Numpy array replace values by index. It takes three arguments: the condition, the .

Numpy array replace values by index Sep 20, 2021 · I intend to replace the value of specific indices based on an array of indices. This should be superior to using numpy arrays directly especially for very large matrices in both dimensions, if not Create a NumPy ndarray Object NumPy is used to work with arrays. where for loop? Asked 3 years, 7 months ago Modified 3 years, 7 months ago Viewed 2k times Feb 26, 2024 · Introduction In the world of Python data science and numerical computing, NumPy reigns as a foundational package. copyto(arr, vals, where=mask), the difference is that place uses the first N elements of vals, where N is the number of True values in mask, while copyto uses the elements where mask is True. index () to grab the indices but this also takes too much time. Categorical(a The Numpy library in Python comes with a number of useful functions and methods to work with and manipulate the data in arrays. Here is a simple example of I'm trying to do: import numpy as np from Nov 8, 2017 · I want to replace values in a 2D numpy array based on following dictionary in python: code region 334 0 4 22 8 31 12 16 16 17 24 27 28 18 32 21 36 I have a NumPy array like: a = np. Sep 24, 2020 · How to replace each value in the n x n array with column index if value >= 1 otherwise with row index. It allows us to treat lists as arrays and perform efficient element-wise operations. all to check if all the columns have a True value for your comparison, then using the created mask to replace the values: Oct 22, 2014 · I am new to Numpy and want to replace part of a matrix. In example for a list I have a 2D NumPy array. Among its comprehensive Sep 9, 2021 · I have another array which carries the values that I want to replace inside matrix. choice # random. Mar 16, 2021 · I have the value of the new elements in a tuple/numpy array and the indexes of the elements of a that needs to be replaced in another tuple/numpy array. Feb 7, 2023 · Learn, how to replace the values of a numpy array using a very fast technique? Submitted by Pranit Sharma, on February 07, 2023 NumPy is an abbreviated form of Numerical Python. clip() function, the numpy. , `np. In ‘raise’ mode, if an exception occurs the target array may still be modified. Using np. Note that this disables indexing with negative numbers. This will replace the values at indices 1 and 3 with 10 and 20, respectively. array([[[255, 101, The first array returned contains the indices along axis 1 in the original array, the second array contains the indices along axis 2. y[index] = REPLACE_WITH So basically I generate a bunch of numbers from 1 to MAX_SIZE-1, and then I want to replace every REPLACE_EVERY_Nth element with REPLACE_WITH. 6 days ago · NumPy is the cornerstone of numerical computing in Python, empowering efficient manipulation of large, multi-dimensional arrays and matrices. Hence, you should For example, you may want to visit the elements of an array in memory order, but use a C-order, Fortran-order, or multidimensional index to look up values in a different array. [0, 1, -1, 3, 4] for integers or [0, 1, nan, 3, 4] for floating point. Its strength lies in aligning data by *dimensions and labels* rather than just numerical indices, making it easier to handle complex datasets. I have two arrays and one list with indices Array source (array 01): x = np. 30017675, 130. where () The np. str_ or numpy. 4 days ago · This becomes problematic when your array contains multiple elements with the same minimum value—you might miss important indices! In this blog, we’ll explore why `np. array (<<your code comes here>>) Say, you want to replace a few elements (say from index 2 to 5) of the above NumPy array my_orig_arr2 by a value -6, you can do so by following the below steps: my_orig_arr2 [2 : 5] = -6 15 hours ago · NumPy is the backbone of numerical computing in Python, powering everything from data analysis to machine learning. Numpy is a vast library in python which is used for almost every kind of scientific or mathematical operation. array([1, 2, 2, 2]) original_array = np. shape, dtype=np. int32) smaller_array = np. It takes three arguments: the condition, the Sep 2, 2015 · x = numpy. Below are different methods, arranged from most efficient to least efficient. I tried the following. where a powerful tool for Parameters: conditionarray_like, bool Where True, yield x, otherwise yield y. nan_to_num, except in reverse. Oct 16, 2025 · NumPy is a fundamental library in Python for scientific computing. Categorical(a Jul 23, 2012 · x = x[~numpy. It provides an efficient interface for working with large, multi-dimensional arrays and matrices. I found some answers using numpy. Alternative pandas solution: import pandas as pd pd. cols_to_replace = np. Conditional transformation: It performs element-wise operations, selecting values from one array or another based on a condition. These can arise from operations like division by zero, overflow (e. Using Slicing to Update Multiple Array Elements You can also update multiple elements in an array using slicing. argmin` falls short, and walk through step-by-step methods to find **all indices** of minimum values in both Python lists and NumPy arrays. Sep 7, 2019 · I'm trying to understand the fancy indexing using numpy and still facing some problem. In this blog, we’ll explore practical methods to replace NaNs in a NumPy array with their closest non-NaN neighbors. Iterating thorough each item of a numpy array in python is much slower than iterating through each item of a list. Nov 19, 2018 · I try to use a numpy array to replace the data from a pandas DataFrame (more precisely I want to normalize the data and then set the new columns in the existing DataFrame). Unlike slicing, which offers a way to replace a contiguous range of elements, numpy. Aug 7, 2014 · I am trying to replace part of a 2D numpy array named "S" as a function of i and j. Sep 28, 2018 · Changed value in numpy array based on index and on criteria Asked 7 years, 1 month ago Modified 7 years, 1 month ago Viewed 4k times In this case, if the index arrays have a matching shape, and there is an index array for each dimension of the array being indexed, the resultant array has the same shape as the index arrays, and the values correspond to the index set for each position in the index arrays. The original 2d-array is this: A= [[0. Their implementations are different. ma as ma bigger_array = np. Replace value at a specific index in list Create an example list, Aug 4, 2010 · {4: 0, 9: 5, 14: 10, 19: 15, 20: 0, 21: 1, 22: 2, 23: 3, 24: 0} I would like to replace some of the elements according to the map above. put # numpy. The numpy array is really large, and only a small subset of the elements (occurring as keys in the dictionary) will be replaced with the corresponding values. The numpy. What is the fastest way to do this? numpy. isnan() function to check if a value is nan or not) in a Numpy array with ones. By leveraging integer and boolean indexing, you can perform complex data manipulations with ease, making NumPy a powerful tool for data analysis and scientific computing. 47348838]) I need the array values to replace the b column, with the index number remaining the same: Index a Replacing selected elements of a NumPy array by a specific value Create a new array my_orig_arr2 having following elements (2, 9, 17, 13, 1, 4, 20, 57). Method 2: Using np. Y contains true/false. array([[1,2,3], [4,5,6], [7,8,9]]) array([[1, 2, 3], [4, 5, 6], [7, 8, 9]]) Array with new Nov 4, 2013 · I have a large numpy array that I need to manipulate so that each element is changed to either a 1 or 0 if a condition is met (will be used as a pixel mask later). Aug 16, 2019 · No need to create b or c, you can use np. zeros(graph2. Returns: outndarray An array with elements from x where condition is True, and elements from y elsewhere. , netCDF files, climate data, or any dataset with coordinates like time, latitude, or longitude). bytes_. 2. array([10, Sep 25, 2024 · In this example, we create a NumPy array with values from 1 to 5. . The number is likely to change as different arrays are processed because e For example, you may want to visit the elements of an array in memory order, but use a C-order, Fortran-order, or multidimensional index to look up values in a different array. May 16, 2024 · Replacing values in NumPy arrays by index involves specifying the indices of elements you want to replace and assigning new values to them. Oct 9, 2016 · Is there a numpy method which is equivalent to the builtin pop for python lists? Popping obviously doesn't work on numpy arrays, and I want to avoid a list conversion. unique and have it return the inverse: d = np. Importantly this also allows you to do things like: 6 days ago · NumPy is the cornerstone of numerical computing in Python, empowering efficient manipulation of large, multi-dimensional arrays and matrices. Importantly this also allows you to do things like: Mar 18, 2017 · Understand the above 3D array as three slices of 3x3 array, with the top most array having index 0 and center one having index 1 and the bottom one having index 2. The highest value in x[0] is therefore x[0, 1, 2]. Aug 18, 2021 · The NumPy slice assignment operation doesn’t need the same shape on the left and right side because NumPy will use broadcasting to bring the array-like data structure providing the replacement data values into the same shape as the array to be overwritten. So here is an example of what I'd like to do. Sep 27, 2025 · In NumPy, you can access or modify array elements at specific indices. ‘clip’ mode means that all indices that are too large are replaced by the index that addresses the last element along that axis. array([[0,1,2,3,4,5],[0,-1,2,3,-4,-5],[0,-1,-2,-3,4,5]]) I'd like to scan all the columns of x and replace the values of each column if they are equal to a specific value. array([1,2,3,4,5]) # a is a subset of row: a = np. However, during data preprocessing or mathematical operations, you may encounter a common issue: **positive infinity (`inf`) values** in your NumPy arrays. 0), along a given axis (e. Whether you choose to filter, replace, or delete NaN values, the choice depends on the specific requirements of your operation and the integrity of your dataset. random. Using numpy array When dealing with larger datasets or arrays, numpy provides a powerful alternative for replacing index elements. 20181675, 109. Here’s a concise definition from Wes McKinney: This practice of replacing explicit loops with array expressions is commonly referred to as vectorization. This section is just an overview of the various options and issues related to indexing. put() enables you to replace elements at arbitrary positions with a single function call. Apr 8, 2015 · Replace slice of a numpy array with values from another array Asked 10 years, 7 months ago Modified 10 years, 7 months ago Viewed 9k times Oct 9, 2025 · If elements in a NumPy array don’t meet a certain condition, you replace them with another value (like 0, -1, or NaN), while keeping the elements that satisfy the condition unchanged. The actual size of the numpy array is 514 by 504 and of the list is 8. Is there a simple way of replacing all negative values in an array with 0? I'm having a complete block on how to do it using a NumPy array. Index cannot be "sliced indexed" I need to first copy the index/column array, replace some of the elements in the copy based on the mask and then have the setter set the complete new value. I am looking for some sort of replace(old, new) function. my_orig_arr2 = np. A common task in data preprocessing, analysis, or cleaning is modifying specific elements of an array—*especially* when you need to target elements **not** in a predefined list of indices. S. The key point is: if the index arrays have a matching shape, and there is an index array for each dimension of the array being indexed, the resultant array has the same shape as the index arrays, and the values correspond to the index set for each position in the index arrays. where() is an incredibly powerful function that acts as a conditional selector for NumPy arrays. Whether you're filtering data, replacing values, or handling missing values, where() provides an efficient alternative to traditional loops. minimum() function, and the indexing method. How do I replace all values in it greater than a threshold T = 255 with a value x = 255? A slow for-loop based method would be: Dec 5, 2024 · Explore effective ways to replace elements in a NumPy array that exceed a specified threshold, optimizing performance and maintaining data integrity. int32) Now, I only want the first three values of the smaller array to replace those of a certain section of the bigger array, but masking doesn't do what I'd hoped it would do: As pd. For example, Try it in your browser! import numpy as np import numpy. I want to only replace values In this case, if the index arrays have a matching shape, and there is an index array for each dimension of the array being indexed, the resultant array has the same shape as the index arrays, and the values correspond to the index set for each position in the index arrays. The assignment operator = then assigns 1. isnan(x)] Explanation The inner function numpy. However, I want to replace all 1's by 0 and all -1's by 1. put() function allows you to replace elements in an array at specified indices with new values. Jun 22, 2021 · Array indexing refers to any use of the square brackets ( []) to index array values. The following is the syntax – Jul 23, 2012 · x = x[~numpy. What is the np. Jul 12, 2025 · Finally, the list res is printed, which contains the elements from a ordered according to the indices in b. place # numpy. ones((2,2), dtype=np. -1 like in the example), I would just create a list or an array from the dictionary once where the keys are the array indices and then use that for an efficient Numpy fancy indexing routine. Index the same ndarray multiple times efficiently # It must be kept in mind that basic indexing produces views and advanced indexing produces copies, which are computationally less efficient. Oct 16, 2017 · 3 I want to replace the values in a given numpy array (A) at a given index (e. What is Vectorization? Vectorization is a powerful ability within NumPy to express operations as occurring on entire arrays rather than their individual elements. However, real-world data is rarely perfect—you’ll often encounter `NaN` (Not a Number) and `Inf` (Infinity) values in NumPy matrices. x, yarray_like Values from which to choose. Steps to set all the zeros to nan in Numpy You can use boolean indexing to set all the zeros in a Numpy array to nan. zeros((4,4), dtype=np. Try it in your browser! See full list on pythonguides. array([[-1,1,-1],[-1,1,1]]) My array only contains two different values: -1 and 1. A common task in data preprocessing, analysis, or cleaning is modifying specific elements of an array— especially when you need to target elements not in a predefined list of indices. Hence, you should Feb 2, 2024 · There are three main methods that can be used to replace values inside a NumPy array in Python, the numpy. This will work for sure, but I was looking for a more convenient way to do it. Apr 2, 2015 · I want to find and replace multiple values in an 1D array / list with new ones. array( Parameters: conditionarray_like, bool Where True, yield x, otherwise yield y. 5) finds the indices for which the conditions are True and & combines them. For example, I have two matrices, A, B generated by numpy Feb 4, 2025 · numpy. int8) May 8, 2019 · Replace values in a 3d numpy array Asked 6 years, 6 months ago Modified 6 years, 6 months ago Viewed 10k times May 28, 2018 · How to replace values of an array with another array using numpy in Python Asked 6 years, 11 months ago Modified 6 years, 11 months ago Viewed 4k times Apr 29, 2025 · Learn how to create a 2D NumPy array and use boolean indexing to replace elements that meet a certain condition with a specified value. The left and right operands don’t have the Nov 24, 2021 · @hpaulj Yes, this is because you add arrays of different shapes, so the numpy. 0 to the values on those indices. To get the indices of each maximum or minimum value for each (N-1)-dimensional array in an N-dimensional array, use reshape to reshape the array to a 2D array, apply argmax or argmin along axis=1 and use unravel_index to recover the index of the values per slice: The thing is, I have a 2d numpy array and I'd like to replace some of its values at random positions. unique(a, return_inverse = True)[1] >>> d array([1, 0, 1, 2, 1]) For reference: return_inverse : bool, optional If True, also return the indices of the unique array (for the specified axis, if provided) that can be used to reconstruct ar. where () function in NumPy provides a concise way to replace elements based on a condition. The resulting array will be [1, 2, 3, 0, 0]. choice(a, size=None, replace=True, p=None) # Generates a random sample from a given 1-D array I have three numpy arrays: row = np. E. e 1 and 3? Then, I want to put another values instead of them. The array object in NumPy is called ndarray. 0. put () function allows you to insert values into an array at designated positions, supporting all array dimensions (1-D, 2-D, 3-D). I need to replace certain numbers (given in a list) with specific alternatives (also given in a corresponding list) I wrote the following code (which seems to For example, you may want to visit the elements of an array in memory order, but use a C-order, Fortran-order, or multidimensional index to look up values in a different array. Follow our step-by-step guide. Apr 6, 2022 · How to replace values in array numpy using np. -2) with a given value (e. com Mar 15, 2017 · Efficient numpy arrays are of uniform type; thus it's better to use a sentinel value, e. The next example shows how to replace every other value of a 1D array with the same value. For example: Suppose the condition is “keep only numbers greater than 3”. Jan 23, 2024 · In this tutorial, we’ve learned how to replace elements in NumPy arrays based on conditions using a variety of methods. Given S as: I have an array: ([ 137. In this tutorial, we will look at how to set all the zeros in a Numpy array to nan with the help of some examples. A multidimensional vector in numpy is contiguous while python treats them as a list of lists. This dual functionality makes np. Those indices inside [ ] acts as the input indices for arr[ ] and gives you the elements for which the condition holds. isnan returns a boolean/logical array which has the value True everywhere that x is not-a-number. In general, vectorized array operations will often be one or two (or more Mar 1, 2024 · NumPy offers several efficient ways to remove or replace NaN values in arrays, ranging from simple to more advanced techniques. The result is the indices at which the values fulfill these conditions. Oct 6, 2014 · I think the best answer to your particular question is to use a matrix type object. Is it possible to modify the numpy. 0) and (arr<1. The following examples demonstrate replacing a value at a specific index in a list or array in Python. place(arr, mask, vals) [source] # Change elements of an array based on conditional and input values. Below is an example of using python to do what I want. choice function in order to make it return the index of the chosen element? Basically, I want to create a list and select elements randomly without replac numpy. But since there are negative values, too, this won't work. where function in NumPy serves two primary purposes: 1. One common operation when working with NumPy arrays is replacing specific values within the array. It takes three arguments: the condition, the The first array returned contains the indices along axis 1 in the original array, the second array contains the indices along axis 2. argwhere(s == matrix) for s in substitutes] Then I do: 15 hours ago · NumPy is the cornerstone of numerical computing in Python, enabling efficient manipulation of large arrays and matrices. Dec 29, 2020 · a = array([[ 5, 3, 2, 0], [ 0, 4, 6, 7], [ 1, 2, 7, 3]]) Please note, in real application my arrays are large, over 30k elements and several thousands of them. put is roughly equivalent to: In this case, if the index arrays have a matching shape, and there is an index array for each dimension of the array being indexed, the resultant array has the same shape as the index arrays, and the values correspond to the index set for each position in the index arrays. This method is straightforward and easy to read. Oct 26, 2015 · In a given array I want to replace the values by the index of this value in an other array (which doesn't contain duplicates). However, one common frustration for users is encountering the Aug 19, 2021 · a = np. Even better if the replaced value would map into other 1d array and return the value from it. Of course I can loop over my array, check the value of every field and replace it. I have tried to solve the problem I have odd -ve values from given array need to understand how to replace +v String functionality # The numpy. Lastly, we use this logical array to index into the original array x, in order to retrieve just the non-NaN values. x, y and condition need to be broadcastable to some shape. Jun 12, 2012 · How can I remove some specific elements from a numpy array? Say I have import numpy as np a = np. substitutes = np. find_nearest(array, value) 2 days ago · Xarray is a powerful Python library for working with labeled multi-dimensional arrays (e. Jun 5, 2012 · You can achieve this with a much higher performance using np. nanmean and np. Feb 15, 2023 · NumPy Array - Replace NaN values with average of columns Suppose that we are given a numpy ndarray that contains some nan values, we need to replace each nan value with the average of its column. array([[[255, 101, For example, you may want to visit the elements of an array in memory order, but use a C-order, Fortran-order, or multidimensional index to look up values in a different array. 2 days ago · Xarray is a powerful Python library for working with labeled multi-dimensional arrays (e. Jul 23, 2025 · Explanation: We accessed the element at index 2 (which is 3) and assigned a new value 10 to it. Although NumPy doesn't have a built - in `replace` function like some other data manipulation libraries, we can achieve value ValueError: assignment destination is read-only I guess that's because I can't really write into a numpy array. This code for example aims to replace all the negative values (where the value is equal to the column number) to 100: Mar 3, 2021 · Out[15]: array([[4, 5, 6], [8, 5, 4]]) Question: Is there a pythonic way to conditionally replace elements in the array with a number from their index value? For example, how could I replace all the even numbers with their row number? I can accomplish this rather clumsily with a separate array of just row numbers as follows. Feb 20, 2024 · An intuitive way to replace values in a Numpy array is through basic indexing, which involves specifying conditions for which indices to replace. We then use boolean indexing to select the elements that are greater than 3 and assign them a new value of 0. Can anyone help me in identifying first the In this case, if the index arrays have a matching shape, and there is an index array for each dimension of the array being indexed, the resultant array has the same shape as the index arrays, and the values correspond to the index set for each position in the index arrays. It is used for different types of scientific operations in python. There are many options to indexing, which give NumPy indexing great power, but with power comes some complexity and the potential for confusion. May 26, 2017 · @JammyDodger A bit late, but numpy "arrays" are represented as a contiguous 1D vector in memory while python "arrays" are just lists. All I know is the index of the valu Nov 14, 2025 · For example, in a time series of temperature readings, a missing value at 2:00 PM might be best replaced with the closest reading at 1:59 PM or 2:01 PM. Array A contains indices for rows in X that need replacement, with the value -1. Aug 17, 2017 · I have an unsorted array of numbers. Unfortunately this does not create a view on the original array so I can not replace its values. 0), equivalently: A[:,:,0,:]=0 Problem is the input array A may come in as 3D or 4D or other shapes, so for 3D data I would need A[:,0,:]=0 If it's 5D: A[:,:,:,0,:]=0 Currently I'm using an exec() to get The first array returned contains the indices along axis 1 in the original array, the second array contains the indices along axis 2. array([ 0, 1, 2, 3, 4, 5, 6, 7, 8, 10, 11, 13, 14, 16, 17, 18, 19, 21, 22]) Find the indices of each of the substitutes inside matrix (multiple occurrences are possible): indices = [np. However, one common frustration for users is encountering the Jul 15, 2022 · Assuming the indices to skip are few, why not create a new numpy array of zeros and fill the values from the original array like: newgraph = np. One practical solution is to replace NaN values with the average of their respective columns. There are about 8 million element May 3, 2023 · Numpy array — sometimes we want to filter or apply some conditions to filter some values for a Numpy array. put(a, ind, v, mode='raise') [source] # Replaces specified elements of an array with given values. This method is useful if you want to replace a range of elements in the array. Hence, you should I have three numpy arrays: row = np. Sep 27, 2025 · In real-world datasets, missing values (NaN) are common and can cause errors in calculations or visualizations. These values can break calculations (e. Oct 22, 2018 · (arr>1. It is itself an array which is a May 13, 2024 · In Python, lists and numpy arrays are mutable and you can modify list or array elements directly by updating the indexes. Whether your Oct 22, 2017 · If the map didn't contain negative values (e. Mar 18, 2017 · Understand the above 3D array as three slices of 3x3 array, with the top most array having index 0 and center one having index 1 and the bottom one having index 2. Mar 23, 2016 · The addition will keep the non-zero elements as they are, logical_and will create a mask that multiplied by a constant will add 0 values where the array is non-zero and CONSTANT otherwise. Use boolean indexing to replace all instances of NaN (use numpy. Apr 2, 2010 · How do I find the nearest value in a numpy array? Example: np. mean()` returns `NaN` if the data contains `NaN`), crash models, or skew results. We can create a NumPy ndarray object by using the array() function. 55021238, 125. I want to replace certain columns in a multi-dimensional array with the value that I have. Parameters: conditionarray_like, bool Where True, yield x, otherwise yield y. It provides a high - performance multidimensional array object and tools for working with these arrays. Understanding how to manipulate NumPy arrays is vital for any task involving numerical data in Python. arange(30) I know that I can replace the values located at positions indices=[2,3,4] using for instance fancy indexing: a[indices] = 999 But how to replace the v The key point is: if the index arrays have a matching shape, and there is an index array for each dimension of the array being indexed, the resultant array has the same shape as the index arrays, and the values correspond to the index set for each position in the index arrays. I have also tried similar iterative methods, and using list. array ( [1,2,3,4,5,6,7,8,9]) I then want to remove 3,4,7 from a. Here is example: import numpy as np array = np. The indexing works on the flattened target array. I have an array: ([ 137. I have tried for loops but these take a long time to compute. numpy. Since we want the opposite, we use the logical-not operator ~ to get an array with True s everywhere that x is a valid number. How do I do this efficiently in NumPy? May 8, 2014 · I two numpy arrays, both M by N. put is roughly equivalent to: is it possible to get values from numpy array based on list of indexes like i. array must resort the python -like behavior, which is exactly storing numbers in list objects. Finding indices: It returns the indices of elements in an array where a specified condition is True. A sparse matrix should be the most performant in terms of storing large numbers of these matrices of large sizes in a memory friendly way, given that most of the matrix is populated with zeroes. X contains random values. , `1e309` in float64), or incorrect data @JoshLee - True, but using general list processing techniques on numpy arrays is very inefficient. g. choice to create a mask for the array. Jul 23, 2025 · These examples illustrate how advanced indexing techniques in NumPy can be used to change specific values in an array efficiently. where Function in NumPy? The np. Either remove or replace values with another number/data. After updating, the array reflects the change and the element at index 2 is now 10. strings module provides a set of universal functions operating on arrays of type numpy. P. take This vectorized approach uses NumPy functions to calculate column means while I am looking to replace a number with NaN in numpy and am looking for a function like numpy. Similar to np. array([1, 5]) # b is an array that I use to change some elements in the first row array: b = np. nguffxb wztp igcdje zracey ntdkaa usapcb gozpd ybwqe gdgfwt mjil uxij vdopj kwhqn rtmbfe esl