Lecture 2b Dr. Robert D. Kent

 Structured program development  Program control

 Structured program development ◦ Sequential nature of instruction logic ◦ Non-sequential instruction logic ◦ Control structures  Repetition  Decision  Selection

 Previously, we discussed program design from the perspectives of ◦ Top-Down Bottom-Up Stepwise-Refinement  In practice, all of these techniques are used ◦ However, the Top-Down approach emphasizes the recognition of processing modules, or units of logic, that must always be processed as complete units  Sometimes these units may be single statements  Usually they are expressed as compound statements { } ◦ The goal of structured programming is to apply formal language control structures that determine the execution of processing units.

 At the level of the machine (CPU hardware) ◦ The RAM address of the next instruction to be executed is stored in a CPU register (storage unit) called the Program Counter (PC) ◦ Machine language instructions (encoded bit strings) are loaded from RAM, using the address stored in the PC, to an Instruction Register (IR) in the CPU ◦ While the IR is being decoded, the PC is incremented by the length (in bytes) of the current instruction.  This last step assumes that the next instruction (in sequence) automatically follows the current instruction in RAM. The DEFAULT mode of process execution is SEQUENTIAL. Statement_1 ; Statement_2 ;..... Statement_N-1 ; Statement_N ;

 Hardware designers have long understood the need to perform logical steps “out of order” ◦ This is called non-sequential logic ◦ Must be able to change the value stored in the CPU Program Counter (PC) register  Requires hardware instructions for this purpose  This is called branching logic

 Forward Branch ◦ if process cond FALSE TRUE TO DO or NOT TO DO ? That is the question!

 Forward Branch ◦ if else cond Tprocess FALSE TRUE Fprocess EITHER OR

 Backward Branch (Code repetition) ◦ Pre-Form: while / for cond process FALSE TRUE Do ONCE or REPEATEDLY

 Backward Branch (Code repetition) ◦ Post-Form: do while cond process FALSE TRUE

 In the C language, several formal control structures have been designed ◦ Repetition  while do-while for ◦ Decision  if if-else ◦ Selection  if-else if-else switch ◦ Each of these structures guide how the compiler generates proper machine language codes that preserve the semantics of the control logic

Multiple selection and For control structures

 Program control ◦ Multiple selection  If-else if-else  switch ◦ Repetition  do-while  for ◦ Break and Continue

 Multiple selection logic arises when a choice between more than two possible outcomes may happen  C provides two control structures to deal with these situations ◦ if-else if-else ◦ switch

 Problem: ◦ Part of a calculator program requires the user to input a value from 1 to 4 indicating his/her choice of the operation to perform on two values A and B (assume A, B already entered)  RESULT: C = A operation B ; ◦ The interpretation of the inputs is defined as  1- Add  2- Subtract  3- Multiply  4- Divide

 Solution using if-else if-else : ◦ printf ( “Enter operation code >” ) ; scanf ( “%d”, &Code ) ; if ( Code == 1 ) C = A + B ; else if ( Code == 2 ) C = A – B ; else if ( Code == 3 ) C = A * B ; else C = A / B ; A bit difficult to understand.

 Solution using if-else if-else : ◦ printf ( “Enter operation code >” ) ; scanf ( “%d”, &Code ) ; if ( Code == 1 ) C = A + B ; else if ( Code == 2 ) C = A – B ; else if ( Code == 3 ) C = A * B ; else C = A / B ; Much easier. REMEMBER ! Indentation is only for programmers, compilers do not understand indents.

 Solution using switch : ◦ printf ( “Enter operation code >” ) ; scanf ( “%d”, &Code ) ; switch ( Code ) { case 1 : C = A + B ; break ; case 2 : C = A – B ; break ; case 3 : C = A * B ; break ; case 4 : C = A / B ; break ; default : printf ( “Error in input\n” ) ; break ; }

 Solution using switch : ◦ printf ( “Enter operation code >” ) ; scanf ( “%d”, &Code ) ; switch ( Code ) { case 1 : C = A + B ; break ; case 2 : C = A – B ; break ; case 3 : C = A * B ; break ; case 4 : C = A / B ; break ; default : printf ( “Error in input\n” ) ; break ; }

 Repetition logic may be of two forms ◦ Pre-condition testing : enter, or re-enter, the loop body if the condition is true. ◦ Post-condition testing : enter the loop body in all cases (performing the body a minimum of once), then repeat the loop body only if the condition is true.  C supports three forms of repetition control structures ◦ while ◦ do-while ◦ for

 while ( condition_expression ) statement ;  while ( condition_expression ) { statement1 ; statementN ; }

 do statement ; while ( condition_expression ) ;  do { statement1 ; statementN ; } while ( condition_expression ) ;

 for ( init_stmt ; cond_expr ; update_stmt ) statement ;  for ( init_stmt ; cond_expr ; update_stmt ) { statement1 ; statementN ; }

 Example: Find the sum of all integers from 1 to 10.  int Sum = 0, k ; for ( k = 1 ; k <= 10 ; k++ ) Sum = Sum + k ;

 Example: Find the sum of all integers from 1 to 10.  int Sum, k ; for ( k = 1, Sum = 0 ; k <= 10 ; k++ ) Sum = Sum + k ;

 C defines two instruction statements that cause immediate, non-sequential alteration of normal sequential instruction processing  Break Logic ◦ Execution of a break ; statement at any location in a loop- structure causes immediate exit from the loop-structure  Continue Logic ◦ Execution of a continue ; statement at any location in a loop-structure causes execution to continue at the beginning of the loop structure (at the next loop iteration) while skipping the remaining statements.

 Break Logic ◦ Execution of a break ; statement at any location in a loop- structure causes immediate exit from the loop-structure  for ( k = 0 ; k < 10 ; k++ ) { if ( k == 5 ) break ; printf ( “%d, ”, k ) ; }  Produces output : 0, 1, 2, 3, 4

 Continue Logic ◦ Execution of a continue ; statement at any location in a loop-structure causes execution to continue at the beginning of the loop structure (at the next loop iteration) while skipping the remaining statements.  for ( k = 0 ; k < 5 ; k++ ) { if ( k == 3 ) continue ; printf ( “%d, ”, k ) ; }  Produces output : 0, 1, 2, 4

 Data ◦ Types, Declarations, Literal values ◦ Input/Output specification codes  Operators  Structured program development ◦ Selection / Decision control logic  If If – else  Switch  Control structures ◦ Repetition (Loop) control logic  Counters Sentinels ◦ While Do-While ◦ For

 Reading ◦ Chapters 1 – 4 (All sections) ◦ Midterm Examination 1 will focus on concepts and techniques from  Chapters 1-3, completely  Chapter 4.1 – 4.4, 4.7, 4.8  Some material presented in Lecture 2 slides may not be tested at this time, but will be tested later  Students are responsible for all material presented in the Lecture slides, as well as all assigned reading and Laboratory exercises and Examinations.