Atoi With Error Checking
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Strtol Error
community of 4.7 million programmers, just like you, helping each other. Join them; it only takes a minute: Sign up atoi — how to identify the difference between zero and error? up vote 29 down vote
Atoi If Not A Number
favorite 2 http://www.cplusplus.com/reference/clibrary/cstdlib/atoi/ Return Value On success, the function returns the converted integral number as an int value. If no valid conversion could be performed, a zero value is returned. If the correct value is out of the range of representable values, INT_MAX or INT_MIN is returned. So how I differ between atoi("poop") and atoi("0") and atoi("0000000") Yes I can loop and check for all zeroes in case I get 0 how to use strtol in c result, but isn't there a better way? Notice: I use ANSI C89 c share|improve this question edited Aug 31 '13 at 4:33 Jonathan Leffler 437k61507821 asked Jan 15 '12 at 17:25 Nahum Litvin 3,20952857 6 cplusplus.com is wrong about atoi, it does not detect errors at all. Don't trust that site. –Fred Foo Mar 5 '13 at 16:48 The C89, C99 and C11 standards say nothing about the value returned by atoi() when the correct value is out of range. The C++11 standard says even less (it lists atoi in two tables, and that's all!). The statements from cplusplus.com are essentially wishful thinking and/or common implementations — not guaranteed by any standard. –Jonathan Leffler Aug 31 '13 at 4:37 TL;DR: Use en.cppreference.com/w/cpp/string/byte/atoi instead. –Kuba Ober Sep 3 '15 at 16:17 add a comment| 3 Answers 3 active oldest votes up vote 24 down vote accepted That's one of the reasons atoi is sometimes considered unsafe. Use strtol / strtoul instead. And if you have it use strtonum. The function atoi is more dangerous than you might think. The POSIX standard says: If the value cannot be represented, the behavior is undefined. The C99 standard says this also: 7.20.1 The functions atof, atoi, atol, and atoll
1 2 > Thread Tools Search this Thread Rate Thread #1 31-Mar-2008, 17:40 JustinFox Junior Member Join Date: Mar 2008 Posts: 59 atoi() error catching? CPP / C++ /
Strtonum
C Code:
printf("Please select from the following:\n"); printf("1. Human strtol vs atoi vs. Human\n"); printf("2. Human vs. NPC\n"); printf("Your choice (1 or 2)? "); char choice[20]; scanf("%s",&choice); strtol errno int result; if( !(result = atoi(choice)) == NULL) printf("Please enter numbers only!")will this work for testing if a user's input is numeric? Because result will be null if the atoi() http://stackoverflow.com/questions/8871711/atoi-how-to-identify-the-difference-between-zero-and-error function fails right? Thanks, Justin Fox #2 01-Apr-2008, 00:30 WaltP Outstanding Member Join Date: Feb 2004 Location: Midwest US Posts: 3,435 Re: atoi() error catching? Nope. If you enter 12x3 it'll work fine, converting the 12 to an integer. __________________ Definition: Politics Latin, from poly meaning many and tics meaning blood sucking parasites -- Tom Smothers #3 01-Apr-2008, 01:21 mamntc02 Junior Member Join https://www.gidforums.com/t-17778.html Date: Mar 2008 Location: Barcelona - Catalonia Posts: 63 Re: atoi() error catching? Hi, The int atoi( const char *str ) function converts str into an integer, and returns that integer. str should start with whitespace or some sort of number, and atoi() will stop reading from str as soon as a non-numerical character has been read. Regards __________________Please, correct me if I'm wrong, and sorry for my english ;) #4 01-Apr-2008, 05:20 L7Sqr Awaiting Email Confirmation Join Date: Jul 2005 Location: constant limbo Posts: 234 Re: atoi() error catching? strtol should be used in place of atoi. From the man page Quote: The call atoi(str) shall be equivalent to: (int) strtol(str, (char **)NULL, 10) except that the handling of errors may differ. If the value cannot be represented, the behavior is undefined. You can provide a second argument to strtol (instead of that NULL) to find out what the last unconverted character is (including the NULL terminator in the case of a full parse). Consider the following example, pay close attention to the return value for the first example CPP / C++ / C Code:
#include#include int main () { char< Programming Boards C Programming atoi return error Getting started with C or C++ | C Tutorial | C++ Tutorial | C and C++ FAQ | Get a compiler | Fixes for common problems Thread: atoi return error Thread Tools Show http://cboard.cprogramming.com/c-programming/27572-atoi-return-error.html Printable Version Email this Page… Subscribe to this Thread… Display Linear Mode Switch to Hybrid Mode Switch to Threaded Mode 10-30-2002 #1 rotis23 View Profile View Forum Posts Registered User Join Date Aug 2002 Posts 351 atoi return error ok, i've https://bytes.com/topic/c/answers/222669-how-can-i-know-whether-atoi-function-call-succeed got one for ya! i've just looked at a refernce for the atoi function in stdlib.h. it states that atoi returns a 0 if it cannot convert the char*. how, then, is "0" supposed to be converted with confirmation? therefore, aoti error checking cannot be used to check for a valid int in a character array. is the best way to achieve this to convert to ascii and test for the int range for each char in the array? or is there a function that can do this? TIA, rotis23 10-30-2002 #2 Magos View Profile View Forum Posts Visit Homepage Confused Join Date Sep 2001 Location Sweden Posts 3,145 You could check every character in the string if it's a number: Code: #define FirstDigit atoi with error 48 bool IsNumeric(char* String) { bool Result = true; for(int i=0; i
FirstDigit + 9)) Result = false; } return Result; } (There might be a standard function doing this already) MagosX.com Give a man a fish and you feed him for a day. Teach a man to fish and you feed him for a lifetime. 10-30-2002 #3 Monster View Profile View Forum Posts Me want cookie! Join Date Dec 2001 Posts 680 Or just use the isdigit function... Code: #include bool IsNumeric(char* String) { char *ptr = String; while(*ptr && isdigit(*ptr)) ptr++; return (*ptr ? false : true); } 10-30-2002 #4 Dave_Sinkula View Profile View Forum Posts Just Lurking Join Date Oct 2002 Posts 5,005 Use strtol instead of atoi. One way to think of atoi might be as follows. Code: int atoi(const char *s) { return (int)strtol(s, NULL, 10); } So if you want the error detection, use strtol but do error handling using the second parameter instead of passing it NULL. 10-30-2002 #5 rotis23 View Profile View Forum Posts Registered User Join Date Aug 2002 Posts 351 i think i might just check manually - using an ascii range. aswell as atoi, strtol returns a 0 if an error occurs. i presume the second parameter will not point to NULL if it finds a non-numerical char. hence, the error check. is isdigit part of standard c? thanks guys. 10-30-2002 #6 Dave_S help? Post your question and get tips & solutions from a community of 418,434 IT Pros & Developers. It's quick & easy. how can i Know whether atoi function call succeed? P: n/a Anonymousgoogledeja Hi all, since the function atof, atoi, _atoi64, atol returned value are Return Values Each function returns the double, int, __int64 or long value produced by interpreting the input characters as a number. The return value is 0 (for atoi and _atoi64), 0L (for atol), or 0.0 (for atof) if the input cannot be converted to a value of that type. The return value is undefined in case of overflow. how can I tell if these function calls fail or succeed? thanks. Baumann@Pan Nov 15 '05 #1 Post Reply Share this Question 15 Replies P: n/a Robert Gamble Anonymousgoogledeja wrote: Hi all, since the function atof, atoi, _atoi64, atol returned value are Return Values Each function returns the double, int, __int64 or long value produced by interpreting the input characters as a number. The return value is 0 (for atoi and _atoi64), 0L (for atol), or 0.0 (for atof) if the input cannot be converted to a value of that type. The return value is undefined in case of overflow. how can I tell if these function calls fail or succeed? _atoi64 is not a standard function but for the rest, you cannot portably tell if they succeed or fail. Some systems may set errno on failure but this is not required. This is a good reason to use strto* functions instead. Robert Gamble Nov 15 '05 #2 P: n/a Jack Klein On 21 Aug 2005 19:55:47 -0700, "Anonymousgoogledeja"
wrote in comp.lang.c: Amazing, you actually printed part of the documentation yet you missed the most important point! Hi all, since the function atof, atoi, _atoi64, atol returned value are Return Values Each function returns the double, int, __int64 or long value produced by interpreting the input characters as a number. The return value is 0 (for atoi and _atoi64), 0L (for atol), or 0.0 (for atof) if the input cannot be converted to a value of that type. The return value is ^^^^^^^^^^^^^^^^^^^ undefined in case of overflow. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ how can I tell if these function calls fail or succeed? Actually the text you quoted it not quite correct. If you call one of the ato... functions and the converted value is outside the range