qsort() sample program in C++
By: Norman Chap
At times you may want to sort a table or an array; qsort() provides a quick and easy way to do so. The hard part of using qsort() is setting up the structures to pass in.
qsort() takes four arguments. The first is a pointer to the start of the table to be sorted (an array name works just fine), the second is the number of elements in the table, the third is the size of each element, and the fourth is a pointer to a comparison function.
The comparison function must return an int, and must take as its parameters two constant void pointers. void pointers aren't used very often in C++, as they diminish the type checking, but they have the advantage that they can be used to point to items of any type. If you were writing your own qsort() function, you might consider using templates instead. Listing below illustrates how to use the standard qsort() function.
1: /* qsort example */ 2: 3: #include <iostream.h> 4: #include <stdlib.h> 5: 6: // form of sort_function required by qsort 7: int sortFunction( const void *intOne, const void *intTwo); 8: 9: const int TableSize = 10; // array size 10: 11: int main(void) 12: { 13: int i,table[TableSize]; 14: 15: // fill the table with values 16: for (i = 0; i<TableSize; i++) 17: { 18: cout << "Enter a number: "; 19: cin >> table[i]; 20: } 21: cout << "\n"; 22: 23: // sort the values 24: qsort((void *)table, TableSize, sizeof(table[0]), sortFunction); 25: 26: // print the results 27: for (i = 0; i < TableSize; i++) 28: cout << "Table [" << i << "]: " << table[i] << endl; 29: 30: cout << "Done." << endl; 31: return 0; 32: } 33: 34: int sortFunction( const void *a, const void *b) 35: { 36: int intOne = *((int*)a); 37: int intTwo = *((int*)b); 38: if (intOne < intTwo) 39: return -1; 40: if (intOne == intTwo) 41: return 0; 42: return 1; 43: } Output: Enter a number: 2 Enter a number: 9 Enter a number: 12 Enter a number: 873 Enter a number: 0 Enter a number: 45 Enter a number: 93 Enter a number: 2 Enter a number: 66 Enter a number: 1 Table[0]: 0 Table[1]: 1 Table[2]: 2 Table[3]: 2 Table[4]: 9 Table[5]: 12 Table[6]: 45 Table[7]: 66 Table[8]: 93 Table[9]: 873 Done.
Analysis: On line 4, the standard
library header is included, which is required by the qsort() function.
On line 7, the function sortFunction() is declared, which takes the
required four parameters.
An array is declared on line 13 and filled by user input on lines 16-20. qsort()
is called on line 24, casting the address of the array name table to be
a void*.
Note that the parameters for sortFunction are not passed to the call to qsort(). The name of the sortFunction, which is itself a pointer to that function, is the parameter to qsort().
Once qsort() is running, it will fill the constant void pointers a and b with each value of the array. If the first value is smaller than the second, the comparison function must return -1. If it is equal, the comparison function must return 0. Finally, if the first value is greater than the second value, the comparison function must return 1. This is reflected in the sortFunction(), as shown on lines 34 to 43.
Archived Comments
1. Thanks
View Tutorial By: Bad Programmer at 2011-11-23 10:43:49
2. thanks
for your clear and precise explanation :)
View Tutorial By: Tausiq at 2009-09-05 07:27:49
3. good!
View Tutorial By: rey.ryan at 2009-03-14 05:49:28
4. but include<iostream> is main function.
View Tutorial By: glen at 2009-02-03 22:19:22
5. How do use qsort to sort lines in text file?
View Tutorial By: kenny at 2008-11-18 08:59:55
6. DO NOT USE QSORT IN A C++ CODE.
If this is a tutorial on C++, then try std::sort in t
View Tutorial By: paercebal at 2008-08-13 05:06:45
Comment on this tutorial
- Data Science
- Android
- AJAX
- ASP.net
- C
- C++
- C#
- Cocoa
- Cloud Computing
- HTML5
- Java
- Javascript
- JSF
- JSP
- J2ME
- Java Beans
- EJB
- JDBC
- Linux
- Mac OS X
- iPhone
- MySQL
- Office 365
- Perl
- PHP
- Python
- Ruby
- VB.net
- Hibernate
- Struts
- SAP
- Trends
- Tech Reviews
- WebServices
- XML
- Certification
- Interview
categories
Related Tutorials
Calculating total based on the given quantity and price in C++
Sorting an array of Strings in C++
Matrix using nested for loops in C++
Compute the square root of the sum of the squares of an array in C++
Calculate average using Two-Dimensional Array in C++
Two-Dimensional Array Manipulation in C++
Compiling and Linking Multiple Source Files in C++
Escape Sequences for Nonprintable Characters in C++
Using the Built-in Arithmetic Types in C++