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Counting words in a line
Input: a line
Output: the number of words
in the line
#include <stdio.h>
int main()
{
char c, line[80];
int i, num = 0, word = 0;
gets(line);
e.g. “ This is a test. “ output is 4
How do you count words?
Data structure: line[], i, c, num, word
for (i = 0; (c = line[i])!= '\0'; ++i)
if (c ==' ')
word = 0;
else if (word == 0)
{
word = 1;
num++;
}
printf("There are %d words in the
line\n", num);
Algorithm:
1. Get an input string line[], num = 0,
word = 0
2. For i from 0 to strlen(line) step 1
3.
if line[i] = ‘ ‘ word = 0
4.
else if word = 0 then
5.
word =1, num = num + 1
6. Print num
What’s the running time of the algorithm?
The running time is O(n), where n is the length of the
input string
}
Ex. Write the a program to count the number of words in a file
CP104 Introduction to Programming
String
Lecture 27 __ 1
Find for a string in a line
Input: a line and a string
Output: the position of the string in the line if found;
otherwise not found
e.g. input: “This is a test.”, test
output “test” is found at position 10
How do you do the search?
Data structure: line[80], i, to_find[20], substr[20], found,
position
Algorithm:
1.
Get an input line, and string to_find
find_len = strlen(to_find), found = 0, i = 0;
2.
While !found && I <= strlen(line)-find_len do
3.
strncpy(substr, &line[i], find_len)
4.
strcmp(substr, to_find) = 0
5.
found = 1
6.
else increase i = i +1
7.
If found then output i, else output not fou
What’s the running time of this algorithm?
The running time is O(n), where n is the length of the input
string
#include <stdio.h>
#include <string.h>
#define LINE_LEN 80
int main() {
char line[LINE_LEN], to_find[20], substr[20];
int i = 0, find_len, found = 0;
printf("Input a line>\n");
gets(line);
printf("Input a searching string>\n");
gets(to_find);
find_len = strlen(to_find);
while (!found && i <= strlen(line)-find_len) {
strncpy(substr, &line[i], find_len);
substr[find_len] = '\0';
if (strcmp(substr, to_find) == 0)
found = 1;
else ++i;
}
if (found)
printf("The string is found at position %d.", i);
else
printf(“Not found");
}
Ex. Write the a program to count the number of a given word in a file
CP104 Introduction to Programming
String
Lecture 27 __ 2
String Sorting
Input: a sequence of words
Output: the sequence of the words in
increasing order
e.g. input: this, is, a, test
output: a, is, test, this
Application: dictionary
How do you store the words
e.g. Use str[50][20] to hold 50 words
How do you do the sorting?
Bubble sort, selection sort
The running time is O(n2), where n is the
number of words.
CP104 Introduction to Programming
String
void selection_sort(char words[][20], int n)
{
int fill, index_of_min;
char temp[20];
for (fill = 0; fill < n; ++fill) {
index_of_min = find_min(words, fill, n-1);
if (fill != index_of_min) {
strcpy(temp, words[index_of_min]);
strcpy(words[index_of_min], words[fill]);
strcpy(words[fill], temp);
}
}
}
int find_min(char words[][20], int fill, int n){
int i, cur_min_posi;
cur_min_posi = fill;
for (i = fill+1; i <= n; ++i)
if (strcmp(words[i], words[cur_min_posi])<0)
{cur_min_posi = i;}
return (cur_min_posi);
}
Lecture 27 __ 3
More comments on running of an algoirthm
• The running time of an algorithm is defined to be
the number of basic operations done by the
algorithm.
• Example:
– A linear time algorithm is one with running time O(n), ie.
Cn, where n is the data length, and C is a constant.
• Simple search, find max, find min, are all linear algorithms.
– A logarithmic time algorithm is one with running time
O(log n).
• Binary search algorithm is a logarithmic time algorith
CP104 Introduction to Programming
String
Lecture 27 __ 4
Comparison and exchange in selection sort
CP104 Introduction to Programming
String
Lecture 27 __ 5
More problems about string operation
•
•
•
•
•
Count the number of a particular string in a file
Delete a substring from a file
Copy a string into a particular position of a file
Insert a string in a given position of a file
Copy one file to another file
CP104 Introduction to Programming
String
Lecture 27 __ 6
Sentinel-Controlled Loop for String Input
CP104 Introduction to Programming
String
Lecture 27 __ 7
3n+1 problem
Consider the following algorithm:
1. input n
2. print n
3. if n = 1 then STOP
4.
if n is odd then n = 3n+1
5.
else n = n/2
6. GOTO 2
For example: given 13 the following sequnce of n will be output
for n: 40 20 10 5 16 8 4 2 1
write a program test all input integers n < 10000.
Conjecture: the algorithm above will terminate (when a 1 is
printed) for any integer input value.
CP104 Introduction to Programming
String
Lecture 27 __ 8
An Array of Pointers
CP104 Introduction to Programming
String
Lecture 27 __ 9
Two Orderings of One List Using an Array of Pointers (I)
CP104 Introduction to Programming
String
Lecture 27 __ 10
Two Orderings of One List Using an Array of Pointers (II)
CP104 Introduction to Programming
String
Lecture 27 __ 11
Two Orderings of One List Using an Array of Pointers (III)
CP104 Introduction to Programming
String
Lecture 27 __ 12
Structure Chart for Text Editor Program (I)
CP104 Introduction to Programming
String
Lecture 27 __ 13
Text Editor Program (II)
CP104 Introduction to Programming
String
Lecture 27 __ 14
Text Editor Program (III)
CP104 Introduction to Programming
String
Lecture 27 __ 15
Text Editor Program (IV)
CP104 Introduction to Programming
String
Lecture 27 __ 16
Text Editor Program (V)
CP104 Introduction to Programming
String
Lecture 27 __ 17
Text Editor Program (VI)
CP104 Introduction to Programming
String
Lecture 27 __ 18
Text Editor Program (VII)
CP104 Introduction to Programming
String
Lecture 27 __ 19
Sample Run of Text Editor Program
CP104 Introduction to Programming
String
Lecture 27 __ 20