CODEWITHSUNDEEP
c++strcmp
JayKay
JayKay

Reputation: 183

If statement with negative values

In an if statement like this:

    if(strcmp(str1,str2))

strcmp() can return a negative value, and if it does, does the if statement consider it to be TRUE or FALSE.

Upvotes: 0

Views: 8572

Answers (4)

Cory Gross
Cory Gross

Reputation: 37206

The strcmp function when used with an if statement can lead to some very unreadable code. The following statement says to call foo if and only if str1 is different than str2.

if (strcmp(str1, str2)) foo();

Some would argue this is somewhat unexpected and unreadable, but it's due to the fact that strcmp is not really meant to be used this way as it does not return a bool. If you check out this great reference you'll see that it returns an integral value which is meant to indicate the relationship between the strings. strcmp can tell you much more than simply whether or not two strings are the same. According to the reference strcmp returns:

  • 0 if both strings are equal
  • A positive value if the first character that does not match has a greater value in str1 than in str2
  • A negative value if the first character that does not match has a greater value in str2 than in str1

Because an if statement will coerce any non-zero integer value into a boolean value of true the if statement if (strcmp(str1, str2)) foo() will always interpret true and execute foo, except when str1 and str2 are equal (in which case the if statement will interpret false and foo will not be executed).

The more common way to use strcmp with an if statement is to combine your call to strcmp with a binary comparison within the if statement, testing against an integral value:

if (strcmp(str1, str2) == 0) foo(); // foo executed iff str1 and str2 are equal
if (strcmp(str1, str2) != 0) foo(); // foo executed iff str1 and str2 are not equal

You might also use strcmp with an if statement to sort strings, strcmp is ideal for this because it returns based on the first unmatching character of the string. You could use it with something like the following (untested code):

bool swapped = false;
do {
    for (i = 1; i < numStrings; i++) {
        if (strcmp(str[i-1], str[i]) > 0) {
            swap(i-1, i);
            swapped = true;
        }
    }
} while (swapped);

Upvotes: 1

Mats Petersson
Mats Petersson

Reputation: 129454

C converts any non-zero value to true and only zero is false.

Note that if(strcmp(str1, str2)) is NEARLY always "wrong". You either want one of:

  • if (strcmp(str1, str2) == 0) to detect that two strings are equal
  • if (strcmp(str1, str2) > 0) to detect that str1 > str2
  • if (strcmp(str1, str2) < 0) to detect that str1 < str2

The only reason for if(strcmp(str1, str2)) is for detecting that str1 is different from str2 - which is nearly always not what you want.

And of coruse, in C++, you probably shouldn't be using strcmp at all - since you have std::string that you can just write what you want as a comparison, e.g. if (str1 > str2) or if (str1 != str2), and it's immediately obvious to most people what it means without further questions.

Upvotes: 0

Pete Becker
Pete Becker

Reputation: 76418

When an integer value is converted to a Boolean value, 0 is false and any other value is true.

Upvotes: 0

Greg Hewgill
Greg Hewgill

Reputation: 993861

In C++, the if statement treats any nonzero value as true. A negative value is not zero, so it will be considered true. The following two statements are equivalent:

if (strcmp(str1, str2))

if (strcmp(str1, str2) != 0)

Upvotes: 12

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