CSC Programming for Science Lecture 10: Boolean Expressions & More If ­ Else Statements

Today’s Goal After today, should have better understanding of how to write if-else statements  Will finish this discussion on Friday

Goal When Writing Code Good programs work  Focus must be creating working algorithm  No code can overcome incorrect algorithm Always design solution on paper first  Start coding only after design is complete Quick poll: How many did this in lab?

Boolean Expression In C, any expression can be a boolean expression  Simply a matter of how expression is used  If value is 0, C treats it as false  If value is not zero, C treats it as true  What would the following expressions be if x = 3? x == 3 (x + 3) != ((5 * 2) – 4) !x x = 0 x = 4

Operators Most functions and all operators compute value  Result can be used, ignored, or assigned (saved)  Is not saved unless variable assigned its value Numeric operators are familiar & comfortable  +, -, /, *, %, =, --, ++, …  Depending on inputs compute a char, short, int, long, float, double, or long double Relational operators just another operator  ==, !=,, =  Result of these always smallest possible integer type

Operators Result Result of numeric operation well defined / 4 10 / 4 x = 4 sqrt(x) Result of relational operator not as defined  Follows simple rules, but true not just 1 value 7 <= 5 (10 / 4) == 2 pow(9, 0.5) != 0.5

Boolean Operators Like relational operators, compute integer  Operands do not have to be boolean Boolean is NOT a datatype in C!  But operand handled with C rules of boolean

Boolean Operators &&  AND  Result is “true” if both operands is “true” ||  OR  Result is “true” if either operand is “true” !  NOT  Result is “true” when operand is “false”  So if operand is zero, result is not zero  And if operand is not-zero, result is zero

Spacing in a Program C ignores almost all spaces in a program  Includes where newlines are included  The few exceptions: pre-processor statements on own line text in quotes must be on one line This leads to some very… interesting code

Any Guesses? char _ [3141 ],__3141[3141];_314159[31415],_3141[31415];main(){register char* _3_141,*_3_1415, *_3__1415; register int _314,_31415,__31415,*_31, _3_14159,__3_1415;*_ =__31415=2,_ [0][_ ]=1[__3141]=5;__3_1415=1;do{_3_14159=_314=0,__ ;for( _31415 =0;_31415<(3,14-4)*__31415;_ )_31415[_3141]=_314159[_31415]= - 1;_3141[*_314159=_3_14159]=_314;_3_141=_ __3_1415;_3_1415= __3_1415 +__3141;for(_31415 = __3_1415 ;_31415;_ ,_3_141 ++,_3_1415++){_314 +=_314<<2 ;_314<<=1;_314+= *_3_1415;_31 =_ _314; if(!(*_31+1) )* _31 =_314 / __31415,_314 [_3141]=_314 % __31415 ;* ( _3__1415=_3_141 )+= *_3_1415 = *_31;while(* _3__1415 >= 31415/3141 ) * _3__1415+= - 10,(*--_3__1415 )++;_314=_314 [_3141]; if ( ! _3_14159 && * _3_1415)_3_14159 =1,__3_1415 = 3141-_31415;}if( _314+(__31415 >>1)>=__31415 ) while ( ++ * _3_141==3141/314 )*_3_141--=0 ;}while(_3_14159 ) ; { char * __3_14= "3.1415"; write((3,1) (--*__3_14,__3_14 ),(_3_ ,++_3_14159)) ; } for ( _31415 = 1; _31415< ;_ )write( 31415% 314-( 3,14),_ [ _31415 ] + " ","314" [ 3]+1)-_314; puts((*_ =0,_ )) ;_314= *" ";}

Indentation Traditionally indent code within set of braces ({}) Use consistent indentation size (I use 3 space) Nothing is more annoying than looking for the next line.

If statement First evaluates expression in parenthesis If value of expression is “true”, executes next statement  If expression “false”, execution continues after statement Very common bug  ; is statement (a boring statement, but…) if (a == b) ; printf(“A is equal to B\n”);

Longer Statements What if more than 1 statement needed? Add opening brace (“{“) after expression  Indent all statements you want to include Add closing brace (“}”) after statements to be run when expression is true if (sqrt(x) == 3.0) { printf(“The value of x is 9.0 or %lf\n”, x); printf(“Square root of x is 3.0 or %lf\n”, sqrt(x)); }

A Modest Proposal if (a == b); Hanging semi-colon is a common bug  Also very easy to miss when debugging Adding (unneeded) braces does no harm  Worst-case scenario: a little extra typing Use braces on all if – else statement  Prevent hanging semi-colon bug  Name more variables stewardesses to make up for extra typing

else statement May want to perform one of two actions Add else clause after if  Value of expression is “true”, execute if statements  Otherwise, execute statements in else if (boolean expression) { statement;... } else { statement;... } Execution returns as normal once finished

Handling Multiple Options Sometimes want more than two options Add “else if” after if & before else if (a > 3) { printf(“a = %d & greater than 3\n”, a); } else if (b > 3) { printf(“b = %d & greater than 3\n”, b); } else if (c > 3) { printf(“c = %d & greater than 3\n”, c); } else { printf(“Is everything less than 3?\n”); }

if/else if/else Statements First evaluate if expression and when “true” execute if statements  Otherwise evaluate first else if expression and when “true” execute its statements  Otherwise evaluate next else if expression and when “true” execute statements  If no expression is “true” execute else statements Include as many else if as you want else is optional

Your Turn Get into groups and complete daily activity

For Next Lecture Read Sections 3.1 – 3.3 of book  Do not need to understand all the details  But important knowing what is not understood Continue week #4 weekly assignment Get lab #3 up and running First programming assignment on web  Due 2 weeks from this Friday