1. Department of Computer Science Western Michigan University
CS 1023 Intro to Engineering Computing 3: Computer Programming Conditions – Asking Questions if- and if-else-statements
Mark Kerstetter
Department of Computer Science
Western Michigan University
© Spring 2012

2. Conditions Generally used to control what to do next?
Expressions that evaluate to True or False
In C Programs conditions …
Follow keyword if
Follow keyword while
Contained within ( and )
Relational Operations: >=, >, = =, != , <, <=
Compare numeric expressions
Compare characters and character strings
Boolean (Logical) Operations: ! (not), && (and), | | (or)
Only used with logical expressions, i.e., those whose values are True or False
Never used with numeric or character expressions
Important!
= does not compare it assigns
= = compares
=> and =< and =! are not legal

3. Selection Statements if and if-else Decision Statements
Flow Control Statements – Making Decisions What statement(s) to perform next?
Two basic instructions – if… and if…else…
if-statement
if (condition) { statement(s) to execute if condition is true ; }
if-else-statements
if (condition) { statement(s) to execute if condition is true ; } else { statement(s) to execute if condition is false ; } ;
{ and } needed to surround multiple statements. Does no harm to use them for single statements.
Do not place semicolon after a condition. The if and if-else statements include the statements to do next! ;
Caution! A common mistake when writing if and if-else statements is to place a semicolon immediately at the end of the first line of the statement thus creating an empty statement.

4. Examples of if-statement
if ((nbr % 2) == 0 ) evenCount++;
totalNbrs = totalNbrs + 1;
Count Even Numbers
if ((nbr % 2) == 0 ) evenCount++;
else
oddCount++;
totalNbrs = totalNbrs + 1;
Count Even & Odd Numbers
if ( (side1 > 0) && (side2 > 0) ) {
area = side1 * side2;
perimiter = 2*(side1 + side2); }
else
{ printf(“Cannot compute area or perimeter.\n”); }
Multiple Instructions in Then-part

5. Examples of if-statement
Nested if-else-statements
if ((s1<s2+s3) && (s2<s1+s3) && (s3<s1+s2)) { printf(“legal triangle\n”) ; if ( (s1==s2) && (s2==s3) ) { printf(“equilateral triangle\n”) ; } else if (((s1==s2)&&(s1!=s3)) || (((s1==s3)&&(s1!=s2) || ((s2==s3)&&(s1!=s2))) printf(“isosceles triangle\n”) ; else printf(“scalene triangle\n”) ; } else printf(“not a triangle\n”) ;