C - Check if given argument is natural number

Question

How should I correctly check if given argument is a natural number in C? I am a very beginner when it comes to C... I already have compared that argument to 1 and 2 by atoi(argv[1]) == 1 ..., but when I pass let's say 1.2137 as an argument, atoi cuts it to 1. Thanks for any help.

`atoi` stands for "Ascii to Integer", meaning only whole numbers, if you want floating point numbers, you can use `atof` — Matthew, Dec 06 '20 at 15:57
Or better yet, [`strtod`](https://en.cppreference.com/w/c/string/byte/strtof) for a function with some validation. — Some programmer dude, Dec 06 '20 at 15:57
That's right, but I do not want floating point numbers, just want to check if it is not a float — siwakw7, Dec 06 '20 at 15:58
[strtol()](http://www.cplusplus.com/reference/cstdlib/strtol/) si a more advanced function that provides the capability to check the input. — Roberto Caboni, Dec 06 '20 at 15:58
Alternatively you can traverse `argv[1]` string making sure that all characters are digits (use `isdigit()`). Then you will discard input transalated to 0 by `atoi()` — Roberto Caboni, Dec 06 '20 at 16:01

score 1 · Accepted Answer · answered Dec 06 '20 at 16:33

1

You can either use long strtol(const char* nptr, char** endptr, int base) from the header stdlib.h to check if your whole string can be converted to a number:

char* end;
strtol( argv[1], &end, 10 );
if( *end == '\0' ){
    printf( "String is a natural number\n" );
} else {
    printf( "String is not a natural number\n" );
}

Another way would be to check for characters that are not '+', '-' or digits

bool valid = true;
for( const char* it = argv[1]; *it; ++it ){
    if(!(( *it >= '0' && *it <= '9' ) || *it == '+' || *it == '-' )){
        valid = false;
    }
}
printf( "String is%s a natural number\n", valid ? "" : " not" );

answered Dec 06 '20 at 16:33

Orcthanc

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1
6

Note that `strtol()` skips leading whitespace, so if the OP wants to reject strings with leading whitespace they would need to supplement `strtol()` with a test for that. – John Bollinger Dec 06 '20 at 16:46
Negative integer numbers don't belong to the _natural numbers_ set, which is made up only by positive integer numbers. – Roberto Caboni Dec 06 '20 at 17:04

C - Check if given argument is natural number

1 Answers1