COMP1511 Week 06 Tutorial Questions

Your tutor has asked a lab pair to present their week 5 work.

Discuss the good, the bad and the ugly aspects of their code.

Please be gentle in any criticism - we are all learning!

Did you blog last week? What is this week's blogging theme?
1. Encode and decode a message with a Caesar cipher.
2. The shift is the key for a Caesar Cipher - how many bits are in it?
3. How would you crack a Caesar Cipher?
4. Encode and decode a message with a Substitution cipher.
5. The letter mapping is the key for a Substitution cipher - how many bits are in it?
6. How would you crack a Substitution Cipher (assume the plain text is English)?
Write a program sum_digits.c which reads characters from its input and counts digits.
When the end of input is reached it should print a count of how many digits occured in its input and their sum.
The only functions you can use are getchar and printf.
For example:
```
./sum_digits
1 2 3 o'clock
4 o'clock rock
Input contained 4 digits which summed to 10
./sum_digits
12 twelve 24 twenty four
thirty six 36
Input contained 6 digits which summed to 18
```

Write a program letter_triangle.c that read an positive integer n and outputs a triangle of letters of height n as below. For example:

./letter_triangle
Enter height: 3
  A
 BCB
DEFED
./letter_triangle
Enter height: 7
      A
     BCB
    DEFED
   GHIJIHG
  KLMNONMLK
 PQRSTUTSRQP
VWXYZABAZYXWV
./letter_triangle
Enter height: 10
         A
        BCB
       DEFED
      GHIJIHG
     KLMNONMLK
    PQRSTUTSRQP
   VWXYZABAZYXWV
  CDEFGHIJIHGFEDC
 KLMNOPQRSRQPONMLK
TUVWXYZABCBAZYXWVUT

Write a program input_statistics.c that for the characters provided on standard input.:

outputs the number of white-space characters (spaces, tabs and new lines)
outputs the numbers of words word (any contiguous sequence of non-white-space characters), and
outputs the length of the shortest word
outputs the length of the longest word

For example:

./input_statistics
    "Beauty is truth, truth beauty," -- that is all
    Ye know on earth, and all ye need to know.
Input contains 27 blanks, tabs and new lines
Number of words: 19
Length of shortest word: 2
Length of longest word: 8
./input_statistics
And here is another example with only one line of input!!!!!!!!!
Input contains 11 blanks, tabs and new lines
Number of words: 11
Length of shortest word: 2
Length of longest word: 14

```
#include <stdio.h>

int main(void) {
    char str[10];
    str[0] = 'H';
    str[1] = 'i';
    printf("%s", str);
    return 0;
}
```
1. What will happen when the above program is compiled?
2. How do you correct the program?
Revision questions
The remaining tutorial questions are primarily intended for revision - either this week or later in session.
Your tutor may still choose to cover some of the questions time permitting.

How many ints can the array matrix below hold?

#include <stdio.h>

#define N_ROWS 12
#define N_COLUMNS 15

int main(void) {
    int matrix[N_ROWS][N_COLUMNS];

Write nested for loops that sets every element of matrix. Each element should be set to the product of its two indices.

Write nested for loops that print the elements of matrix plus sums of each row and sums of each column.

The output of your code should look like this:

a.out
    0    0    0    0    0    0    0    0    0    0    0    0    0    0    0 |    0
    0    1    2    3    4    5    6    7    8    9   10   11   12   13   14 |  105
    0    2    4    6    8   10   12   14   16   18   20   22   24   26   28 |  210
    0    3    6    9   12   15   18   21   24   27   30   33   36   39   42 |  315
    0    4    8   12   16   20   24   28   32   36   40   44   48   52   56 |  420
    0    5   10   15   20   25   30   35   40   45   50   55   60   65   70 |  525
    0    6   12   18   24   30   36   42   48   54   60   66   72   78   84 |  630
    0    7   14   21   28   35   42   49   56   63   70   77   84   91   98 |  735
    0    8   16   24   32   40   48   56   64   72   80   88   96  104  112 |  840
    0    9   18   27   36   45   54   63   72   81   90   99  108  117  126 |  945
    0   10   20   30   40   50   60   70   80   90  100  110  120  130  140 | 1050
    0   11   22   33   44   55   66   77   88   99  110  121  132  143  154 | 1155
---------------------------------------------------------------------------
    0   66  132  198  264  330  396  462  528  594  660  726  792  858  924

A student has written this program to read ints until the end-of-input. It counts how many numbers it reads categorized by their last digit:

#include <stdio.h>

#define N 10

int main(void) {
    int digitCount[N];
    int x, lastDigit;

    while (scanf("%d", &x) == 1) {
        lastDigit = x % N;
        digitCount[lastDigit] = digitCount[lastDigit] + 1;
    }
    lastDigit = 0;
    while (lastDigit < N) {
        printf("%d numbers with last digit %d read\n", digitCount[lastDigit], lastDigit);
        lastDigit = lastDigit + 1;
    }

    return 0;
}

It works on the students laptop:

gcc -Wall -O last_digit.c
a.out
42 121 100 11
<cntrl-d>
1 numbers with last digit 0 read
2 numbers with last digit 1 read
1 numbers with last digit 2 read
0 numbers with last digit 3 read
0 numbers with last digit 4 read
0 numbers with last digit 5 read
0 numbers with last digit 6 read
0 numbers with last digit 7 read
0 numbers with last digit 8 read
1 numbers with last digit 9 read

print counts of how many numbers read.

But when run at uni fails

dcc last_digit.c
a.out
42 121 100 11
<cntrl-d>
778121076 numbers with last digit 0 read
7632239 numbers with last digit 1 read
-2032569224 numbers with last digit 2 read
32727 numbers with last digit 3 read
0 numbers with last digit 4 read
0 numbers with last digit 5 read
-2032409578 numbers with last digit 6 read
32727 numbers with last digit 7 read
-21600000 numbers with last digit 8 read
32767 numbers with last digit 9 read

Why doesn't the code work at uni .

Why doesn't dcc detect an error?

Fix the code (make sure you understand how it works - its an common & useful programming pattern).

What is the effect of each of the following statements? What are the initial values in the arrays?

int nums1[10];

int nums2[] = {0,1,2,3,4,5,6,7,8,9};

int nums3[10] = {0,2,4,6,8,-2};

int nums4[10] = {0};

int nums5[2][10] = {{0,1,2,3,4,5,6,7,8,9},
                    {10,20,30,40,50,60,70,80,90,100}};

What would the output of the following fragment of code be - given the array definitions above?

int i;
printf("%d\n",nums2[3]); 
printf("%d\n",nums3[5]); 
printf("%d\n",nums5[0][1]); 
printf("%d\n",nums5[1][0]); 
nums1[0] = nums2[1] + 10 ;
printf("%d\n",nums1[0]);    
i = 0;
printf("%d\n",nums1[i]);

What is wrong with the following piece of code - given the above array definitions?

printf("%d\n",nums2[10]);   

printf("%d\n",nums5[2][0]);

printf("%d\n",nums5[1][10]);

Strlen is a function that returns the length of a string. Write your own C function to do the same.
```
        int myStrlen(char *string);
    
```
Write a C function that is given two strings and returns 1 if they are both equal and 0 otherwise. This is a simplified version of strcmp, provided in the C library. Try writing one yourself.
```
        int myStrCmp(char *string1, char *string2);
    
```
Write a C function that is given two strings and returns 1 if the first begins with the second (and 0 otherwise). For example, "APPLE" begins with "APP".
```
        int beginsWith(char *string1, char *string2);
    
```
Write a C function that is given two strings and returns 1 if the first is a substring of the second (and 0 otherwise). For example, "APP", "E", "PL" are all substrings of "APPLE".
```
        int isSubstring(char *substring, char *string);
    
```

Revision questions