Nested if Statements: Revision Notes (Computer Science A 2.4)

Recap

A nested if is an if, if-else or if-else-if inside another. A multiway selection (if-else-if) chooses one of several options.

Structure	How it runs
Nested if	Inner condition evaluated only if the outer condition lets execution reach it
if-else-if chain	Conditions tested top to bottom; the first true one runs; the rest are skipped
Trailing else	Runs only if every earlier condition was false; optional
Separate ifs	Every condition tested; several bodies can run

Key relationships

Idea	Rule
Bodies that can run in a chain	At most one
Reaching a later condition	All earlier conditions were false
Order of overlapping conditions	Most demanding (most specific) first
Dangling else	Pairs with the nearest unmatched if; braces fix it
Boundary values	Test the exact boundary of each band (`<=` versus `<`)
Chain versus nest	Same quantity in bands: chain. Dependent second question: nest

Assumptions

Conditions in a chain are Boolean expressions evaluated in written order.
Every path assigns a value to any variable used after the structure, or the code will not compile.
Braces are used around every body unless stated.

Mistakes to avoid

Writing the bands in the wrong order, so a low band catches high values.
Replacing else if by if, so later bodies overwrite earlier ones.
Reading an else as belonging to the outer if because of indentation.
Forgetting that a nested condition may never be evaluated.
Using < when the boundary belongs in the band (or the reverse).
Leaving off the final else and leaving a variable unassigned.

Worked mini-example

int t = 18;
String s;
if (t < 0) {
    s = "freezing";
} else if (t < 15) {
    s = "cold";
} else if (t < 25) {
    s = "mild";
} else {
    s = "hot";
}

Trace for t = 18: 18 < 0 false; 18 < 15 false; 18 < 25 true, so s = "mild". The hot branch is never reached. For t = 15 the answer is also “mild”, because 15 < 15 is false. For t = -4 the first test is true, so the later tests are never evaluated.

Quick self-check

In an if-else-if chain of four conditions, how many bodies can run? Answer: at most one.
If x = 10, what does if (x > 5) { r = 1; } else if (x > 8) { r = 2; } set r to? Answer: 1; the second condition is never tested.
When is the inner condition of a nested if evaluated? Answer: only when the outer condition allows execution to reach it.
In if (a) if (b) X; else Y; which if does the else belong to? Answer: the nearer one, if (b).

Full explanations and diagrams are in the Topic 2.4 study guide.