NSString 遍历字符串的每一个字符并替换相关的字符

主要方法：

• 获取子字符串：substringWithRange:
• 替换字符串：stringByReplacingCharactersInRange:range

for (int i = 0; i<[string length]; i++) {
    //截取字符串中的每一个字符
    NSString *s = [string substringWithRange:NSMakeRange(i, 1)];
    NSLog(@"string is %@",s);
    if ([s isEqualToString:@"m"]) {
        NSRange range = NSMakeRange(i, 1);
        //将字符串中的“m”转化为“w”
        string =   [string stringByReplacingCharactersInRange:range withString:@"w"];
        
    }
}
NSLog(@"%@",string);