Special Interest Group on C++
A student recently brought me a simple introductory-level problem found on the web and asked if their program is “efficient”. Though the problem is simple enough that most C++ beginners are likely to solve it in minutes, I noticed it provides good opportunity to discuss certain efficiency considerations that are not typically discussed in an introductory course.
This post describes the problem and invites students to develop alternative solutions and analyze the runtime space and time (memory and speed) needs of the solutions. I encourage students who have completed a course in C++ to ponder this problem, especially if they have completed multiple C++ courses. The overall goal is to become aware of implementation choices and their consequences.
The problem is to read a whole number greater than zero from the user and print the
number as a word as long as the number does not exceed nine. For example, if the user
enters 1, print the text one; if the user enters 2, print two; and so on. If the
input number exceeds nine, just print the text greater than nine.
Example runs of a program might look as follows in a Microsoft Windows environment, where
2 and 15 are values the user inputs at run time, and as-text.exe is the executable
file’s name:
C:\>as-text.exe
Enter a whole number greater than 0: 2
Number as text: two
C:\>as-text.exe
Enter a whole number greater than 0: 15
Number as text: greater than nine
Obviously, at the point when selection statements are introduced in the course, cascaded
selection (if...else if... or switch) solves the problem just fine. However, by
course completion, students will have learned enough to develop cleverer solutions, and
not every clever solution is more efficient than the original simple solution in terms of
runtime space and time.
Each exercise asks to develop two versions of a program. I recommend using this pre-configured Compiler Explorer link to develop the two versions. I also recommend placing the text of your analysis in a GitHub gist or repo.
For simplicity, omit validating the state of the input stream object std::cin in all
programs developed for the exercises.
Write a baseline C++ program using just selection statements to solve the problem and analyze its runtime space and time needs. Then revise the program and analyze the space and time needs of the revised program in relation to the baseline. If you are able to write only the baseline version, please do attempt to analyze its space and time needs.
Both versions of the program should have only one function: the obligatory main.
Write a function as_text which receives a number and returns the text version of the
number using the same logic described in Section 1. Do not perform any user
interaction (input or output) in the function. In main, read a number from the user,
use function as_text to obtain the text version of the number, and print the text
version.
Develop two programs: one where the as_text function corresponds to the baseline
version developed for Exercise 1; another where as_text corresponds to the revised
version developed for Exercise 1.
Write a note on how the runtime space and time needs of the programs in this exercise differ from those of their corresponding programs in Exercise 1. Also describe what (if any) changes you made to the programs of Exercise 1 due to observations as you developed the programs for this exercise. Include a rationale for any changes made.
Note: This exercise simply asks to move the conversion logic that is in main
function in Exercise 1 to its own function such that main now performs only user
interaction and calls the new function as_text.
Ask questions, give feedback, and discuss this post on Twitter. The Twitter link is specific to this post. We greatly appreciate all discussion on the post being only at the post-specific tweet.
Submit solutions by DM on Twitter (only by DM, please) so as to avoid spoilers. Please provide Compiler Explorer links to code. We prefer textual answers in the form of GitHub gists, files in a repo, or other form where we can just follow a link and open the content in a browser.