Python: how to find value in list smaller than target

Question

For example I have a non-ordered list of values [10, 20, 50, 200, 100, 300, 250, 150]

I have this code which returns the next greater value:

def GetNextHighTemp(self,  temp,  templist):
    target = int(temp)
    list = []
    for t in templist:
        if t != "":
            list.append(int(t))
    return str(min((abs(target - i), i) for i in list)[1])

e.g. If temp = 55, it will return '100'.

But how can I get the lesser of the value? That is how to get it to return '50'?

Thank you.

EDIT - now working

def OnTWMatCurrentIndexChanged(self):
    self.ClearTWSelectInputs()
    material = self.cb_TW_mat.currentText()
    temp = self.txt_design_temp.text()
    if material != "":
        Eref = self.GetMaterialData(material,  "25",  "elast")
        if Eref and Eref != "":
            Eref = str(float(Eref) / 1000000000)
            self.txt_TW_Eref.setText(Eref)
        else:
            self.txt_TW_Eref.setText("194.8")
            self.ShowMsg("No temperature match found for E<sub>ref</sub> in material data file. Value of 194.8 GPa will be used.",  "blue")
    if material != "" and temp != "":
        if self.CheckTWTemp(material,  temp):
            dens = self.GetMaterialData(material,  temp,  "dens")
            self.txt_TW_dens.setText(dens)
            elast = self.GetMaterialData(material,  temp,  "elast")
            elast = str(float(elast) / 1000000000)
            self.txt_TW_Et.setText(elast)
            stress = self.GetMaterialData(material,  temp,  "stress")
            stress = str(float(stress) / 1000000)
            self.txt_TW_stress_limit.setText(stress)
        else:
            self.ShowMsg("No temperature match found for " + temp + "&#x00B0; C in material data file. Extrapolated data will be used where possible or add new material data.",  "blue")
            dens = self.GetExtrapolatedMaterialData(material,  temp,  "dens")
            self.txt_TW_dens.setText(dens)
            elast = self.GetExtrapolatedMaterialData(material,  temp,  "elast")
            elast = str(float(elast) / 1000000000)
            self.txt_TW_Et.setText(elast)
            stress = self.GetExtrapolatedMaterialData(material,  temp,  "stress")
            stress = str(float(stress) / 1000000)
            self.txt_TW_stress_limit.setText(stress)
    else:
        self.ClearTWSelectInputs()

def CheckTWTemp(self, matvar, tempvar):
    for material in self.materials:
        if material.attrib["name"] == matvar:
            temps = material.getiterator("temp")
            for temp in temps:
                if int(temp.text) == int(tempvar):
                    return True
            return False

def GetMaterialData(self, matvar, tempvar, tag):
    for material in self.materials:
        if material.attrib["name"] == matvar:
            temps = material.getiterator("temp")
            for temp in temps:
                if temp.text == tempvar:
                    value = temp.find(tag)
                    return value.text

def GetExtrapolatedMaterialData(self, matvar, tempvar, tag):
    try:
        templist = QStringList()
        for material in self.materials:
            if material.attrib["name"] == matvar:
                temps = material.getiterator("temp")
                for temp in temps:
                    templist.append(temp.text)
        templist.sort()
        target = int(tempvar)
        x1 = max(int(t) for t in templist if t != '' and int(t) < target)
        x2 = min(int(t) for t in templist if t != '' and int(t) > target)
        y1 = float(self.GetMaterialData(matvar, str(x1), tag))
        y2 = float(self.GetMaterialData(matvar, str(x2), tag))
        x = target
        y = y1 - ((y1 - y2) * (x - x1) / (x2 - x1))
        return str(y)
    except Exception, inst:
        return "0"

Answer 1

A better and much faster (code and cpu wise) way is to use bisect module which does binary search but for that you will need to sort the list first, here is the sample usage:

import bisect

mylist = [10, 20, 50, 200, 100, 300, 250, 150]
mylist.sort()

index = bisect.bisect(mylist, 55)
print "Greater than target", mylist[index]
print "Smaller than or equal to target", mylist[index-1]

output:

Greater than target 100
Smaller than or equal to target 50

Also you will need to check the returned index, if it is 0 it means you have passed target lower than the lowest

Answer 2

Edit: Ah, I used templist instead of list -- hence the confusion. I didn't mean it to be a one-line function; you still have to do the conversions. (Of course, as Mike DeSimone rightly points out, using list as a variable name is a terrible idea!! So I had a good reason for being confusing. :)

To be more explicit about it, here's a slightly streamlined version of the function (fixed to test properly for an empty list):

def GetNextHighTemp(self, temp, templist):
    templist = (int(t) for t in templist if t != '')
    templist = [t for t in templist if t < int(temp)]
    if templist: return max(templist)
    else: return None                   # or raise an error

Thanks to Mike for the suggestion to return None in case of an empty list -- I like that.

You could shorten this even more like so:

def GetNextHighTemp(self, temp, templist):
    try: return str(max(int(t) for t in templist if t != '' and int(t) < int(temp)))
    except ValueError: return None      # or raise a different error