© Janice Regan, CMPT 102, Sept CMPT 102 Introduction to Scientific Computer Programming Conditional Statements Control Structures

© Janice Regan, CMPT 102, Sept Flowcharts  Flowcharts use some basic symbols  To start or end a function  To contain calculations  To make decisions  To connect different parts of an algorithm

© Janice Regan, CMPT 102, Sept Control Structures  Three methods of processing a program  In sequence  Branching  Looping  Branch: Altering the flow of program execution by making a selection or choice  Loop: Altering the flow of program execution by repetition of a particular block of statement(s)

© Janice Regan, CMPT 102, Sept Selection Based on Branching  One-Way Selection  One alternative: “Things to do” if a condition is true  C implementation: if statement  Two-Way Selection  Two alternatives: “Things to do” if the condition is true, a different “things to do” if the condition is false  C implementation: if-else statement  Compound Statements  Allows a group of statements to be considered as a block  That block can replace a single statement in any selection structure

© Janice Regan, CMPT 102, Sept Selection Based on Branching  Multiple Selections  A series of different “things to do” if a series of conditions hold  C implementation: if-elseif-else structure  Alternate C implementation: Nested if statements  Switch Structures  A series of different “things to do” depending on the value of a particular variable

© Janice Regan, CMPT 102, Sept Decisions  Use a decision statement when an action is to be taken only if a particular condition holds  The condition which must hold may be logical or relational expression or a Boolean variable. The value of the condition must be True or False  Each possible path through a condition statement will contain a sequence of steps to be executed  The condition and the sequences of steps that are executed for each outcome of the condition statement form a selection structure.  A selection structure is a type of control structure

© Janice Regan, CMPT 102, Sept Flowchart: one way selection  A simple decision uses a decision box to hold the condition (Boolean value: relational or logical expression)  The sequence of statements is held in a sequence box or boxes condition Statement 1; Statement n; T F ⋮

© Janice Regan, CMPT 102, Sept Example of one-way selection  if statements  An action is taken only if a particular condition is true. The action is described by a single C statement.  Example: if (examScore < 50) fprintf(“Printing address of failing student so they can be contacted to arrange tutoring: is %s”, Address);

© Janice Regan, CMPT 102, Sept Compound/Block Statement  Only one statement following the if statement is part of the if control structure  Must use a compound statement, implemented in C as { }, to include more than one statement in a list of things to do if the condition is true  A compond statement can also be called a "block" statement  You should always use a block statement { }  Even if the block contains just one statement  Using the { } if there is only one statement is a style decision, not a requirement of C

© Janice Regan, CMPT 102, Sept One way selection  Do a series of actions only if a given condition holds  If the condition does not hold skip the actions if ( condition ) { /* Series of actions to be taken */ /* when the condition is TRUE */ action 1; ⋮ action n; }

© Janice Regan, CMPT 102, Sept Compound Statement in Action  Note indenting in this example: if (myScore > yourScore) { printf(“My score was higher than yours “); difference = myScore – yourScore; printf(“I got %d more points than you did”, difference) }

© Janice Regan, CMPT 102, Sept Flowchart: two way selection  A selection structure uses a decision box, and sequence boxes. There may be multiple sequence boxes along each path condition Statement 1; Statement n; T F Statement 1; Statement n; ⋮ ⋮

© Janice Regan, CMPT 102, Sept Example of two-way selection  if-else statements  Choice of two alternate statements based on condition expression  Example: if (examScore > 50) myCourseGrade = “PASS”; else myCourseGrade = “FAIL”;

© Janice Regan, CMPT 102, Sept Compound/Block Statement  Only one statement following the if statement is part of the if control structure  Only one statement following the else statement is part of the control structure  Must use a compound statement { } to include more than one statement  Style Pointer: Each block (after the if and after the else) should have block statement even if the block contains just one statement

© Janice Regan, CMPT 102, Sept Two way selection  Complete one of two possible series of actions  First series of actions is complete if condition is true  Second series of actions is completed if condition is false if (condition) {//Series of actions to be taken when the condition is TRUE action 1; ⋮ action n; } else { // Series of actions to be taken when the condition is FALSE action 1; ⋮ action n; }

© Janice Regan, CMPT 102, Sept if-else Statement Syntax Selection structure for C  Formal syntax:  if ( ) else  Note each alternative is only ONE statement!  To have multiple statements execute in either branch  use compound statement

© Janice Regan, CMPT 102, Sept Flowchart for multiple selection condition2 Statement 1; Statement n; T F Statement 1; Statement n; condition F Statement 1; Statement n; T ⋮ ⋮ ⋮

© Janice Regan, CMPT 102, Sept Example of Multiple selection  if-else statements  Choice of two alternate statements based on condition expression  Example: if (examScore > 80) myCourseGrade = “A”; else if (examScore > 60) myCourseGrade = “C”; else myCourseGrade = “FAIL”;

© Janice Regan, CMPT 102, Sept Compound/Block Statement  Only one statement following the if statement is part of the if control structure  Only one statement following the else statement is part of the control structure  Only one statement following the else if statement of the control structrue  Must use a compound statement { } to include more than one statement  Style Pointer: Each block should have block statement even if it contains one statement

© Janice Regan, CMPT 102, Sept Multiple Selections (else if) if (condition1) {// Series of actions to be taken when condition 1 is TRUE action 1; action n; } else if (condition 2) { // actions to be taken when condition 1 is FALSE and condition2 is TRUE action 1; action n; } else { // Series of actions to be taken when condition 1and condition 2 are FALSE action 1; action n; }

© Janice Regan, CMPT 102, Sept Multiway if-else: Example  Not new, just different indenting  Avoids "excessive" indenting  Syntax:

© Janice Regan, CMPT 102, Sept CMPT 102 Programming Style  When using decision structures (if statements) you should always use the { } regardless of whether you use a single or multiple statements in each branch of the if statement.  The opening and closing brackets for the block should each be written on their own line. (examples follow)  WHY?  As your code evolves it is common to add statements (functionality) within a decision statement.  When you add statements and forget to add the {} to indicate the extent of the block unexpected things happen. The resulting problems can be difficult to find.

© Janice Regan, CMPT 102, Sept Defining the condition  Each decision statement is based upon a condition  The condition is an expression with a logical value (true or false)  The condition is a boolean expression and may be  A relational expression (a type of logical expression) Two numerical values combined using a binary relational operator (a simple relational expression) A more complex relational expression  Another type of logical expression Two logical values combined with a binary logical operator One logical value One logical value operated on by a unary logical operator A more complex logical expression

© Janice Regan, CMPT 102, Sept Binary Relational Operators in C  <less than  <=less than or equal to  > greater than  >= greater than or equal to  == equal to  !=not equal to  Evaluated left to right Binary Equality Operators in C

© Janice Regan, CMPT 102, Sept Binary Logical Operators  &&Logical AND  ||Logical OR  !Not  Evaluated left to right  Arguments of logical operators have values of true or false Unary Logical Operators

© Janice Regan, CMPT 102, Sept Truth Table && The && (And) operator EXPRESSION1 EXPRESSION2 EXPRESSION1 && EXPRESSION2 T T T T F F F T F F F F

© Janice Regan, CMPT 102, Sept Truth Tables || The || (Inclusive Or) operator EXPRESSION1 EXPRESSION2 EXPRESSION1 || EXPRESSION2 T TT T FT F TT F F F

© Janice Regan, CMPT 102, Sept Truth Tables ^ The ^ (Exclusive or) operator EXPRESSION1 EXPRESSION2 EXPRESSION1 ^ EXPRESSION2 T T F T F T F TT F FF

© Janice Regan, CMPT 102, Sept Truth Tables ! The ! (Not) operator EXPRESSION1 ! EXPRESSION1 T F F T

© Janice Regan, CMPT 102, Sept Precedence of operators in C  ( ) []. innermost first  (pre) + - ! ~(unary) (right to left)  * / %  + -  >=  == !=  &&  ||  = += -= *= /= %=(right to left)

© Janice Regan, CMPT 102, Sept Precedence Examples  Arithmetic before logical  x + 1 > 2 || x || ( x + 1) < -3  Short-circuit evaluation  (x >= 0) && (y > 1)  Be careful with increment operators! (x > 1) && x<y++  In C Boolean values (true and false) are represented as integers  All non-zero integer values  true  Zero value  false

© Janice Regan, CMPT 102, Sept Expressions with relational operators  Value of a relational expression (expression including a relational or binary equality operator) is true or false.  Arguments of a relational operator are numerical (or character)  A < C  (A + B) >= CLet A=9, B=5, C=2  A * B <= C  A % C == A % B  A != -C  A > B

© Janice Regan, CMPT 102, Sept Expressions: logical operators  Value of a logical expression (expression including a logical operator) is true or false.  Arguments of the logical operator are also true or false Let A=9, B=5, C=2 Then  (C = C  (A C  !(A < B) || B<A  A < B && B < C++

© Janice Regan, CMPT 102, Sept Common Pitfalls in C  Operator "=" vs. operator "=="  One means "assignment" (=)  One means "equality" (==)  These two operators are VERY different  Example of common error: if (x = 12)  Note operator used! Do_Something else Do_Something_Else