Checking for bad input

CSCI 201 – Computer Science 1Lab assignment 4: Nested FOR loops and Input/Output with Files
Due on Friday February 2.
Read sections 5.10, 5.11 and 5.12 of the text as you work on this assignment
Objective: Become familiar with designing a solution using a for loop. Learn how to create a
nested for loop in Raptor and in C++.
This assignments deal with files and nested loops.
Input: The program prompts for the name of an input file and the name of an output file. The
input file contains two positive (non-zero) integers, m and n, each less than 20, followed by a
character.
Output: A file containing an m × n rectangle made using the specified character.
For example, if the input file contained the numbers 4 and 6 and the character ⋆ as shown below,
4 6*
the output file would show 4 lines of 6 stars each, as shown below:
******
******
******
******
This program will start by opening the input file and reading the numbers and the specified character. Thereafter a nested for loop will generate the pattern. The following questions are designed
to step you through the process of creating the nested loop.
Question 1: Using Raptor, create a flowchart for the following process:
write a specified character (denoted ch) n times on the output stream and move the cursor to the
beginning of the next line. (In Raptor, the character must be put in double quotes. When writing,
uncheck the “end current line” box to stay on the same line; write an empty string with the box
checked to go to the next line.)
Question 2: Created a nested loop in Raptor. The outer loop will run the inner loop a specified
number of times. The result will be rectangular block of characters. Example for constructing a
nested Raptor loop can be found here. Note that when you draw the nested loop in Raptor, the
outer loop is created first, and the inner loop is placed inside it.
Question 3: Implement your algorithm in C++ using the for loops. Create three sample data
files. In a script session, do the following:
1. cat the program file and compile it using g++.
2. For each data file you created, display the file, run the program and display the resulting output
file.
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What to submit. Create a folder named Lab4 in your CourseFiles folder and upload the Raptor
flowcharts and your script to this.
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CSCI 201 – Computer Science 1
Homework Assignment 3
Objectives. The objectives of this assignment are:
1. Learn to design a do while loop and implement it in C++.
2. Learn to create functions in C++.
3. Learn how functions can be employed to encapsulate a part of the process, and thereby simplify
the code in the main program.
In this homework, we modify and extend the program completed in Lab 3 to incorporate some
additional features.
Part I.
Due date: Monday Jan 29.
Using a do-while loop. Recall that in a post-test loop, the loop body must execute once before
the exit condition is checked. Therefore, to use a post-test loop, we have to change the way we
implement the strategy. This can be done as follows:
initialize total as zero;
initialize operator as ’+’;
read num;
process num (add or subtract from total, depending on operator);
read next operator;
If (operator is ’=’) exit and print total.
read num;
…
Question 1: Find the repetition in the above sequence and construct a flowchart with a post-test
loop (the exit condition should be checked at the end of the loop) using Raptor.
Question 2: Translate this into C++ code, compile and test in a script session.
Question 3 (reflection): Compare the pretest and post-test versions. In this case, which one
feels more natural and why? What are the pros and cons of using one over the other?
What to submit: Raptor file for Q 1, script file, .cpp file and answer to Q 3 in the Hwork3Part1
folder within your CourseFiles folder.
Part II.
Due date: Thursday, February 1.
Checking for bad input. There are several kinds of errors that can occur when the program is
being used, and the mechanism to handle each one is different. In this assignment we look at two
kinds of errors:
1. Input errors when entering the operator (character input).
2. Input errors when entering the operand (number input).
Dealing with bad characters. Consider a situation where the user inputs some character other
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than + or – for an operator. Since the input will always be an ASCII character, the program can
read it into a variable of type char and check it with an if statement. We would like that the
program should continue operating properly in the event of bad input, and allow the user to correct
the mistake as often as needed. That is accomplished by printing an error message and asking the
user to give the input again. A sample run looks something like this:
Welcome to your friendly neighborhood accumulator! Please input
your expression, one token at a time, starting with an operand and
type in ‘‘=’’ when completed.
3.0
*
Bad character. Please input +, – or =.
)
Bad character. Please input +, – or =.
+
2.0
-5.0
=
10.0
Thank-you for using your friendly neighborhood accumulator!
Question 4: Write a C++ function with the following header
char getOperator()
that performs just the task of reading an operator, allowing the user to make any number of
mistakes. Create a test program for this function. In CourseInfo/Hwork3/ in CourseFiles, you
will find the file getCodeTest.cpp that gives you a example of how such a function can be written
and tested separately. Create a script file showing the source code, compilation and testing.
Question 5: Copy the above function into either one of your accumulator programs (with the while
loop or the do-while loop) and replace the cin statement(s) that reads the operator with a call to
this function. In CourseInfo/Hwork3/ in CourseFiles, you will find the file colorCounter.cpp that
gives you a example of how this is done. Create a script file showing the source code, compilation
and testing.
Dealing with bad numbers. In the previous error, even when a bad operator is typed in, we
can still read it because it is a character. A more difficult situation arises when the user types in
bad input when an operand was expected. Here, the program is expecting a C++ float constant
but finds something else, say, an alphabet. A sample run looks like this:
Welcome to your friendly neighborhood accumulator! Please input
your expression, one token at a time, starting with an operand and
type in ‘‘=’’ when completed.
3.0
*
Bad character. Please input +, – or =.
2
)
Bad character. Please input +, – or =.
+
Need a float value; please try again.
a
Need a float value; please try again.
2.0
-5.0
=
10.0
Thank-you for using your friendly neighborhood accumulator!
Using the cin object to catch bad input. When cin finds unexpected input, i.e., an alphabet
instead of a real number, it does the following:
ˆ The program continues as if the read statement was not executed, i.e., the program will
continue to execute with whatever value was stored in the variable.
ˆ The special boolean variable, also called cin, becomes false.
Our strategy therefore is to check boolean variable cin after the read operation, to detect bad
input. After detecting the bad input the clear operation is invoked on cin to fix the input stream
and the ignore operation is used to flush out the input pipeline. See the file UsingClearCin.cpp
in CourseInfo/Hwork3/ in CourseFiles for an example. In that program, the command
cin.ignore(200, ’\n’);
tells the system to ignore the first 200 characters or the end-of-line(denoted by ’\n’), whichever
comes first. (This is usually enough, but we can still get some strange output; try, for instance, the
input “25a” at the first prompt when you run UsingClearCin.cpp. We cannot fix everything!)
Question 6: Write a C++ function with the following header
float getOperand()
that performs just the task of reading an operand, and allowing the user to make any number of
mistakes. This function will return a float value.Create a test program for this function. Create a
script file showing the source code, compilation and testing.
Question 7: Copy the above function into your program for Question 5 of this homework, and
replace the cin statement with a call to this function. Create a script file showing the source code,
compilation and testing.
What to submit: Script files for Questions 4, 5, 6 and 7 and the final .cpp file in the Hwork3Part2
folder, inside your CourseFiles folder.
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