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Decision Making EE2372 Software Design I Dr. Gerardo Rosiles

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Decision Making In addition to looping, the ability to make decisions is an important and powerful feature in programming languages. Decision making elements emulate our thought processes and logic. We saw that there is a special symbol for decisions when using flowcharts. Decision elements allow us to make comparisons, check if a condition is met, or if a group of conditions are met. In C-language we have three types of decision making elements: – if statements – switch statements – The conditional operator

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if statements Similar to the MATLAB if stament Syntax if (expression) program statement Syntax with block of statements if (expression) { program statements } If the expression is TRUE, the program statement or program statements are executed, otherwise they are skipped.

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if statements The expression can be any valid C r-value expression that evaluates to a numerical solution. In C-language – an expression evaluating to zero is defined as FALSE. – an expression evaluating to non-zero is defined as TRUE. Example: int x = 5, y; y = 5; if (x-y){ //x-y = 0 therefore FALSE, program does not enter here printf(“x and y are different”); //etc. }

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if statement expressions As we discussed before, we can form expressions using the arithmetic, relational and logic operators. – Arithmetic: +, -, *, / – Relational: >, >=, <, <=, ==, != – Logical: &&, ||, ! We can form very complex expression which can be confusing: A && B >= 10 || X+Y/2 – 5 != Z && D Which operation goes first? Table A.5 in your book (pg. 440) summarizes the operator precedence. So you can figure it out… However, the intended order may not match the precedence defined by C. The most appropriate approach is to make generous use of parenthesis. A && (B >= 10) || ( (X+Y/2 – 5) != Z) && D Don’t be shy! Others will thank you for showing good programming habits.

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Some variants: if-else In many cases, we will have an alternative to the if statement expression. Hence we can use the if-else construct. if (expression) { program statements } else { alternative program statements }

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Some variants Example: int x = 5, y; y = 5; if (x-y){ //x-y = 0 therefore FALSE, program does not enter // here printf(“x and y are different”); //etc. } else { // the alternative gets executed printf(“x and y are different”); }

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Some variants: nested if statements We can always have multiple if statement nested if ( condition1 is TRUE) if (condition2 is TRUE) if (condition3 is TRUE) { DO SOMETHING} A more practical statement is build using if-else: if (x > 0) if (x > 20 && y != x) { y = x; x = 2*x; } else { x = 20; } We have an if statement … followed by an if-else construct. The else belongs to the inner if OR most recent if.

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Some variants: nested if statements If we want to form an if-else construct with the first if, we need to use brackets to form the correct association : if (x > 0) { if (x > 20 && y != x) { y = x; x = 2*x; } else { x = 20; } … with an if statement inside the if- else construct We have an “outer” if-else construct… BE VERY CAREFUL WITH YOUR INTERPRETATION OF THE IF-ELSE STATEMENTS

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More interesting: the else-if construct Suppose we want to test the value of x against more than two conditions. Each condition is multiple exclusive. We can build a nested if-else construct as follows: if (x <= -10) y = -10; else if ( (x > -10) && (x <= 5) ) y = -5; else if ( (x > -5) && (x < 5) ) y = 0; else if ( (x >= 5) && (x < 10) ) y = 5; else y = 10;

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The switch statement In many cases, the nested if-else statements can be expressed in terms of a switch statement. An expression produces different n values. For each value there is a different case with a specific set of statements. A break statement is included at the end of each case to exit the switch structure. A default case is included for any other values with no specific case. The default case is optional. switch (expression) { case value1: program statement(s); break; case value2: program statement(s); break;... case valuen: program statement(s); break; default: program statement(s); break; }

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switch statement What happens if no break statements are included? If the break statement is not included, the statement the program falls through the next case. This is maybe useful in some cases, but in general you want to break at the end of each case. switch (expression) { case value1: program statement(s); case value2: program statement(s); break; case value3: program statement(s); break; default: program statement(s); break; } expression == value 1 no break statement break terminates switch

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Conditional operator A very particular operator from C-language. A ternary operator implies it uses three expressions and returns a value. Syntax: condition ? expression1 : expression2 If condition is TRUE, evaluate expression1. If condition is FALSE, evaluate expression2.

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Conditional operator - Examples s = (x<0) ? -1 : x*x; minimum min_value = (a < b) ? a : b; sign (nested conditional operators) sign =(num 0) ? 1:0 ) Not a smiley face

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