What does the question mark character ('?') mean?

What does a qu开发者_运维技巧estion mark (?) in C mean?

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? is the first symbol of the ?: conditional operator.

a = (b==0) ? 1 : 0;

a will have the value 1 if b is equal to 0, and 0 otherwise.

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Additionally to other answers, ? can be part of a trigraph.

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This is a ternary Operator which is conditional operator uses like if-else

example

int i=1;
int j=2;
int k;
k= i > j ? i : j;
//which is same as
if(i>j)
  k=i;
else
  k=j;

Usage: Syntax of ?: is

assignment_Variable = Condition ? value_if_true : value_if_false;

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That’s probably a part of the ternary operator:

const int numApples = …;
printf("I have %i apple%s.\n", numApples == 1 ? "" : "s");

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This is a so called conditional operator. You can shorten your if else statement with this operator.

The following link should explain everything

http://www.crasseux.com/books/ctutorial/The-question-mark-operator.html

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It is a conditional operator. For example refer the below link http://en.wikipedia.org/wiki/Conditional_operator

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Its the ternary operator, see http://en.wikipedia.org/wiki/Ternary_operation#C.2C_C.2B.2B.2C_C.23.2C_Objective-C.2C_Java.2C_JavaScript.2C_ActionScript

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Most likely the '?' is the ternary operator. Its grammar is:

RESULT = (COND) ? (STATEMEN IF TRUE) : (STATEMENT IF FALSE)

It is a nice shorthand for the typical if-else statement:

if (COND) {
    RESULT = (STATEMENT IF TRUE);
} else {
    RESULT = (STATEMENT IF FALSE);

as it can usually fit on one line and can improve readability.

Some answers here refer to a trigraph, which is relevant to the C preprocessor. Take a look at this really dumb program, trigraphs.c:

# /* preprocessor will remove single hash symbols and this comment */
int main()
{
    char *t = "??=";
    char *p = "??/"";
    char *s = "??'";
    ??(, ??), ??! ??<, ??>, ??-
    return 0;
}

invoking only the c preprocessor by running gcc -E -trigraphs trigraph.c the output is

int main()
{
 char *t = "#"
 char *p = "\"";
 char *s = "^";
 [, ], | {, }, ~
 return 0;
}

Hopefully that clarifies a little bit what a trigraphs are, and what a '?' "means" in C.