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Conditional Expressions in C

By: Emiley J in C Tutorials on 2007年09月20日 [フレーム]

The statements

 if (a > b)
 z = a;
 else
 z = b;

compute in z the maximum of a and b. The conditional expression, written with the ternary operator ``?:'', provides an alternate way to write this and similar constructions. In the expression

 expr₁ ? expr₂ : expr₃

the expression expr₁ is evaluated first. If it is non-zero (true), then the expression expr₂ is evaluated, and that is the value of the conditional expression. Otherwise expr₃ is evaluated, and that is the value. Only one of expr₂ and expr₃ is evaluated. Thus to set z to the maximum of a and b,

 z = (a > b) ? a : b; /* z = max(a, b) */

It should be noted that the conditional expression is indeed an expression, and it can be used wherever any other expression can be. If expr₂ and expr₃ are of different types, the type of the result is determined by the conversion rules discussed earlier in this chapter. For example, if f is a float and n an int, then the expression

 (n > 0) ? f : n

is of type float regardless of whether n is positive.

Parentheses are not necessary around the first expression of a conditional expression, since the precedence of ?: is very low, just above assignment. They are advisable anyway, however, since they make the condition part of the expression easier to see.

The conditional expression often leads to succinct code. For example, this loop prints n elements of an array, 10 per line, with each column separated by one blank, and with each line (including the last) terminated by a newline.

 for (i = 0; i < n; i++)
 printf("%6d%c", a[i], (i%10==9 || i==n-1) ? '\n' : ' ');

A newline is printed after every tenth element, and after the n-th. All other elements are followed by one blank. This might look tricky, but it's more compact than the equivalent if-else. Another good example is

 printf("You have %d items%s.\n", n, n==1 ? "" : "s");

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Conditional Expressions in C

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