LECTURE 09:if STATEMENTS

PHY 107 – Programming For Science

Announcements

Need to learn concepts: no magic formulas exist Single solution not useful; will not see

problem again Examples from slides/web cannot just be

copied What does scanf do? When should it be

used? How to print out things? What does %lf

mean? Project #1 available later tonight; due

in 3 weeks You will complete 3 larger projects over term Tie everything together; more realistic

problems solved DO NOT LEAVE FOR LAST MOMENT

Today’s Goals

At end of lecture, be able to use if statements Choose what is executed so not just

calculator Understand source of many bugs & how to

avoid Correctly use braces ({ }) and make code

pretty Be able to explain when & why ifs

used

Boolean Values

3 boolean functions can combine into computer Billions of gates combined by engineers to

make CPU Could use, for example: (a && b) || (a && !c)

But how to generate inputs for these functions First need boolean values of true and false Easy in hardware (true is any signal > 3.3V) But how could we do get values within our

program?

Boolean Operators

&& (“AND”: true when both operands true)

|| (“OR”: true when either operand true)

^ (“XOR”: true when only one operand true)

! (“NOT”: true when operand false)

Relational Operators

< (less than) > (greater than) <= (less than of equal to) >= (greater than of equal to) != (inequality ≠) == (equality – if two things have same

value) NOT the same as assignment (=)

Relational Operators

< (less than) > (greater than) <= (less than of equal to) >= (greater than of equal to) != (inequality ≠) == (equality – if two things have same

value) NOT the same as assignment (=)

Relational Operators

< (less than) > (greater than) <= (less than of equal to) >= (greater than of equal to) != (inequality ≠) == (equality – if two things have same

value) NOT the same as assignment (=)

= assigns value== checks equalityC/C++ will NOT

correct your misuse

Code Structures in Programming Sequence

Code Structures in Programming Sequence

Decisiontrue

false

if (…) statement

1st evaluates boolean expression in parenthesis Executes next statement if expression is true

When expression is false, skips past statement(s)int num = -32;if (num < 0) { printf("Sorry Dave, I can't do that.\n");}if ((num < 100) && (num >= 0)) { printf(" ");}if (num >= 0) { printf("%lf=square root of %d", sqrt(num), num); }

if (…) statement

1st evaluates boolean expression in parenthesis

Executes next statement if expression is true When expression is false, skips over the

statement This leads to one very common bug

; is statement (a boring statement, but…)

if (dc > 20);printf("This line is always run\n");

I Want More!

Add opening brace ({) after closing parenthesis Can now write all statements to execute

Add closing brace (}) to show where if ends If expression false, execution restarts at

that point

if (month == 9 && day == 19) { printf("Yarrrr! Avast, matey!”); ninja = false;}

A Modest Proposal

if (a == b); Hanging semi-colon is a common bug

Also easy to miss when writing & debugging No harm is done by adding (unneeded)

braces A little extra typing is worst-case scenario

Use braces everywhere Prevents hanging semi-colon bug

A Modest Proposal

if (a == b); Hanging semi-colon is a common bug

Also easy to miss when writing & debugging No harm is done by adding (unneeded)

braces A little extra typing is worst-case scenario

Use braces everywhere Prevents hanging semi-colon bug Name variables stewardesses to balance

typing

Spacing in a Program

C/C++ ignores almost all spaces in a program This also means where most newlines

placed ignored Symbolic constants & text in quotes are

exceptions This can lead to some very… interesting

code

What Does This Compute? char _3141592654[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;*_3141592654=__31415=2,_3141592654[0][_3141592654 -1]=1[__3141]=5;__3_1415=1;do{_3_14159=_314=0,__31415++;for( _31415 =0;_31415<(3,14-4)*__31415;_31415++)_31415[_3141]=_314159[_31415]= -1;_3141[*_314159=_3_14159]=_314;_3_141=_3141592654+__3_1415;_3_1415=__3_1415 +__3141;for (_31415 = 3141-

__3_1415 ; _31415;_31415-- ,_3_141 ++, _3_1415++){_314 +=_314<<2 ; _314<<=1;_314+= *_3_1415;_31 =_314159+_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_14159 ++,++_3_14159))+ 3.1415926; } for ( _31415 = 1; _31415<3141- 1;_31415++)write( 31415% 314-( 3,14),_3141592654[ _31415 ] + "0123456789","314" [ 3]+1)-_314; puts((*_3141592654=0,_3141592654)) ;_314= *"3.141592";}

Indentation

Traditionally we indent code within braces ({})

Use consistent size to indent (I use 2 spaces)

Indentation

Traditionally we indent code within braces ({})

Use consistent size to indent (I use 2 spaces)

Nothing is

Indentation

Traditionally we indent code within braces ({})

Use consistent size to indent (I use 2 spaces)

Nothing ismore

Indentation

Traditionally we indent code within braces ({})

Use consistent size to indent (I use 2 spaces)

Nothing ismore

annoying

Indentation

Traditionally we indent code within braces ({})

Use consistent size to indent (I use 2 spaces)

Nothing ismore

annoyingthan

Indentation

Traditionally we indent code within braces ({})

Use consistent size to indent (I use 2 spaces)

Nothing ismore

annoyingthan

looking

Indentation

Traditionally we indent code within braces ({})

Use consistent size to indent (I use 2 spaces)

Nothing ismore

annoyingthan

looking

for

Indentation

Traditionally we indent code within braces ({})

Use consistent size to indent (I use 2 spaces)

Nothing ismore

annoyingthan

looking

forthe next

Indentation

Traditionally we indent code within braces ({})

Use consistent size to indent (I use 2 spaces)

Nothing ismore

annoyingthan

looking

forthe next

line.

Your Turn

Get in groups & work on following activity

For Next Lecture

Read pages 99 – 104 for Friday What if we want to have multiple possible

options? How do else & else if statements work?

We will have lab on Thursday

Week #3 weekly assignment due tomorrow If problem takes more than 10 minutes,

TALK TO ME!