3.12 Calling Procedures

1. Exam Points

• Call given procedures:
  • procName (arg1, arg2, …)
• Pass arguments to parameters: procName(arg1, arg2, …)
• Return a value from a procedure: result ← procName(arguments)
• Note: The code segment after a return statement is unreachable.
• Predict output/result of executing procedures.

2. Knowledge Points

(1) Understand Procedures

• A procedure is a named group of programming instructions that may have parameters and return values.
• Example: develop a procedure and call the procedure
  
• Input and output in CSP:
  • Input: procedure INPUT()
    • x ← INPUT()
  • Output: procedure DISPLAY(): display a value, followed by a space
    • DISPLAY(”a”)
    • DISPLAY(10)

(2) Develop and Call Procedures

• Procedures are referred to by different names, such as method or function, depending on the programming language.
• Syntax:
  
• Example 1: a procedure with parameters
  
  
  • 
  • Parameters (形式参数 - count) : input variables of a procedure.
  • Arguments (实际参数 - 5): actual values of the parameters when a procedure is called.
• Example 2: a procedure with a returned value
  
  
  • 
  • Note:
    • The RETURN statement may appear at any point inside the procedure and causes an immediate return from the procedure back to the calling statement.
    • If return(a) is executed, then the statements after return(a) in the procedure will not be executed.
    • The code segment after a return statement is unreachable.
  • Purpose of the return statement:
    • Return a value
    • Return the flow of control back

(3) Note

• A procedure call interrupts the sequential execution of statements, causing the program to execute the statements within the procedure before continuing.
• Once the last statement in the procedure (or a return statement) has executed, flow of control is returned to the point immediately where the procedure was called.
• Example: b is displayed only after the last statement in proc2() is executed.
  

3. Exercises

• Practice
• Solution