Making Choices in C if/else statement logical operators break and continue statements switch statement the conditional operator

The if Statement Format if ( condition ) program statement; or if ( condition ) { program statements; } Flow Condition true? Program statements Yes No

Example Calculating the absolute value of an integer number < 0 number = - number; Yes No printf(“type in your number: “); scanf(“%i”, &number); if ( number < 0 ) number = -number; printf(“\nThe absolute value is %i\n”, number);

The if-else Statement Format if ( condition ) program statement 1; else program statement 2; Flow condition true? program statement 1 Yes No program statement 2

The else if Statement Format if ( condition1 ) program statement 1; else if (condition2) program statement 2; Flow condition1 true? program statement 1 Yes No condition2 true? No program statement 2 Yes

One example if (condition1) program statement 1; else if (condition2) program statement 2; else program statement 3; condition1 true? program statement 1 Yes No condition2 true? No program statement 3 program statement 2 Yes

Boolean Variables Declaration –Boolean variables are represented by integers Value –0 represents false –1 representing true, mostly Any nonzero value represents true in C

Logical Operators Why –Make a decision based on multiple conditions What are they OperatorExampleDescription ||x widthlogical OR &&x >= 0 && x <= widthlogical AND !!(x < 0)logical NOT

Logical OR Returns false only if both expressions are false Example: for (i=5 ; i<=95; i+=5) { if((i 60)) { printf(“%i”, i); } ABA || B True FalseTrue FalseTrue False

Logical AND Returns true only if both expressions are true Example: for (i=5 ; i<=95; i+=5) { if((i > 35) && (i < 60)) { printf(“%i”, i); } ABA && B True False TrueFalse

Logical NOT Inverts the Boolean value of an expression Example: int a = 0; if (!a) { printf(“a is not 0\n”); } a = 1; if (a) { printf(“a is 1\n”); } A!A TrueFalse True

Conditional Expressions Relational expression using ==, !=, >, >=, <, <= –The value of “relational expression” is 1 if the relation is true, and 0 if false. Operators can be combined with logical operators (with decreasing precedence): –! (NOT) –&& (AND) –|| (OR) Rules: –&& and || are evaluated left to right Example: c==' ' || c=='\t' || c=='\n'

Example: Giving Final Grade A(100-90), B(89-80),C(79-70),D(69-60),F(59-0) if (score >= 90) grade = ‘A’; else if (score >= 80) grade = ‘B’; else if (score >= 70) grade = ‘C’; else if (score >= 60) grade = ‘D’; else grade = ‘F’;

Evaluate a simple expression Objective: –Evaluate expression and display results with input in the form: number operator number Example –User input: –Program output: 15

Program #include int main(void) { float v1, v2; char operator; printf("Type in your expression: \n"); scanf("%f %c %f", &v1, &operator, &v2); if ( operator == '+' ) printf("%.2f\n", v1 + v2); else if ( operator == '-' ) printf("%.2f\n", v1 - v2); else if ( operator == '*' ) printf("%.2f\n", v1 * v2); else if ( operator == '/' ) printf("%.2f\n", v1 / v2); return 0; } Need to check v2 == 0?

Corrections Change else if ( operator == '/' ) printf("%.2f\n", v1 / v2); to else if ( operator == '/' ) if ( v2 == 0 ) printf(“Division by zero.\n”); else printf("%.2f\n", v1 / v2);

Watch Out for This!!! The assignment operator “=“ and the equality operator “==“ ARE VERY DIFFERENT, DO NOT CONFUSE THEM! This code compiles and runs, but does not do what you may expect: if (x = 0) { x = 1; } printf(“x = %i\n,x);

How to get out of a loop prematurely? Use the break statement –Skips all subsequent statements in the loop –Exits immediately from the current loop –Continues execution after the loop Or the continue statement –Skip subsequent statements in the current iteration –Continue execution with the next iteration

Example – summation of valid scores int count, n = 10, sum = 0, score; for (count = 0; count < n; count++){ scanf("%d", &score); if (score 100) continue; sum = sum + score; }

Example – summation of valid scores int count, n = 10, sum = 0, score; for (count = 0; count < n, count++) { scanf("%d", &score); if (score 100) { printf(“Bad data. Exiting\n”); break; } sum = sum + score; }

The switch Statement When to use –The value of a variable successively compared against different values Format switch( expression ) { case value 1: program statement 1; break; case value 2: program statement 2; break; ׃ case value n: program statement n; break; default : program statement n+1; break; } == value 1 evaluate expression statement 1 Y N == value 2 == value n N N statement n+1 Y statement 2 Y statement n

Example switch ( operator ) { case '+': printf("%.2f\n", v1 + v2); break; case '-': printf("%.2f\n", v1 - v2); break; case '*': printf("%.2f\n", v1 * v2); break; case '/': if ( v2 == 0 ) printf(“Division by zero.\n”); else printf("%.2f\n", v1 / v2); break; default: printf("Unknown operator!\n"); break; }

Another example switch (month) { case 1: printf("January"); break; case 2:printf("February"); break; case 3: printf("March"); break; case 4: printf("April"); break; case 5:printf("May"); break; case 6:printf("June"); break; case 7:printf("July"); break; case 8:printf("August"); break; case 9:printf("September"); break; case 10:printf("October"); break; case 11:printf("November"); break; case 12:printf("December"); break; default:printf("Invalid month.");break; }

More on switch Statement case value 1: statement 1; case value 2: statement 2; break ; == value 1 statement 1 Y N == value 2 N Y statement 2 Usually, this is a BUG! And one that is often very hard to find! “MISSING BREAK”

The conditional operator Format: condition ? expression1 : expression2 Same as if ( condition ) { expression1; } else { expression2; } Except: the “?” can be used in an expression.