scanf

1. scanf • Reads in information from the keyboard • Examples • scanf(“%d”, &number); • scanf(“%d%d”, &value1, &value2); • WARNINGS! • Don’t forget the & (address of) in front of each variable! • Use the correct percent code for the type of the variable you are using!

2. /* Arup Guha My Second C Program, edited 9/2/03 Computes the number of feet user ran. */ #include <stdio.h> #define YARDS_IN_MILE 1760 #define FEET_IN_YARD 3 int main(void) { int feet_in_mile, num_miles; feet_in_mile = YARDS_IN_MILE*FEET_IN_YARD; printf("How many miles did you run?\n"); scanf("%d", &num_miles); printf("You ran %d feet.\n", feet_in_mile*num_miles); // system(“PAUSE”); Necessary to see screen in DevC++ return 0; }

3. Programming Style • Include ample white space • Indent code in between matching {} • Always write a header comment • Comment each major block of code • Use meaningful variable names • Define constants when appropriate • Be consistent with your style

4. Types of Errors • Compiler Error • Your program has incorrect syntax • Compiler outputs some message pertaining to the problem • Run-Time Error • Some statement in your program forces your computer to crash on an individual execution • Not all executions may create a run-time error • Logic Error • Program runs, but produces incorrect output sometimes.

5. Common Errors • Forgetting a matching double quote • Forgetting a semicolon • Using the incorrect character code in a printf or scanf • Forgetting & (address of) when reading into a variable • Not initializing variables • Incorrect order of assignment statements

6. Rules for Variable Names • Comprised of letters, digits, underscores • Can NOT start with a digit • Usually, variable names are case sensitive • Can NOT be a keyword (such as double) • Although it’s not a rule, variable names should reveal the purpose of the variable.

7. Why use constants? • Symbolic name (eg. WATER_DENSITY) has more meaning that the value (1). • If you ever have to change its value, you can change it in one place. • The compiler does NOT allow you to change its value DURING execution. • Two Different Schools of Thought • Some think constants should ONLY be used for things that never change (PI, E, etc.) • Others think constants can be used for values that might change sometime, but WON’T change within the execution of a program.

8. Increment/Decrement Operators • variable++ or ++variable • Equivalent to variable = variable+1; • There’s a subtle difference between the two forms. (We won’t go into it in this class.) • variable-- or –variable • Equivalent to variable = variable-1;

9. Other Shorthand Assignments • General Form <var> <op> <expr> • Short hand for <var> = <var> <op> <expr> • Examples • x += 10; • money -= 20; • value *= (rate+2); • portion /= (students+staff);

10. Pesky Detail about = • Can be used multiply • x = y = z – 4; is valid • Not good style. • If z were 7 before this statement executed • y would first get set to 3, and this operation would return 3 • Then, x would ALSO get set to 3, the return value of the original operation. • Thus, the associativity is right to left.

11. /* Arup Guha 8/29/07 Circle Program – edited to read in the radius of a circle and the cost for one square foot of it and print out the total cost of the whole circle. */ #include <stdio.h> #define PI 3.14159 int main(void) { double radius, area; double cost, totalcost; printf(“Enter the radius&cost per sqft of your circle.”); scanf(“%lf%lf”, &radius, &cost); area = PI*radius*radius; totalcost = cost*area; printf(“The total cost of the circle is $.2lf\n”, totalcost); // system(“PAUSE”); Necessary to see screen in DevC++ return 0; }