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 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
sum_digits.c
#include <stdio.h>
int
main(void) {
int c;
int digitCount, digitSum, digitValue;
digitCount = 0;
digitSum = 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') {
digitCount = digitCount + 1;
digitValue = c - '0';
digitSum = digitSum + digitValue;
}
c = getchar();
}
printf("Input contained %d digits which summed to %d\n", digitCount, digitSum);
return 0;
}
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 wth 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
input_statistics.c
/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* 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 COMP9021 *
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
#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;
}
#include <stdio.h>
int main(void) {
char str[10];
str[0] = 'H';
str[1] = 'i';
printf("%s", str);
return 0;
}
printf, like many C library functions expects strings to be null-terminated.
In other words printf, expects the array str to contain an element with value 0 (also writen '\0')
which marks the end of the sequence of characters to be printed.
printf will print str[0] ('H'), str[1] then examine
str[2].
Code produced by dcc --valgrind will then stop with an error because
str[2] is uninitialized.
The code with gcc will keep executing and printing element from str until
it encounters one containing '\0'. Often str[2] will by chance contain '\0'
and the program will work correctly.
Another common behaviour will be that the program prints some extra "random" characters.
It is also possible the program will index outside the array which would result in it stopping
with an error if it was compiled with dcc.
If the program was compiled with gcc and uses indices well outside the array it may be terminated by the the operating system because of an illegal memory access.
#include <stdio.h>
int main(void) {
char str[10];
str[0] = 'H';
str[1] = 'i';
str[2] = '\0';
printf("%s", str);
return 0;
}
Your tutor may still choose to cover some of the questions time permitting.
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
#include <stdio.h>
#define N_ROWS 12
#define N_COLUMNS 15
int main(void) {
int matrix[N_ROWS][N_COLUMNS];
int row, column, rowSum, columnSum;
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) {
rowSum = 0;
column = 0;
while (column < N_COLUMNS) {
printf(" %4d", matrix[row][column]);
rowSum = rowSum + matrix[row][column];
column = column + 1;
}
row = row + 1;
printf(" | %4d\n", rowSum);
}
for (column = 0; column < N_COLUMNS; column = column + 1) {
printf("-----");
}
printf("\n");
column = 0;
while (column < N_COLUMNS) {
columnSum = 0;
row = 0;
while (row < N_ROWS) {
columnSum = columnSum + matrix[row][column];
row = row + 1;
}
printf(" %4d", columnSum);
column = column + 1;
}
printf("\n");
return 0;
}
#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 readprint 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 readWhy doesn't the code work at uni .
Why doesn't dcc detect an error?
digitCount.
Their program used values elements of digitCount 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 digitCount 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 - its an common & useful programming pattern).
#include <stdio.h>
#define N 10
int main(void) {
int digitCount[N];
int x, lastDigit;
lastDigit = 0;
while (lastDigit < N) {
digitCount[lastDigit] = 0;
lastDigit = lastDigit + 1;
}
while (scanf("%d", &x) == 1) {
lastDigit = x % N;
if (lastDigit < 0) {
lastDigit = - lastDigit;
}
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;
}
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
int i;
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
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
int myStrlen(char *string);
int myStrCmp(char *string1, char *string2);
int beginsWith(char *string1, char *string2);
int isSubstring(char *substring, char *string);