Programming C for Engineers • An exercise is posted on the web site! • Due in one week • Single submission
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() { 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 value is non-zero • If it is zero, it is false • Therefore, in the following – if (expression) statement statement is executed if expression is non zero.
More about operators • In C, every expression has a numeric value • When using arithmetical operators (+, -, *, /) this is straightforward • 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 evaluates to zero if A is larger than or equal to B, and some non-zero value if A is smaller than B • 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) evaluates to 4 • (y = 0) evaluates to 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 warn us about 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
The ?: operator • expr1 ? expr2 : expr3 • Nicer way to write: • (expr1)? expr2 : expr3 • If expr1 is true (non-zero), expr2 is evaluated. Otherwise, expr3 is evaluated
The ?: operator #include <stdio.h> int main() { int i, j, min; printf("Please enter two numbers: "); scanf("%d%d", &i, &j); min = (i < j)? i : j; printf("The minimum between %d and %d is %d\n", i, j, min); return 0; }
The switch statement • a multiway conditional statement • similar to if-else if-else • allows the selection of an arbitrary number of choices based on an integer value • switch (expression) {case const-expr: statementscase const-expr: statements…default:statements}
The switch statement • expression must have an integer value • when the switch statement is executed: • the expression is evaluated • if a case matches the value of the expression, the program jumps to the first statement after that case label • otherwise, the default case is selected • the default is optional
That grade example again switch (grade/10) { case 10: case 9: printf ("A\n"); break; case 8: printf ("B\n"); break; case 7: printf ("C\n"); break; case 6: printf ("D\n"); break; default: printf ("F\n"); }
Give me a break • when the switch transfers to the chosen case, it starts executing statements at that point • it will “fall through” to the next case unless you “break out” • break causes the program to immediately jump to the next statement after the switch statement
A more interesting example • operation.c
Exercise • Write a program that accepts a number between 1 and 100 from the user. If there is a coin of that value in cents, it should display its name. Otherwise, it should report that there is no such coin • 1 = cent, 5 = nickel, 10 = dime, 25 = quarter, 100 = dollar • Remember to check for the validity of the input!
Solution • coins.c
Debugging • It is virtually impossible to program without errors • Syntax errors are detected by the compiler • However, often a program has no syntax errors and compiles, but still doesn’t perform as desired
Debugging • Debuggers are software tools designed to help find software bugs • Both Visual C and the lcc compiler include a debugger
Debugging • The debugger allows us to – • Execute the program one line at a time • At each step see the values of all variables and expressions • Run the program up to a pre-specified point • And more…
