使用值的 NSArray 有效地创建占位符模板 NSString [英] Efficiently create placeholder template NSString with NSArray of values

问题描述

我正在尝试为 FMDB 制作一个实用方法，该方法将采用 NSArray 值并返回一串占位符，以根据数组中的值数量在 IN 语句中使用.

I am trying to make a utility method for FMDB which will take an NSArray of values and return a string of placeholders for use in an IN statement based on the number of values in the array.

我想不出一种优雅的方式来创建这个字符串，我是不是缺少一些 NSString 实用方法:

I can't think of an elegant way of creating this string, am I missing some NSString utility method:

// The contents of the input aren't important.
NSArray *input = @[@(55), @(33), @(12)];    

// Seems clumsy way to do things:
NSInteger size = [input count];
NSMutableArray *placeholderArray = [[NSMutableArray alloc] initWithCapacity:size];
for (NSInteger i = 0; i < size; i++) {
    [placeholderArray addObject:@"?"];
}

NSString *output = [placeholderArray componentsJoinedByString:@","];
// Would return @"?,?,?" to be used with input

推荐答案

这个怎么样?

NSArray *input = @[@(55), @(33), @(12)];

NSUInteger count = [input count];
NSString *output = [@"" stringByPaddingToLength:(2*count-1) withString:@"?," startingAtIndex:0];
// Result: ?,?,?

stringByPaddingToLength 填充给定的字符串(在本例中为空字符串)通过从模式 @"?," 附加字符到给定的长度.

stringByPaddingToLength fills the given string (the empty string in this case) to the given length by appending characters from the pattern @"?,".

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