The fastest / most efficient string to integer conversion.

Posted on 2008-10-12
Last Modified: 2012-06-27
There are several ways to convert strings to integers in C++.

Can you please tell me which of them is the most efficient / fastest option and why?
Question by:nanderEire
  • 4
  • 2
LVL 84

Expert Comment

ID: 22699238
the fastest would be to convert all strings to the integer 0

Author Comment

ID: 22699274
More specifically, which method to convert a string to its integer representation, ie "1234" to 1234, operates the fastest?.
LVL 84

Expert Comment

ID: 22699368
Some processors have a single machine instruction that does it.
On those processors, using that instruction may be fastest.
If there is enough memory for a table of 2^16 or 2^32 ints, processing the string 2 or 4 bytes at a time may be fastest,
but if the string is the wrong length, that method may try to read beyond the end of the string, which could result in undefined behavior.
Free Tool: IP Lookup

Get more info about an IP address or domain name, such as organization, abuse contacts and geolocation.

One of a set of tools we are providing to everyone as a way of saying thank you for being a part of the community.


Author Comment

ID: 22700766
Thank you for your assistance. I would like to explore the pre-existing functionality before writing my own.

Of the methods which are available to me, sscanf / atoi / strtod / stringstreams etc. which of those performs its task the fastest? What are the pros and cons of each?

LVL 84

Expert Comment

ID: 22700811
well, since atoi does nothing other than convert a string to an integer, it has the potential to be implemented faster than the other functions that do that as well as other things.
LVL 84

Expert Comment

ID: 22700841
On the other hand, it could also be implemented as a special case of one of the other functions, in which case just invoking the special case yourself might involve one less subroutine call than calling it, then letting it call the other function.
And if you want to do any of the other things that the other functions do in addition to just getting an integer, then you might prefer to use the more fitting function.
LVL 39

Accepted Solution

itsmeandnobodyelse earned 250 total points
ID: 22704609
>>>> since atoi does nothing other than convert a string to an integer
That is true but atoi stops converting at the first non-digit without error. In case the string might contain nothing or a non-integer you rarely can use atoi.

If you were sure the string is a number (not negative), you might try the following loop

   int i = 0;
   char* psz = strnum;
   while (*psz != '\0') i = i*10 + *psz++;

As it doesn't have a call and no checks beside of checking for zero-termination, it should be even faster than atoi.


Featured Post

Free Tool: ZipGrep

ZipGrep is a utility that can list and search zip (.war, .ear, .jar, etc) archives for text patterns, without the need to extract the archive's contents.

One of a set of tools we're offering as a way to say thank you for being a part of the community.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

Title # Comments Views Activity
array6 challenfge 6 114
countHi2 challenge 7 59
Problem to error 4 67
VS2015 Redefinition errors 4 68
This article shows you how to optimize memory allocations in C++ using placement new. Applicable especially to usecases dealing with creation of large number of objects. A brief on problem: Lets take example problem for simplicity: - I have a G…
The purpose of this article is to demonstrate how we can use conditional statements using Python.
This tutorial will introduce the viewer to VisualVM for the Java platform application. This video explains an example program and covers the Overview, Monitor, and Heap Dump tabs.
This video will show you how to get GIT to work in Eclipse.   It will walk you through how to install the EGit plugin in eclipse and how to checkout an existing repository.

860 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question