CS 331, Principles of Programming Languages

1. CS 331, Principles of Programming Languages Chapter 3

2. Structured Programming • Historical controversy over readability of computer programs • Undisciplined use of the infamous “goto” statement can lead to programs that are impossible to follow • Modern languages provide control structures that should make programs easier to read

3. Control Structures • Composition • <stmt>; <stmt> • Conditional • if <expr> then <stmt> else <stmt> • Iteration • while <expr> do <stmt>

4. Other Control Structures • Iteration (iterating at least once) • repeat <stmt> until <expr> • Definite Iteration • for <variable> = <expr> to <expr> do <stmt> • Multi-way selection • case <expr> of <label> : <stmt>; <label> <stmt> ;

5. Variations • In C (and C++) we have premature loop exit • break • continue • But composition, if, and while are basic • case can be simulated with nested if • repeat can be simulated with while and an extra boolean variable (how would that work?)

6. Invariants • Invariants are formal statements that describe the state of affairs at some point in your program • they should express your understanding of what’s going on in your program • Bugs live in those parts of your program that you don’t (yet) understand! • the C language assert macro does something like this, e.g. assert(ptr!=0);

7. Loop Invariants • A loop invariant is a logical statement that • is true before a loop starts, and • is true after the loop ends, • no matter how many times (0 or more) the loop is executed • To be useful, the loop invariant has to somehow capture the essense of what the loop is doing

8. Pseudo-code with invariants // see if a list of array elements is in ascending order Boolean testsorted (X[], int lower, int upper) assert ((X != 0) && (lower <= upper)) int I = lower // “is sorted” means that for any m,n in the given range, // m<n implies X[m] <= X[n] invariant (X[lower..I] is sorted) // loop invariant while (I < upper) do if X[I] > X[I+1] then the list isn’t sorted, return false I = I+1 invariant (X[lower..I] is still sorted) // so keep going end invariant (I == upper) and therefore X[lower..upper] is sorted return true

9. Functions and Invariants • Identify what a piece of code can assume • precondition • asserts at the top of a function can do this • and what it is supposed to do • postcondition • asserts at the end of a function (sanity check?) • and every few lines in between