8 Displaying Prompts • When input data is needed in an interactive program, you should use the printf function to display a prompting message , or prompt , that tells the user what data to enter. Printf(“Enter the distance in miles> “);
9 The scanf Function scanf(“%lf”, &miles); function name function arguments format string variable list place holder • When user inputs a value, it is stored in variable miles . • The placeholder type tells the function what kind of data to store into variable miles. • The & is the C address of operator. The & operator in front of variable miles tells the scanf function the location of variable miles in memory.
10 Fig 2.6: Scanning data line Bob char letter_1, letter_2, letter_3; …. Scanf(“%c%c%c”, &letter_1, &letter_2, &letter_3);
11 Assignment Statements • Assignment statement - Stores a value or a computational result in a variable kms = KMS_PER_MILE * miles; • The assignment statement above assigns a value to the variable kms . The value assigned is the result of the multiplication of the constant KMS_PER_MILE by the variable miles .
12 Figure 2.3 Effect of kms = KMS_PER_MILE * miles;
13 Figure 2.2 Memory(a) Before and (b) After Execution of a Program
14 More on Assignments • In C the symbol = is the assignment operator Read it as ”becomes”, ”gets”, or ”takes the value of” rather than ”equals” because it is not equivalent to the equal sign of mathematics. In C, == tests equality. • In C you can write assignment statements of the form: sum = sum + item; where the variable sum appears on both sides of the assignment operator. This is obviously not an algebraic equation, but it illustrates a common programming practice. This statement instructs the computer to add the current value of sum to the value of item ; the result is then stored back into sum .
15 return Statement return (0); • Transfers control from your program to the operating system. • return (0) returns a 0 to the Operating System and indicates that the program executed without error. • It does not mean the program did what it was suppose to do. It only means there were no syntax errors. There still may have been logical errors. • Once you start writing your own functions, you’ll use the return statement to return information to the caller of the function.
16 Reserved Words /* Converts distances from miles to kilometers */ #include <stdio.h> /* printf, scanf definitions */ #define KMS_PER_MILE 1.609
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