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!
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 occurred 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
sum_digits.c
#include <stdio.h>
int main(void) {
int c;
int digit_count, digit_sum, digit_value;
digit_count = 0;
digit_sum = 0;
// getchar returns an int which will contain either
// the ASCII code of the character read or EOF
c = getchar();
while (c != EOF) {
// test if ch is digit, isdigit would be better
if (c >= '0' && c <= '9') {
digit_count = digit_count + 1;
digit_value = c - '0';
digit_sum = digit_sum + digit_value;
}
c = getchar();
}
printf("Input contained %d digits which summed to %d\n", digit_count, digit_sum);
return 0;
}
echo_digits.c which print the digit
characters (only) in its command line arguments.
The only function you can use is printf.
For example:
./echo_digits 42 23skid00 a5a5a5a hi 42 2300 555
echo_digits.c
#include <stdio.h>
int main(int argc, char *argv[]) {
int argument = 1;
while (argument < argc) {
int i = 0;
while (argv[argument][i] != 0) {
int c = argv[argument][i];
if (c >= '0' && c <= '9') {
printf("%c", c);
}
i = i + 1;
}
if (argument < argc - 1) {
printf(" ");
}
argument = argument + 1;
}
printf("\n");
return 0;
}
Your tutor may still choose to cover some of the questions time permitting.
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
letter_triangle.c
// Description: Prompts the user for a strictly positive number N
// and outputs an equilateral triangle of height N.
// The top of the triangle (line 1) is labeled with the letter A.
// For all nonzero p < N, line p + 1 of the triangle is labeled
// with letters that go up in alphabetical order modulo 26
// from the beginning of the line to the middle of the line,
// starting with the letter that comes next in alphabetical order
// modulo 26 to the letter in the middle of line p,
// and then down in alphabetical order modulo 26
// from the middle of the line to the end of the line.
//
// Written by Eric Martin for COMP9021
// modified by Andrew Taylor for 1911
#include <stdio.h>
int main(void) {
int ch, i, j, k;
int height = 0;
printf("Enter height: ");
scanf("%d", &height);
ch = 'A';
i = 1;
while (i <= height) {
/* Displays spaces on the left */
j = 0;
while (j < height - i) {
printf(" ");
j = j + 1;
}
/* Displays letters before middle column */
k = 1;
while (k < i) {
putchar(ch);
/* code of next letter */
ch = (ch - 'A' + 1) % 26 + 'A';
k = k + 1;
}
/* Displays middle column */
putchar(ch);
/* Displays letters after middle column */
k = 1;
while (k < i) {
/* Code of previous letter */
ch = (ch - 'A' + 25) % 26 + 'A';
putchar(ch);
k = k + 1;
}
printf("\n");
/* Code of first letter to be input on next line */
ch = ((1 + i) * i / 2) % 26 + 'A';
i = i + 1;
}
return 0;
}
input_statistics.c that for the
characters provided on standard input:
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
/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* Description: Outputs *
* - the number of blank characters (spaces, tabs and new lines) *
* - the length of the shortest word *
* (any sequence of nonblank characters), and *
* - the length of the longest word *
* (any sequence of nonblank characters) *
* read from standard input. *
* *
* Written by Eric Martin for COMP9021 *
* Modified by Andrew Taylor for COMP1511 *
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
#include <stdio.h>
int main(void) {
int whiteSpaceCount;
int wordMinLength, wordMaxLength, wordCurrentLength;
int wordCount;
int ch;
wordCount = 0;
whiteSpaceCount = 0;
wordCurrentLength = 0;
ch = getchar();
while (ch != EOF) {
// iswhite would be better here
if (ch == ' ' || ch == '\t' || ch == '\n') {
whiteSpaceCount = whiteSpaceCount + 1;
// A complete word has just been read iff wordCurrentLength > 0,
// which is then the length of that word.
if (wordCurrentLength > 0) {
if (wordCount == 0) {
wordMinLength = wordCurrentLength;
} else if (wordCurrentLength < wordMinLength) {
wordMinLength = wordCurrentLength;
} else if (wordCurrentLength > wordMaxLength) {
wordMaxLength = wordCurrentLength;
}
wordCurrentLength = 0;
wordCount = wordCount + 1;
}
} else {
wordCurrentLength = wordCurrentLength + 1;
}
ch = getchar();
}
printf("Input contains %d blanks, tabs and new lines\n", whiteSpaceCount);
if (wordCount == 0) {
printf("No word has been input\n");
} else {
printf("Number of words: %d\n", wordCount);
printf("Length of shortest word: %d\n", wordMinLength);
printf("Length of longest word: %d\n", wordMaxLength);
}
return 0;
}
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 loops that set every element of matrix.
Each element should be set to the product of its two indices.
Write nested 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:
./matrix
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
#include <stdio.h>
#define N_ROWS 12
#define N_COLUMNS 15
int main(void) {
int matrix[N_ROWS][N_COLUMNS];
int row, column, row_sum, column_sum;
row = 0;
while (row < N_ROWS) {
column = 0;
while (column < N_COLUMNS) {
matrix[row][column] = row * column;
row = row + 1;
}
column = column + 1;
}
row = 0;
while (row < N_ROWS) {
row_sum = 0;
column = 0;
while (column < N_COLUMNS) {
printf(" %4d", matrix[row][column]);
row_sum = row_sum + matrix[row][column];
column = column + 1;
}
row = row + 1;
printf(" | %4d\n", row_sum);
}
for (column = 0; column < N_COLUMNS; column = column + 1) {
printf("-----");
}
printf("\n");
column = 0;
while (column < N_COLUMNS) {
column_sum = 0;
row = 0;
while (row < N_ROWS) {
column_sum = column_sum + matrix[row][column];
row = row + 1;
}
printf(" %4d", column_sum);
column = column + 1;
}
printf("\n");
return 0;
}
#include <stdio.h>
#define N 10
int main(void) {
int digit_count[N];
int x, last_digit;
while (scanf("%d", &x) == 1) {
last_digit = x % N;
digit_count[last_digit] = digit_count[last_digit] + 1;
}
last_digit = 0;
while (last_digit < N) {
printf("%d numbers with last digit %d read\n", digit_count[last_digit], last_digit);
last_digit = last_digit + 1;
}
return 0;
}
It works on the students laptop:
gcc -Wall -O last_digit.c a.out 42 121 100 11 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
But when run at uni, it fails:
dcc last_digit.c a.out 42 121 100 11 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?
digit_count.
Their program used values elements of
digit_count which had not previously had values
stored in them.
This is illegal C and can hence produce *any* output.
On the student's laptop by chance the bytes used for the array already had zeros in them. This is not uncommon and leads to illegal C seeming to be correct. But changing compiler flags, machine, or even time of day can affect this.
dcc checks (among other things) that array indices
are valid but doesn't check that array elements have been
initialized.
dcc --valgrind checks that array elements have been
initialized (but not that indices are valid)
The student's program can be fixed by adding a for loop to initialize the elements of the array digit_count to 0.
A less obvious problem is that entering a negative number will produce an (illegal) negative array index (also fixed below).
Fix the code (make sure you understand how it works - it's a common and useful programming pattern).
#include <stdio.h>
#define N 10
int main(void) {
int digit_count[N];
int x, last_digit;
last_digit = 0;
while (last_digit < N) {
digit_count[last_digit] = 0;
last_digit = last_digit + 1;
}
while (scanf("%d", &x) == 1) {
last_digit = x % N;
if (last_digit < 0) {
last_digit = - last_digit;
}
digit_count[last_digit] = digit_count[last_digit] + 1;
}
last_digit = 0;
while (last_digit < N) {
printf("%d numbers with last digit %d read\n", digit_count[last_digit], last_digit);
last_digit = last_digit + 1;
}
return 0;
}
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}};
nums5[0] contains 0,1,2,3,4,5,6,7,8,9. The array at
nums5[1] contains 10,20,30,40,50,60,70,80,90,100
printf("%d\n",nums2[3]); prints 3
printf("%d\n",nums3[5]); prints -2
printf("%d\n",nums5[0][1]); prints 1
printf("%d\n",nums5[1][0]); prints 10
nums1[0] = nums2[1] + 10 ;
printf("%d\n",nums1[0]); prints 11
int i = 0;
printf("%d\n",nums1[i]); prints 11
printf("%d\n",nums2[10]); This is an error.
The indexes in nums2 go from 0..9.
By using an index of 10 we are trying to go past the end of the array
printf("%d\n",nums5[2][0]); This is an error.
The first index in nums5 goes from 0..1.
By using an index of 2 we are trying to go past the end of the array
printf("%d\n",nums5[1][10]); This is an error.
The second index in nums5 goes from 0..9.
By using index of 10 we are trying to go past the end of the array