Question

我有一个声明的列表：

lst = [-1,6,4,13]

并且我想过滤值大于0且小于10的列表元素。我写道：

filterList :: [Int] -> [Int]
filterList lst = filter (>0 && <10) lst

当我运行：t过滤器时，我可以看到：

filter (a -> Bool) -> [a] -> [a]

但是当我运行编译器时，显示的错误是由于filter子句中的“ <10”。这不是我指定要过滤的值大于零且小于10的正确方法吗？

Answer 1

filterList lst =过滤器（> 0 && <10）lst

问题在于，这不是Haskell中的有效表达式。但是，即使我们添加括号来消除解析错误：

filterList lst = filter ((>0) && (<10)) lst -- no parse errors, but types are not matched

我们将收到错误消息，指出类型不匹配，这是正确的，因为&&期望两个参数都为Bool，但是(> 0)和(< 10)是功能Integer -> Bool。

函数似乎缺少参数，但是我们可以提供一些参数：

filterList lst = filter (\x -> (x > 0) && (x < 10)) lst

Answer 2

您可以使用列表推导。

 private void uploadFile() {
    final ProgressDialog progressDialog = new ProgressDialog(this);
    progressDialog.setTitle("Uploading...");
    progressDialog.show();
    Uri audioUri = audioFileUri.fromFile(new File(savepath)); // URI for audio file
    Uri fx = audioFileUri.fromFile(new File(fxpath)); // URI for audio effects


    if (audioFileUri != null) {
        StorageReference imageReference = storageRef.child("images").child(System.currentTimeMillis() + "");
        StorageReference audioRef = storageRef.child("audio").child(System.currentTimeMillis() + ".3pg"); // storage location to firebase.
        StorageReference fxRef = storageRef.child("effects").child(fxpath + ".mp3"); // storage location to firebase


        // Upload for attach effects audio file
        fxRef.putFile(fx).addOnSuccessListener(new OnSuccessListener<UploadTask.TaskSnapshot>() {
            @Override
            public void onSuccess(UploadTask.TaskSnapshot taskSnapshot) {
                progressDialog.dismiss();
                Toast.makeText(getApplicationContext(), "Audio Effect Uploaded! ", Toast.LENGTH_LONG).show();
            }
        }).addOnProgressListener(new OnProgressListener<UploadTask.TaskSnapshot>() {
            @Override
            public void onProgress(UploadTask.TaskSnapshot taskSnapshot) {
                double progress = (100.0 * taskSnapshot.getBytesTransferred())/ taskSnapshot.getTotalByteCount();
                System.out.println("Upload is " + progress + " % done");
            }
        }).addOnPausedListener(new OnPausedListener<UploadTask.TaskSnapshot>() {
            @Override
            public void onPaused(UploadTask.TaskSnapshot taskSnapshot) {
                System.out.println("Upload is paused");
            }
        }).addOnFailureListener(new OnFailureListener() {
            @Override
            public void onFailure(@NonNull Exception e) {
                Toast.makeText(getApplicationContext(), "Audio Effect Failed! ", Toast.LENGTH_LONG).show();
            }
        });

        // Upload attach audio file
        audioRef.putFile(audioUri).addOnSuccessListener(new OnSuccessListener<UploadTask.TaskSnapshot>() {
            @Override
            public void onSuccess(UploadTask.TaskSnapshot taskSnapshot) {
                progressDialog.dismiss();
                Toast.makeText(getApplicationContext(), "Audio Uploaded!", Toast.LENGTH_LONG).show();
            }
        }).addOnFailureListener(new OnFailureListener() {
            @Override
            public void onFailure(@NonNull Exception e) {
                progressDialog.dismiss();
                Toast.makeText(getApplicationContext(), e.getMessage(), Toast.LENGTH_LONG).show();
            }
        });

        // Upload for Image
        imageReference.putFile(audioFileUri)
                .addOnSuccessListener(new OnSuccessListener<UploadTask.TaskSnapshot>() {
                    @Override
                    public void onSuccess(UploadTask.TaskSnapshot taskSnapshot) {
                        progressDialog.dismiss();
                        Toast.makeText(getApplicationContext(), "File Uploaded ", Toast.LENGTH_LONG).show();
                    }
                }).addOnFailureListener(new OnFailureListener() {
            @Override
            public void onFailure(@NonNull Exception e) {
                progressDialog.dismiss();

                //and displaying error message
                Toast.makeText(getApplicationContext(), e.getMessage(), Toast.LENGTH_LONG).show();
            }
        }).addOnProgressListener(new OnProgressListener<UploadTask.TaskSnapshot>() {
            @Override
            public void onProgress(UploadTask.TaskSnapshot taskSnapshot) {
                double progress = (100.0*taskSnapshot.getBytesTransferred()/taskSnapshot
                        .getTotalByteCount());
                progressDialog.setMessage("Uploading Image File"+(int)progress+"%");
            }
        });
    } else {
        progressDialog.dismiss();
        Toast.makeText(getApplicationContext(), "No File Selected.", Toast.LENGTH_LONG).show();
    }
}

或者两次内置的过滤器功能。

[x | x <- lst, x<10, x>0]

根据值范围过滤列表元素

2 个答案: