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else if statement in C

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

The construction

 if (expression)
 statement
 else if (expression)
 statement
 else if (expression)
 statement
 else if (expression)
 statement
 else
 statement

occurs so often that it is worth a brief separate discussion. This sequence of if statements is the most general way of writing a multi-way decision. The expressions are evaluated in order; if an expression is true, the statement associated with it is executed, and this terminates the whole chain. As always, the code for each statement is either a single statement, or a group of them in braces.

The last else part handles the ``none of the above'' or default case where none of the other conditions is satisfied. Sometimes there is no explicit action for the default; in that case the trailing

 else
 statement

can be omitted, or it may be used for error checking to catch an ``impossible'' condition.

To illustrate a three-way decision, here is a binary search function that decides if a particular value x occurs in the sorted array v. The elements of v must be in increasing order. The function returns the position (a number between 0 and n-1) if x occurs in v, and -1 if not.

Binary search first compares the input value x to the middle element of the array v. If x is less than the middle value, searching focuses on the lower half of the table, otherwise on the upper half. In either case, the next step is to compare x to the middle element of the selected half. This process of dividing the range in two continues until the value is found or the range is empty.

 /* binsearch: find x in v[0] <= v[1] <= ... <= v[n-1] */
 int binsearch(int x, int v[], int n)
 {
 int low, high, mid;
 low = 0;
 high = n - 1;
 while (low <= high) {
 mid = (low+high)/2;
 if (x < v[mid])
 high = mid + 1;
 else if (x > v[mid])
 low = mid + 1;
 else /* found match */
 return mid;
 }
 return -1; /* no match */
 }

The fundamental decision is whether x is less than, greater than, or equal to the middle element v[mid] at each step; this is a natural for else-if.

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