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6. Week 3 Thursday: Review and Practise

We’ve covered a lot of content in the weeks leading up to this point, and I think it’s a good time to look back and get some practise in. We are at a critical moment in the course where we have introduced a lot of techniques for data storage, handling input, and data manipulation, but have not yet introduced techniques for controlling our code with “control flow”. Thus our programs are linear and predictable: every time we run our program, it does exactly the same thing, and it cannot respond to different inputs or different scenarios with different code.

Nonetheless, there are ways to emulate control flow with clever applications of integer division, string indexing, and others. Let’s demonstrate this with an example:

SAMPLE RUNS: 
Please enter a time in 24-hour format: 4:37 
4:37 AM 

Please enter a time in 24-hour format: 20:59
8:59 PM 

The pseudocode is profoundly straightforward:

1. Read the hours and minutes from the user’s input.
2. Convert the hours to 12-hour format.
3. Print out the converted time, with AM if the inputted hours were less than 12 or PM if the inputted hours were larger than 12.

(Indeed, this code would be far easier to implement if we could use conditional statements and branching control flow, but alas, I am an incarnation of human evil.)

Step 1 can be performed using the input buffer techniques we discussed last Tuesday:

1cout << "Please enter a time in 24-hour format: "; 
2
3int hours, minutes; 
4char colon; 
5cin >> hours >> colon >> minutes; 

Step 2 appears straightforward as well — we need only take the remainder of the hours after division by 12 to get it into 12-hour format.

1int new_hours = hours % 12; 

I’m putting this in a new variable because I anticipate I’ll need to remember the user’s input to determine if I need to print out “AM” or “PM”. There is a subtle problem — 12:30 will make new_hours become 0 instead of 12. In fact, the above code works for all times except for 00:XX and 12:XX!

Please spend some time thinking carefully about how you can fix this without using if statements.

1int new_hours = hours % 12; 
2int n = ???; 
3new_hours += n * 12; 

1int new_hours = hours % 12; 
2int n = (12 - new_hours) / 12; 
3new_hours += n * 12; 

The last step we have to implement is the output. We’ve converted the hours properly now, and we just need to print out an AM or a PM depending on the time of day. The idea is to store both AM and PM in a string, then mathematically define an index variable that will “point to” the correct part of the string.

1string am_pm = "AMPM"; 
2int index = ???; 
3cout << new_hours << ":" << minutes << " " 
4     << am_pm.substr(index, 2) << endl; 

Please spend some time and try to fill out the ??? in the above snippet.

1int index = (hours / 12) * 2; 

Putting all this code together, we arrive at the following finished product:

 1#include <iostream>     // needed for cin, cout 
 2#include <string>       // needed for strings 
 3
 4using namespace std; 
 5
 6int main() {
 7    // step 1 - procuring input. we need to discard the 
 8    // extra colon in the middle. 
 9    cout << "Please enter a time in 24-hour format: "; 
10
11    int hours, minutes; 
12    char colon; 
13    cin >> hours >> colon >> minutes; 
14
15    // step 2 - convert hours to 12-hour format. 
16    int new_hours = hours % 12; 
17    int n = (12 - new_hours) / 12; 
18    new_hours += 12 * n; 
19
20    // step 3 - produce the output, using substrings to 
21    // obtain an "AM" or "PM" based on the hour. 
22    string am_pm = "AMPM"; 
23    int index = (hours / 12) * 2; 
24    cout << hours << ":" << minutes << " " 
25         << am_pm.substr(index, 2) << endl; 
26
27    return 0; 
28}

1int new_hours = hours % 12; 
2new_hours += 12 * (12 - new_hours) / 12;

A sceptical student may ask, “What’s the point of learning these techniques if they will inevitably be superseded by if-else control flow?” This is true: in the near future, you will learn how to execute one line of code if some condition is met, and another if not. But there are situations where this is impractical.

Consider, for instance, the problem of converting a single English alphabetic character into its NATO phonetic, Morse code, or braille counterparts. Do you really want to write out 26 different lines of cout’s, corresponding to the 26 different letters? More broadly, this type of scenario occurs frequently when performing data analysis on categorical data.

We have thus identified an important design pattern. The setup is, given some input that has numerous possible values, produce some output that takes a different value for each input. While it’s natural to for humans to say, “If input has value n, produce output n” and repeat this for every possible input-output pair, this is a horrible way to write code. Rather, our above solution says to organise the n possible outputs in an indexable fashion (i.e. as substrings of a string), then use the input to figure out which “index of output” to use.

We will be able to utilise this design pattern in its fullest potential when we discuss arrays and vectors. We are currently limited to storing string data and accessing substrings; we cannot yet store n different integer outputs, for instance.

Let’s wrap things up with a few practise problems. In the following, you are only permitted to use the iostream and string libraries (though some of these do not even require strings). In addition, you may not use control flow.

SAMPLE RUNS 
Please enter a time in 12-hour format: 3:54 PM
15:54 

Please enter a time in 12-hour format: 12:12 AM
0:12 

SAMPLE RUNS
Please enter a temperature: 100 C
212 F

Please enter a temperature: 77 F
25 C

SAMPLE RUNS: 
Enter a number between 1 and 7: 5
The fifth day of the week is Thursday! 

Enter a number between 1 and 7: 1 
The first day of the week is Sunday!