Programming basics

Scratch and Python do the same jobs. In Scratch you snap blocks. In Python you type the words. The ideas are identical; only the spelling changes.

Sequence: top to bottom

print("Hello")
print("Welcome to Computing")

Line 1 runs, then line 2. That's sequence. print() shows what's inside the brackets. Every program uses sequence, selection and iteration: you'll meet all three on this page.

The exam boards write it differently again

A variable is a named box in memory that holds one value. The name stays the same. The value inside can change while the program runs.

1. score = 0: make a box called score, put 0 in it. This is assignment.

2. score = score + 10: read the right side first. 0 + 10 = 10. Put 10 back in the box.

3. print(score) shows 10. No quotes: it shows what's in the box, not the word "score".

Every value has a type. The type decides what you can do with it. 7 + 7 adds. "7" + "7" joins.

Input and casting

name = input("What is your name? ")
age = int(input("How old are you? "))
print("Hi " + name + ", next year you'll be " + str(age + 1))

1. input() always gives back a string, even if you type digits. Type 13 and you get "13".

2. int() turns "13" into 13 so you can do maths. Changing type is casting. float("2.5") and str(14) are casting too.

3. str() turns 14 back into text so it can be joined to other text with +.

age = input("Age? ")   # you type 13
print(age + 1)

age = int(input("Age? "))   # 13
print(age + 1)

Python does maths with operators. Most are like a calculator. Two are new and very useful: // (DIV) and % (MOD).

By hand: 17 DIV 5 and 17 MOD 5

1. How many 5s fit into 17? Count up in 5s: 5, 10, 15. The next one, 20, is too big.

2. Three 5s fit. So 17 // 5 = 3. That's DIV.

3. Three 5s make 15. What's left? 17 − 15 = 2. So 17 % 5 = 2. That's MOD.

4. Check: 3 × 5 + 2 = 15 + 2 = 17 ✓. The remainder (2) is always smaller than what you divide by (5).

The even/odd test

n = int(input("Number: "))
if n % 2 == 0:
    print("even")
else:
    print("odd")

Even numbers split into 2 with nothing left, so n % 2 is 0. Odd numbers always leave 1.

A comparison asks a question. The answer is always True or False: a Boolean. Every if and every while runs on one.

Join questions with and, or, not. These are the logic gates from Topic 9.

1. The code: age = 13, then if age >= 11 and age <= 14:

2. Swap in the value: 13 >= 11 and 13 <= 14.

3. Work out each side: True and True.

4. and needs both sides True. They are, so the answer is True: age 13 is in KS3.

Selection lets the program choose a path. Python checks each condition from the top. The first True one wins, and the rest are skipped.

score = int(input("Score out of 50: "))
if score >= 40:
    print("Gold badge!")
elif score >= 25:
    print("Bronze badge")
else:
    print("Try the Respawn quest")

1. Check the if. True? Run its indented lines and skip the rest.

2. False? Check the elif ("else if"). You can have as many elifs as you like.

3. Nothing true? Run the else. It has no condition: it catches everything left.

A for loop repeats a set number of times. It's a count-controlled loop: Scratch's "repeat 5". range() makes the list of numbers to count through.

for i in range(5):
    print("Lap", i)

range(5) gives 0, 1, 2, 3, 4: it starts at 0 and stops before 5. Five numbers, so five laps. Each lap, i takes the next number.

A while loop repeats as long as a condition is True. It's a condition-controlled loop. Use it when you don't know how many laps you'll need.

count = 3
while count > 0:
    print(count)
    count = count - 1
print("Lift off!")

A loop that waits for the right answer

password = ""
while password != "dragon":
    password = input("Password: ")
print("Access granted")

Repeats while the condition is True. As soon as the user types dragon, password != "dragon" is False and the loop ends. It might run once, or a hundred times.

A string is a row of characters. Each character has a numbered position called an index. The first index is 0.

name = "Sofia"
for letter in name:
    print(letter)

A for loop can walk along a string one character at a time. Same loop, different thing to count through.

Every programmer makes errors every day. The skill is knowing which kind you have, because each kind shows up at a different time.

Good programmers run code in their head before running it on the computer. Step through these programs. Watch which line runs, what's in each box, and what appears on screen.

The traffic-light build

The crossing-button traffic light uses everything on this page in one real program: variables (how many crossings), selection (was the button pressed?), iteration (the endless main loop, and a countdown).

# The same logic as the Arduino build, in Python
crossings = 0
while True:                      # iteration: for ever
    if button_pressed():         # selection
        crossings = crossings + 1    # variable
        lights("amber")
        lights("red")
        for s in range(5, 0, -1):    # countdown 5..1
            print(s)
    else:
        lights("green")

The mistakes nearly every beginner makes. Read the left side, then the right.

Quick questions first. Then endless practice: a wrong answer shows you the working, step by step.