CODEWITHSUNDEEP
carraysstringcomparisonargv
Roger1988
Roger1988

Reputation: 11

Fail to check the second character in argv[] in C

I would like to check the arguments in argv[], but it fails to check the second character. For example,

I can do that:

int main(int argc, char *argv[]){
  if (*argv[1] == "A")
    printf("Hello: %s\n", argv[1]);
}

However, I can't check argv[1] when I change "A" to "AB" like this:

if (*argv[1] == "AB")
  printf("Hello: %s\n", argv[1]);
}

Upvotes: 0

Views: 1085

Answers (4)

Adam Zalcman
Adam Zalcman

Reputation: 27233

On this line:

if (*argv[1] == "AB")

you're comparing a char to char*. These are different types. Also, even if the first operand was char* you should still not use == to compare strings since it merely compares the pointer values. Use strncmp() instead

if (strncmp("AB", argv[1], 2) == 0)

This condition is true if two first characters of argv[1] are "AB", e.g. if argv[1] is "ABC". If you want to check that argv[1] is exactly "AB" use strcmp() like this

if (strcmp("AB", argv[1]) == 0)

Note that it is fine to use == to compare single characters:

if (argv[1][0] == 'A')

Also, before you assume argv[1] is valid you should check argc to ensure you have actually been given an argument.

Upvotes: 1

Michael Krelin - hacker
Michael Krelin - hacker

Reputation: 143099

You should stop comparing pointers and characters, this isn't healthy.

You can do *argv[1]=='A'. And when it comes two two characters, you should use strncmp.

Upvotes: 0

asaelr
asaelr

Reputation: 5456

*argv[1] is a char. you should not compare it to string (char*). If you want you check if it's A, do if (*argv[1]=='A') If you want to check the whole argument, do it like strcmp(argv[1],"AB")

Upvotes: 0

unwind
unwind

Reputation: 399863

Strings are compared with strcmp() in C, almost never with ==:

if(strcmp(argv[1], "AB") == 0)
  printf("the second argument is AB\n");

Note that 0 is returned when the compared strings are equal.

Upvotes: 3

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