Programming constructs

Both lines are perfectly valid, so nothing is reported. The program runs to completion and simply produces the wrong answer.

The thing the loop's test depends on never changes — most often because the counter is increased inside a branch that does not always run, so the test can never become false.

Set up before the loop — totals at 0, counters at their start. Do the work for one item inside. Print or return the finished result after, since printing inside shows a running value rather than the final one.

Two parts of a program each hold a resource the other is waiting for, so neither can move. It is prevented by having every part claim shared resources in the same fixed order.

Is every value set before it is used? Does the thing the loop tests always change? Is the result used after it is complete rather than during? Those three catch nearly every sequence error.

A chain runs exactly one branch and skips the rest once a match is found. Separate ifs test every condition, so several may run or none. Use a chain for mutually exclusive cases and separate ifs for independent ones.

The chain stops at the first true test. If mark >= 50 is tested before mark >= 70, every mark of 70 or more matches the first test, so the later branch is unreachable.

and requires both sides to be true; or requires at least one; not reverses the result. They are the same logic as the AND, OR and NOT gates, with the same truth tables.

The second half is a bare piece of text rather than a comparison, and non-empty text counts as true — so the whole condition is always true. Each side of an or must be a full comparison.

== compares two values and produces a Boolean; = assigns a value to a variable. Writing = where == belongs is a classic error.

Ask whether you know how many times it must repeat before it starts. Yes means a counted loop (for); no means a conditional one (while), which repeats as long as a condition holds.

Five times, taking 0, 1, 2, 3 and 4. It starts at 0 and stops before 5 — the same rule as a string slice, and the same source of off-by-one errors.

Yes. The condition is tested before the first pass, so if it is already false the body never runs at all. That is correct behaviour and often exactly what is wanted.

Before the loop. Setting it inside resets it on every pass, so the final answer would be just the last value rather than the accumulated one.

The inner loop runs completely for every single pass of the outer loop, so the total work is rows times columns. That is where quadratic complexity comes from.

A named block of code that does one job, is given data through parameters, and hands back a result with return. Written once, it can be called anywhere and fixed in one place.

The parameter is the name in the function's definition — the placeholder. The argument is the actual value supplied when the function is called.

Reusable: one definition serves every caller. Maintainable: a fix applies everywhere at once and faults trace to one function. Testable: each function can be tested on its own. And work can be split between people.

A function told what it needs works anywhere, can be tested on its own, and cannot be broken by something changing elsewhere in the program.

A returned value can be stored, compared or displayed by the caller, and the function can be tested. A function that only prints can do none of those.