Additional Programming Techniques

String manipulation

Common patterns

# Build a username from a name
first = "Aisha"
last = "Khan"
username = (first[0] + last).lower()      # akhan

# Count the vowels in a word
word = input("Enter a word: ")
count = 0
for letter in word.lower():
    if letter in "aeiou":
        count = count + 1
print("Vowels:", count)

# Reverse a string
print(word[::-1])

# Check a palindrome
clean = word.lower().replace(" ", "")
if clean == clean[::-1]:
    print("Palindrome")

Arrays and lists

An array stores many values of the same type under one name, accessed by an index starting at 0.

scores = [12, 45, 3, 78, 20]

print(scores[0])        # 12, the first item
print(scores[4])        # 20, the last item
print(len(scores))      # 5

scores[2] = 99          # change the third item
scores.append(60)       # add to the end

Working through an array

total = 0
highest = scores[0]

for value in scores:
    total = total + value
    if value > highest:
        highest = value

print("Total:", total)
print("Average:", total / len(scores))
print("Highest:", highest)

Notice highest is initialised to the first element, not to 0. Initialising to 0 breaks if all the values are negative.

Two dimensional arrays

A 2D array is a grid, accessed with two indexes: row then column.

board = [["X", "O", "X"],
         ["O", "X", "O"],
         ["X", "O", "X"]]

print(board[1][2])      # row 1, column 2, which is "O"

for row in board:
    for cell in row:
        print(cell, end=" ")
    print()

File handling

Programs need to store data between runs, and that means reading from and writing to files.

# Writing to a file
file = open("scores.txt", "w")      # "w" overwrites the whole file
file.write("Aisha,42
")
file.write("Ben,37
")
file.close()

# Appending, which adds without deleting what is there
file = open("scores.txt", "a")
file.write("Chris,55
")
file.close()

# Reading the whole file line by line
file = open("scores.txt", "r")
for line in file:
    parts = line.strip().split(",")
    name = parts[0]
    score = int(parts[1])
    print(name, "scored", score)
file.close()

The three modes

Always close the file when finished, so that data is properly written and the file is released.

OCR reference language

myFile = open("scores.txt")
while NOT myFile.endOfFile()
    print(myFile.readLine())
endwhile
myFile.close()

SQL

SQL, Structured Query Language, is used to search and manipulate data in a database. J277 requires SELECT, FROM and WHERE.

SELECT which columns you want FROM which table WHERE which rows to include

Examples

Given a table called Students:

Select everything:

SELECT * FROM Students

Select specific columns:

SELECT Name, Grade FROM Students

Select with a condition:

SELECT Name FROM Students WHERE Year = 11

Multiple conditions:

SELECT Name FROM Students WHERE Year = 11 AND Grade >= 8

Text values need quotation marks, numbers do not:

SELECT * FROM Students WHERE Name = 'Aisha'

Subroutines and random numbers

Subroutines

A subroutine is a named block of code that performs a specific task and can be called from anywhere in a program.

def calculate_area(width, height):
    return width * height          # a FUNCTION, it returns a value

def show_welcome(name):
    print("Welcome,", name)        # a PROCEDURE, it returns nothing

area = calculate_area(5, 3)        # area is 15
show_welcome("Aisha")

Parameters are the variables listed in the definition. Arguments are the actual values passed in when it is called.

Why subroutines matter

• Avoid repetition. Write the code once and call it many times.
• Easier to test. Each subroutine can be tested on its own.
• Easier to maintain. Fix a bug in one place rather than in ten copies.
• Reusable in other programs.
• Support decomposition, since each sub problem becomes a subroutine.
• Improve readability, because a well named call explains itself.

Local and global variables

A local variable exists only inside the subroutine where it is created. A global variable is available throughout the whole program.

Local variables are preferred because they cannot be changed accidentally by other parts of the program, and their memory is freed when the subroutine finishes.

Random numbers

import random

dice = random.randint(1, 6)          # a whole number from 1 to 6 inclusive
card = random.choice(["A", "K", "Q"]) # a random item from a list

dice = random(1, 6)

Random numbers are used in games, simulations, shuffling, sampling and generating test data.

Key terms you must be able to define

Array

A data structure holding many values of the same type under one name, accessed by index.

Index

The position of an item within an array, counting from 0.

2D array

An array of arrays, forming a grid accessed by row and column.

Subroutine

A named block of code that performs a specific task and can be called from elsewhere.

Parameter

A variable listed in a subroutine definition that receives a value when it is called.

Argument

The actual value passed into a subroutine when it is called.

Local variable

A variable that only exists within the subroutine in which it is declared.

Global variable

A variable available to every part of the program.

Concatenation

Joining strings together end to end.

SQL

Structured Query Language, used to search and manipulate data in a database.

Append mode

Opening a file so that new data is added to the end without erasing existing content.

Flashcards

Say the definition out loud before you flip the card. Retrieving it from memory is what builds recall, reading it again is not.

Reach the top grade on this topic

This topic covers a lot of ground, so target the marks that are most reliably available.

SQL is nearly free marks. Three keywords, one structure. Learn SELECT ... FROM ... WHERE ... and practise ten queries. Quote text values, do not quote numbers, and only select the columns you were asked for.

File modes matter. If the question says add a record without losing existing data, the answer is append mode. Write mode erases the file.

Explain subroutines by benefit, not definition. "A subroutine avoids repeating the same code, so if the calculation changes it only has to be edited in one place, which reduces the chance of introducing errors."

Initialise from the data, not from zero. When finding a maximum or minimum in an array, start from the first element. This is a favourite exam trap.

• Write a SELECT query with a WHERE clause for any given table
• Read a file line by line and split each line into fields
• Write a function and call it, and explain why it is a function not a procedure
• Loop through a 2D array using row and column indexes in the right order

Mistakes that cost marks here