Programming logic — variables, conditions, loops

Three concepts carry all of programming:

• Variable — a named box that can hold a value: score = 0.
• Condition — a question that is true or false: score > 10.
• Loop — do something several times: repeat 5 times.

To trace a program means to walk through it line by line, writing down what every variable contains. It is the single most important skill for finding faults.

total = 0
for i from 1 to 4:
    if i is even:
        total = total + i

The trace:

i	even?	total
1	no	0
2	yes	2
3	no	2
4	yes	6

Answer: 6. Draw the table every time you are unsure — the brain loves to skip steps.