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Control Structures

Learn about the fundamental control structures in programming, including sequencing, decision-making, and iteration. Understand how these structures allow programs to read input, calculate, store data, compare and branch, and write output.

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Control Structures

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  1. Control Structures CPS120: Introduction to Computer Science Lecture 5

  2. 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

  3. 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

  4. 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)

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

  6. 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.

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

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

  9. 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

  10. Entry false true Exit True statement a IF-THEN Test condition p

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

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

  13. 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

  14. 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.

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

  16. 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.

  17. Entry Exit No Yes Test condition p DO UNTIL Loop Loop statement a

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