Midterm Practice Questions¶

Write a program that calculates the percentage score on an exam. It should ask the user how much the exam is out of, and then ask what score the user got on it. Then it prints the percentage score.

Here’s a sample run:
```
What is the exam out of? 34
What is your score? 29.5
You scored 86.7647%
```
Your program should do the following error-checking:
- If the user enters 0, or less, for what the exam is out of, then use cmpt::error to throw an exception.
- If the user enters less than 0 for their score, then use cmpt::error to throw an exception. Note that there is no highest possible score, and so, for instance, someone could get a score higher than what the exam is out of (and so they will get a % over 100).
Suppose a, b, and c are all variables of type int. They have been created, but we don’t know the exact value any of them contain.

Write a fragment of code that prints “yes” if either a, b, and c are all different, or they are all the same. Otherwise, print “no”.

Re-write the following loops as equivalent while-loops:

// loop 1
for(int i = 0; i < 100; i++) {
    cout << i*i;
}

// loop 2
for(int j = 750; j >= 0; j-=2) {
    cout << j;
}

Write a fragment of code that is an infinite loop that prints “hello” forever. Do it in the following ways:
1. Using a for-loop
2. Using a while-loop
3. Without using an explicit loop
Write a function that prints a fancy title on the screen. For example, if your program has this statement:
```
title("Geometry Helper");
```
Then this text should appear on the screen:
```
---------------
Geometry Helper
---------------
```
The passed-in string should be sandwiched between lines of '-' characters that are the same length as the string.
Suppose you’re a hacker trying to break into a digital safe protected by a 3-digit combination of the form abc, where a, b, and c are all between 0 and 9 (inclusive).
1. Write a program that prints all 3-digit safe combinations on the screen, in order from smallest to biggest, like this:
```
000
001
002
...
567
568
...
998
999
```
2. It turns out that any combination whose 3rd digit is a 3 will trigger an alarm. So, write a program that prints all 3-digit safe combinations on the screen, in order from smallest to biggest, that don’t end with a 3:
```
000
001
002
004
...
361
362
364
...
998
999
```
3. A new version of the safe triggers an alarm if the final digit of the combination is an odd number. Write a program that prints all 3-digit safe combinations on the screen, in order from smallest to biggest, that don’t end with an odd digit:
```
000
002
004
006
...
362
364
366
...
994
996
998
```
4. An even newer version of the safe triggers an alarm if one or more of the digits in the combination are odd. Write a program that prints all 3-digit safe combinations on the screen, in order from smallest to biggest, that don’t contain an odd digit:
```
000
002
004
...
008
020
022
...
086
088
200
202
204
...
884
886
888
```
5. Due to customer complaints about the previous version of the safe having too many alarm-triggering combinations, the safe was modified with a software update such that any combination with exactly one odd digit will trigger an alarm. Write a program that prints all 3-digit safe combinations on the screen, in order from smallest to biggest, that contain either 0, 2, or 3 odd digits:
```
000
002
004
006
...
008
011
013
015
017
...
097
099
101
103
105
...
110
111
112
113
114
115
116
...
997
998
999
```

Write a function called set_to_next_letter that works as follows:

char c = 'a';
set_to_next_letter(c);
cout << c;               // prints 'b'
set_to_next_letter(c);
cout << c;               // prints 'c'
set_to_next_letter(c);
cout << c;               // prints 'd'

c = 'z';
set_to_next_letter(c);
cout << c;  // prints 'a'

Notice that if c is 'z', then set_to_next_letter(c) sets c to be 'a', i.e. it “wraps around”.

You can assume that only lowercase letters in the range 'a' to 'z' will be passed to set_to_next_letter.

The following questions are based on this table of hexadecimal and decimal digits:

Hex Digit Decimal Digit

0 0

1 1

2 2

3 3

4 4

5 5

6 6

7 7

8 8

9 9

a, A 10

b, B 11

c, C 12

d, D 13

e, E 14

f, F 15
1. Write a function that returns true if a character is a hex digit, and false otherwise. It should have this header:
```
bool is_hex(char c)
```
  For example, is_hex('2'), is_hex('e'), and is_hex('F') all return true, while is_hex('$'), is_hex('g'), and is_hex('-') all return false.
2. Write a test driver program that asks the user to enter a character, and then prints whether or not the character is a hex digit. The test driver loop should stop when the user types the '!' character. For example:
```
--> a
a is a hex digit
--> 2
2 is a hex digit
--> $
$ is not a hex digit
--> !
(done)
```
3. Re-write the following function so that it behaves exactly the same, but it does not use a switch statement:
```
int hex_to_int(char c) {
   switch (c) {
      case '0': return 0;
      case '1': return 1;
      case '2': return 2;
      case '3': return 3;
      case '4': return 4;
      case '5': return 5;
      case '6': return 6;
      case '7': return 7;
      case '8': return 8;
      case '9': return 9;
      case 'a': case 'A': return 10;
      case 'b': case 'B': return 11;
      case 'c': case 'C': return 12;
      case 'd': case 'D': return 13;
      case 'e': case 'E': return 14;
      case 'f': case 'F': return 15;
      default:
          cmpt::error("unknown hex digit");
          return -1; // to satisfy compiler
      }
 }
```
4. Write a function that takes a single int n as input, and returns the char that corresponds to the hex digit of n. If n is less than 0 or greater than 16, then use the cmpt::error function to throw an exception.

Midterm Practice Questions¶

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Hex Digit	Decimal Digit
0	0
1	1
2	2
3	3
4	4
5	5
6	6
7	7
8	8
9	9
a, A	10
b, B	11
c, C	12
d, D	13
e, E	14
f, F	15