node to every other in a weighted graph, always going on from the closest node, with heapq.
- Topological sort Orders courses so that each comes after the courses it needs, with Kahn's algorithm: count each one's prerequisites, then use a queue.
- Union-find Puts friends into groups with union-find, where merging two groups and asking if two people share one each take almost constant time.
Dynamic programming
Problems solved by remembering the answers to smaller problems.
- Coin change Finds the fewest coins that add up to an amount by solving every smaller amount first, or -1 when no mix of the coins makes it.
- Fibonacci with memoization Works out Fibonacci numbers with recursion, remembering each answer in a dict so that no number is ever calculated twice.
- 0/1 knapsack Finds the most value that fits in a bag with a weight limit, taking each item at most once, by filling in a table.
- Longest common subsequence Finds the longest run of letters that appears in the same order, not always side by side, in both of two strings, with a table.
Backtracking
Trying every choice and undoing the ones that fail: permutations, subsets, N-Queens.
- Permutations Lists every ordering of a list's items by choosing an item for each place in turn, then undoing the choice to try the next one.
- Subsets Lists every subset of a list's items (the power set) by deciding for each item in turn whether to include it or skip it.
- N-Queens Places N queens on an N×N chessboard so that no two attack each other, trying each column row by row and backing up from dead ends.