Control Structures CPS120: Introduction to Computer Science Lecture 5

What Can a Program Do? A program can only instruct a computer to: –Read Input –Sequence –Calculate –Store data –Compare and branch –Iterate or Loop –Write Output

Fundamental Programming Concepts Assignment of values to a variable Iteration (Looping) –Over a set of set of statements –With respect to a logical expressions (conditions) Delegation of sub-tasks to functions / procedures

The Structure Theorem The Structure Theorem states that any algorithm can be built from three basic control structures. One-after-another (Sequence) Decision-making (Selection) –Making choices between 2 or more alternatives Repetition (Iteration) –Concerned with repetitive tasks (and the termination conditions of loops)

C++ Control Structures 1."Sequence statements" are imperatives 2."Selection" is the "if then else" statement –AND, OR, NOT and parentheses ( ) can be used for compound conditions 3."Iteration" is satisfied by a number of statements –"while" –" do " – "for" 4.The case-type statement is satisfied by the "switch" statement. –CASE statements are used for most non-trivial selection decisions

Sequence Control Structures Sequence control structures direct the order of program instructions. The fact that one instruction follows another—in sequence—establishes the control and order of operations.

Calculate A program can instruct a computer to perform mathematical operations. Add 1 to Counter

Store A program will often instruct a computer to store intermediate results. Place 1 in Counter

Compare and Branch A program can instruct a computer to compare two items and do something based on a match or mismatch which, in turn, redirect the sequence of programming instructions. –There are two forms: –IF-THEN –IF-THEN-ELSE

IF-THEN Test condition p falsetrue Entry Exit True statement a

IF-THEN-ELSE falsetrue Entry Exit Test condition p “true” statement a “false” statement a

Iterate A program loop is a form of iteration. A computer can be instructed to repeat instructions under certain conditions.

Iteration Control Structures Iteration control structures are looping mechanisms. Loops repeat an activity until stopped. The location of the stopping mechanism determines how the loop will work: Leading decisions Trailing decisions

Leading Decisions If the stop is at the beginning of the iteration, then the control is called a leading decision. The command DO WHILE performs the iteration and places the stop at the beginning.

DO WHILE Loop No Yes Entry Exit Test condition p Loop statement a

Trailing Decisions If the stop is at the end of the iteration, the control mechanism is called a trailing decision. The command DO UNTIL performs the iteration and puts the stop at the end of the loop.

DO UNTIL Loop Loop statement a NoYes Entry Test condition p Exit