Selection Statements

Selection Statements • Selects statements to execute based on the value of an expression • The expression is sometimes called the controlling expression • Selection statements: • if statement • switch statement

Selection statements: if • used to execute conditionally a statement or block of code. if (expression) statement • If expression is true, statement is executed (what is true?). • statement can be replaced by a block of statements, enclosed in curly braces.

An example /* This program displays the absolute value of a number given by the user */ #include <stdio.h> int main(void) { double num; printf("Please enter a real number: "); scanf("%lf", &num); if (num<0) num = -num; printf("The absolute value is %g\n", num); return 0; }

If-else statement if (expression) statement1 else statement2 • if expression is true, statement1 is executed. • if expression is false, statement2 is executed • both statements can be (and very often are) replaced by blocks of statements (“compound statements”)

An example (fragment) int first, second, min; /* … */ if (first < second) { min = first; printf ("The first number is smaller than the second.\n"); } else { min = second; printf ("The second number is smaller than the first\n"); } printf("The smaller number is equal to %d\n", min);

True or false • In C, every expression has a numeric value • An expression is ‘true’ when its non-zero • If it is zero, it is false • Therefore, in the following – if (expression) statementstatement is executed if expression is non zero.

More about operators • In C, every expression has a numeric value • When using arithmetical operators (+, -, *, /) this is straight forward • The value of A+B is the sum of A and B • And so on…

More about operators • Expressions with relational operators (<, <=, etc’) have values as well (intuitively, we are used to thinking about them as ‘true’ or ‘false’) • A < B equals zero if A is larger than or equal to B (false), and some non-zero value if A is smaller than B (true) • The exact non-zero value varies (and is not important for that matter)

Relational operators • They are – • A == B (Note the difference from A = B) • A != B • A < B • A > B • A <= B • A >= B • The value of the expression is non-zero if it’s true, zero if it’s false

An example int a, b; printf("Enter two Numbers\n"); scanf("%d%d", &a, &b); if (a == b) { printf("The numbers equal %d\n", a); printf("The expression a == b is %d\n", a == b); } else { printf("The numbers are not equal\n"); printf("The expression a == b is %d\n", a == b); }

The assignment operator = • The assignment operator is also an operator. Hence, expressions involving it have a numeric value • This value equals to whatever appears on the right of the assignment operator • For example – • (x = 4) equals 4 • (y = 0) equals 0

A very common mistake • Very often a programmer might confuse between the equality operator and the assignment operator - • if (x==4) … • if (x=4) … • The second is usually a mistake, but legal in C so the compiler doesn’t call it!

Examples • val.c, eqn_sign.c

Logical operators • Allows to evaluate two or more expressions - • !A – ‘not’ - True when A is not, and vice versa. • A && B – ‘and’ - True when both A and B are true • A || B – ‘or’ (inclusive or) - True when either A or B (or both) are true

A silly example #include <stdio.h> int main(void) { int grade; printf("Please enter your grade: "); scanf("%d", &grade); if (grade<0 || grade>100) printf("This is not a valid grade!\n"); else printf("This is indeed a grade.\n"); return 0; }

Else-if • if statements distinguish between exactly 2 cases and execute different code in each case • The else-if construction allows for a multi-way decision

Else-if if (expression) statement else if (expression) statement elseif (expression) statement else statement

An example if (grade >= 90) printf ("A\n"); else if (grade >= 80) printf ("B\n"); else if (grade >= 70) printf ("C\n"); else if (grade >= 60) printf ("D\n"); else printf ("F\n");

Validating input • When getting input from the user, it is highly recommended to check whether it is valid. • If it’s not, you should display an appropriate message and return a non-zero value. • For example – if (grade<0 || grade>100) { printf(“Invalid input!\n”); return 1; }

The return keyword • For now, used to terminate the program and return a value to the operating system • If the program is successful the return value should be zero, non-zero otherwise • The exact nature of this keyword will become clear in the future

Exercise • Input – • An English letter • Output – • If input is a lowercase letter – the corresponding uppercase letter • If input is an uppercase letter - corresponding lowercase letter • Note – • Remember to check for input validity!

Solution • switch_case.c

Exercise (difficult!) • Write a program such that – • Input – a 3-digit number • Output – the same number with digits sorted • Example – if input is 132, output should be 123 • Note – if input is not a 3-digit number, display an error message and exit!

Solution • Sort_digits.c

Loops • Used to repeat the same instruction(s) over and over again. • C provides some flexible ways of deciding how many times to loop, or when to exit a loop. • for, while, do-while loops.

While loops while (condition) { statement(s); } The statements are executed as long as condition is true When the condition is no longer true, the loop is exited.

Example - factorial #include <stdio.h> int main(void) { int i,n,fact = 1; printf("Enter a number\n"); scanf("%d", &n); i=1; while (i<=n) { fact = fact*i; i++; } printf("the factorial is %d\n", fact); return 0; } This is a counter Every iteration i is incremented by 1. Equivalent to i=i+1.

Example – fibonacci series fibonacci.c

Fibonacci – step by step fib1 fib2 fib_next lim fib1 = 0; fib2 = 1; printf("%d ", fib1); while(fib2 < lim) { printf("%d ", fib2); fib_next = fib1 + fib2; fib1 = fib2; fib2 = fib_next; } printf("\n"); 0 1 --- 5 Screen 0

Fibonacci – step by step fib1 fib2 fib_next lim fib1 = 0; fib2 = 1; printf("%d ", fib1); while(fib2 < lim) { printf("%d ", fib2); fib_next = fib1 + fib2; fib1 = fib2; fib2 = fib_next; } printf("\n"); 0 1 --- 5 Screen 0 1

Fibonacci – step by step fib1 fib2 fib_next lim fib1 = 0; fib2 = 1; printf("%d ", fib1); while(fib2 < lim) { printf("%d ", fib2); fib_next = fib1 + fib2; fib1 = fib2; fib2 = fib_next; } printf("\n"); 0 1 1 5 Screen 0 1

Fibonacci – step by step fib1 fib2 fib_next lim fib1 = 0; fib2 = 1; printf("%d ", fib1); while(fib2 < lim) { printf("%d ", fib2); fib_next = fib1 + fib2; fib1 = fib2; fib2 = fib_next; } printf("\n"); 1 1 1 5 Screen 0 1

Fibonacci – step by step fib1 fib2 fib_next lim fib1 = 0; fib2 = 1; printf("%d ", fib1); while(fib2 < lim) { printf("%d ", fib2); fib_next = fib1 + fib2; fib1 = fib2; fib2 = fib_next; } printf("\n"); 1 1 1 5 Screen 0 1 1

Fibonacci – step by step fib1 fib2 fib_next lim fib1 = 0; fib2 = 1; printf("%d ", fib1); while(fib2 < lim) { printf("%d ", fib2); fib_next = fib1 + fib2; fib1 = fib2; fib2 = fib_next; } printf("\n"); 1 2 2 5 Screen 0 1 1 2

Fibonacci – step by step fib1 fib2 fib_next lim fib1 = 0; fib2 = 1; printf("%d ", fib1); while(fib2 < lim) { printf("%d ", fib2); fib_next = fib1 + fib2; fib1 = fib2; fib2 = fib_next; } printf("\n"); 2 3 3 5 Screen 0 1 1 2 3

Selection Statements